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Why do we need to fork to create new processes? | 1,402,510,190,000 |
In Unix whenever we want to create a new process, we fork the current process, creating a new child process which is exactly the same as the parent process; then we do an exec system call to replace all the data from the parent process with that for the new process.
Why do we create a copy of the parent process in the first place and not create a new process directly?
|
The short answer is, fork is in Unix because it was easy to fit into the existing system at the time, and because a predecessor system at Berkeley had used the concept of forks.
From The Evolution of the Unix Time-sharing System (relevant text has been highlighted):
Process control in its modern form was designed and implemented within a couple of days. It is astonishing how easily it fitted into the existing system; at the same time it is easy to see how some of the slightly unusual features of the design are present precisely because they represented small, easily-coded changes to what existed. A good example is the separation of the fork and exec functions. The most common model for the creation of new processes involves specifying a program for the process to execute; in Unix, a forked process continues to run the same program as its parent until it performs an explicit exec. The separation of the functions is certainly not unique to Unix, and in fact it was present in the Berkeley time-sharing system, which was well-known to Thompson. Still, it seems reasonable to suppose that it exists in Unix mainly because of the ease with which fork could be implemented without changing much else. The system already handled multiple (i.e. two) processes; there was a process table, and the processes were swapped between main memory and the disk. The initial implementation of fork required only
1)
Expansion of the process table
2)
Addition of a fork call that copied the current process to the disk swap area, using the already existing swap IO primitives, and made some adjustments to the process table.
In fact, the PDP-7's fork call required precisely 27 lines of assembly code. Of course, other changes in the operating system and user programs were required, and some of them were rather interesting and unexpected. But a combined fork-exec would have been considerably more complicated, if only because exec as such did not exist; its function was already performed, using explicit IO, by the shell.
Since that paper, Unix has evolved. fork followed by exec is no longer the only way to run a program.
vfork was created to be a more efficient fork for the case where the new process intends to do an exec right after the fork. After doing a vfork, the parent and child processes share the same data space, and the parent process is suspended until the child process either execs a program or exits.
posix_spawn creates a new process and executes a file in a single system call. It takes a bunch of parameters that let you selectively share the caller's open files and copy its signal disposition and other attributes to the new process.
|
How do keyboard input and text output work? | 1,402,510,190,000 |
Suppose I press the A key in a text editor and this inserts the character a in the document and displays it on the screen. I know the editor application isn't directly communicating with the hardware (there's a kernel and stuff in between), so what is going on inside my computer?
|
There are several different scenarios; I'll describe the most common ones. The successive macroscopic events are:
Input: the key press event is transmitted from the keyboard hardware to the application.
Processing: the application decides that because the key A was pressed, it must display the character a.
Output: the application gives the order to display a on the screen.
GUI applications
The de facto standard graphical user interface of unix systems is the X Window System, often called X11 because it stabilized in the 11th version of its core protocol between applications and the display server. A program called the X server sits between the operating system kernel and the applications; it provides services including displaying windows on the screen and transmitting key presses to the window that has the focus.
Input
+----------+ +-------------+ +-----+
| keyboard |------------->| motherboard |-------->| CPU |
+----------+ +-------------+ +-----+
USB, PS/2, … PCI, …
key down/up
First, information about the key press and key release is transmitted from the keyboard to the computer and inside the computer. The details depend on the type of hardware. I won't dwell more on this part because the information remains the same throughout this part of the chain: a certain key was pressed or released.
+--------+ +----------+ +-------------+
-------->| kernel |------->| X server |--------->| application |
+--------+ +----------+ +-------------+
interrupt scancode keysym
=keycode +modifiers
When a hardware event happens, the CPU triggers an interrupt, which causes some code in the kernel to execute. This code detects that the hardware event is a key press or key release coming from a keyboard and records the scan code which identifies the key.
The X server reads input events through a device file, for example /dev/input/eventNNN on Linux (where NNN is a number). Whenever there is an event, the kernel signals that there is data to read from that device. The device file transmits key up/down events with a scan code, which may or may not be identical to the value transmitted by the hardware (the kernel may translate the scan code from a keyboard-dependent value to a common value, and Linux doesn't retransmit the scan codes that it doesn't know).
X calls the scan code that it reads a keycode. The X server maintains a table that translates key codes into keysyms (short for “key symbol”). Keycodes are numeric, whereas keysyms are names such as A, aacute, F1, KP_Add, Control_L, … The keysym may differ depending on which modifier keys are pressed (Shift, Ctrl, …).
There are two mechanisms to configure the mapping from keycodes to keysyms:
xmodmap is the traditional mechanism. It is a simple table mapping keycodes to a list of keysyms (unmodified, shifted, …).
XKB is a more powerful, but more complex mechanism with better support for more modifiers, in particular for dual-language configuration, among others.
Applications connect to the X server and receive a notification when a key is pressed while a window of that application has the focus. The notification indicates that a certain keysym was pressed or released as well as what modifiers are currently pressed. You can see keysyms by running the program xev from a terminal. What the application does with the information is up to it; some applications have configurable key bindings.
In a typical configuration, when you press the key labeled A with no modifiers, this sends the keysym a to the application; if the application is in a mode where you're typing text, this inserts the character a.
Relationship of keyboard layout and xmodmap goes into more detail on keyboard input. How do mouse events work in linux? gives an overview of mouse input at the lower levels.
Output
+-------------+ +----------+ +-----+ +---------+
| application |------->| X server |---····-->| GPU |-------->| monitor |
+-------------+ +----------+ +-----+ +---------+
text or varies VGA, DVI,
image HDMI, …
There are two ways to display a character.
Server-side rendering: the application tells the X server “draw this string in this font at this position”. The font resides on the X server.
Client-side rendering: the application builds an image that represents the character in a font that it chooses, then tells the X server to display that image.
See What are the purposes of the different types of XWindows fonts? for a discussion of client-side and server-side text rendering under X11.
What happens between the X server and the Graphics Processing Unit (the processor on the video card) is very hardware-dependent. Simple systems have the X server draw in a memory region called a framebuffer, which the GPU picks up for display. Advanced systems such as found on any 21st century PC or smartphone allow the GPU to perform some operations directly for better performance. Ultimately, the GPU transmits the screen content pixel by pixel every fraction of a second to the monitor.
Text mode application, running in a terminal
If your text editor is a text mode application running in a terminal, then it is the terminal which is the application for the purpose of the section above. In this section, I explain the interface between the text mode application and the terminal. First I describe the case of a terminal emulator running under X11. What is the exact difference between a 'terminal', a 'shell', a 'tty' and a 'console'? may be useful background here. After reading this, you may want to read the far more detailed What are the responsibilities of each Pseudo-Terminal (PTY) component (software, master side, slave side)?
Input
+-------------------+ +-------------+
----->| terminal emulator |-------------->| application |
+-------------------+ +-------------+
keysym character or
escape sequence
The terminal emulator receives events like “Left was pressed while Shift was down”. The interface between the terminal emulator and the text mode application is a pseudo-terminal (pty), a character device which transmits bytes. When the terminal emulator receives a key press event, it transforms this into one or more bytes which the application gets to read from the pty device.
Printable characters outside the ASCII range are transmitted as one or more byte depending on the character and encoding. For example, in the UTF-8 encoding of the Unicode character set, characters in the ASCII range are encoded as a single bytes, while characters outside that range are encoded as multiple bytes.
Key presses that correspond to a function key or a printable character with modifiers such as Ctrl or Alt are sent as an escape sequence. Escape sequences typically consist of the character escape (byte value 27 = 0x1B = \033, sometimes represented as ^[ or \e) followed by one or more printable characters. A few keys or key combination have a control character corresponding to them in ASCII-based encodings (which is pretty much all of them in use today, including Unicode): Ctrl+letter yields a character value in the range 1–26, Esc is the escape character seen above and is also the same as Ctrl+[, Tab is the same as Ctrl+I, Return is the same as Ctrl+M, etc.
Different terminals send different escape sequences for a given key or key combination. Fortunately, the converse is not true: given a sequence, there is in practice at most one key combination that it encodes. The one exception is the character 127 = 0x7f = \0177 which is often Backspace but sometimes Delete.
In a terminal, if you type Ctrl+V followed by a key combination, this inserts the first byte of the escape sequence from the key combination literally. Since escape sequences normally consist only of printable characters after the first one, this inserts the whole escape sequence literally. See key bindings table? for a discussion of zsh in this context.
The terminal may transmit the same escape sequence for some modifier combinations (e.g. many terminals transmit a space character for both Space and Shift+Space; xterm has a mode to distinguish modifier combinations but terminals based on the popular vte library don't). A few keys are not transmitted at all, for example modifier keys or keys that trigger a binding of the terminal emulator (e.g. a copy or paste command).
It is up to the application to translate escape sequences into symbolic key names if it so desires.
Output
+-------------+ +-------------------+
| application |-------------->| terminal emulator |--->
+-------------+ +-------------------+
character or
escape sequence
Output is rather simpler than input. If the application outputs a character to the pty device file, the terminal emulator displays it at the current cursor position. (The terminal emulator maintains a cursor position, and scrolls if the cursor would fall under the bottom of the screen.) The application can also output escape sequences (mostly beginning with ^[ or ^]) to tell the terminal to perform actions such as moving the cursor, changing the text attributes (color, bold, …), or erasing part of the screen.
Escape sequences supported by the terminal emulator are described in the termcap or terminfo database. Most terminal emulator nowadays are fairly closely aligned with xterm. See Documentation on LESS_TERMCAP_* variables? for a longer discussion of terminal capability information databases, and How to stop cursor from blinking and Can I set my local machine's terminal colors to use those of the machine I ssh into? for some usage examples.
Application running in a text console
If the application is running directly in a text console, i.e. a terminal provided by the kernel rather than by a terminal emulator application, the same principles apply. The interface between the terminal and the application is still a byte stream which transmits characters, with special keys and commands encoded as escape sequences.
Remote application, accessed over the network
Remote text application
If you run a program on a remote machine, e.g. over SSH, then the network communication protocol relays data at the pty level.
+-------------+ +------+ +-----+ +----------+
| application |<--------->| sshd |<--------->| ssh |<--------->| terminal |
+-------------+ +------+ +-----+ +----------+
byte stream byte stream byte stream
(char/seq) over TCP/… (char/seq)
This is mostly transparent, except that sometimes the remote terminal database may not know all the capabilities of the local terminal.
Remote X11 application
The communication protocol between applications an the server is itself a byte stream that can be sent over a network protocol such as SSH.
+-------------+ +------+ +-----+ +----------+
| application |<---------->| sshd |<------>| ssh |<---------->| X server |
+-------------+ +------+ +-----+ +----------+
X11 protocol X11 over X11 protocol
TCP/…
This is mostly transparent, except that some acceleration features such as movie decoding and 3D rendering that require direct communication between the application and the display are not available.
|
Easy command line method to determine specific ARM architecture string? | 1,402,510,190,000 |
I'm trying to write a script which will determine actions based on the architecture of the machine. I already use uname -m to gather the architecture line, however I do not know how many ARM architectures there are, nor do I know whether one is armhf, armel, or arm64.
As this is required for this script to determine whether portions of the script can be run or not, I am trying to find a simple way to determine if the architecture is armhf, armel or arm64. Is there any one-liner or simple command that can be used to output either armhf, armel, or arm64?
The script is specifically written for Debian and Ubuntu systems, and I am tagging as such with this in mind (it quits automatically if you aren't on one of those distros, but this could be applied in a much wider way as well if the command(s) exist)
EDIT: Recently learned that armel is dead, and arm64 software builders (PPA or virtual based) aren't the most stable. So I have a wildcard search finding arm* and assuming armhf, but it's still necessary to figure out a one liner that returns one of the three - whether it's a Ubuntu/Debian command or a kernel call or something.
|
On Debian and derivatives,
dpkg --print-architecture
will output the primary architecture of the machine it’s run on. This will be armhf on a machine running 32-bit ARM Debian or Ubuntu (or a derivative), arm64 on a machine running 64-bit ARM.
On RPM-based systems,
rpm --eval '%{_arch}'
will output the current architecture name (which may be influenced by other parameters, e.g. --target).
Note that the running architecture may be different from the hardware architecture or even the kernel architecture. It’s possible to run i386 Debian on a 64-bit Intel or AMD CPU, and I believe it’s possible to run armhf on a 64-bit ARM CPU. It’s also possible to have mostly i386 binaries (so the primary architecture is i386) on an amd64 kernel, or even binaries from an entirely different architecture if it’s supported by QEMU (a common use for this is debootstrap chroots used for cross-compiling).
|
Why doesn't cp have a progress bar like wget? | 1,402,510,190,000 |
Please note that I don't ask how. I already know options like pv and rsync -P.
I want to ask why doesn't cp implement a progress bar, at least as a flag ?
|
The tradition in unix tools is to display messages only if something goes wrong. I think this is both for design and practical reasons. The design is intended to make it obvious when something goes wrong: you get an error message, and it's not drowned in not-actually-informative messages. The practical reason is that in unix's very early days, there still were teleprinters; that is, the output from programs would be printed on paper, and you don't want to print progress bars.
Whatever the reason, the tradition of only displaying useful messages has stuck in the unix world. Modern tools have sometimes introduced progress bars; in rsync's case, the main motivation is that rsync is often performed over the network, and networks are a lot flakier than local disks, so the progress bar is more useful. The same reasoning applies to wget.
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Will a Linux executable compiled on one "flavor" of Linux run on a different one? | 1,402,510,190,000 |
Will the executable of a small, extremely simple program, such as the one shown below, that is compiled on one flavor of Linux run on a different flavor? Or would it need to be recompiled?
Does machine architecture matter in a case such as this?
int main()
{
return (99);
}
|
It depends. Something compiled for IA-32 (Intel 32-bit) may run on amd64 as Linux on Intel retains backwards compatibility with 32-bit applications (with suitable software installed). Here's your code compiled on RedHat 7.3 32-bit system (circa 2002, gcc version 2.96) and then the binary copied over to and run on a Centos 7.4 64-bit system (circa 2017):
-bash-4.2$ file code
code: ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV), dynamically linked (uses shared libs), for GNU/Linux 2.2.5, not stripped
-bash-4.2$ ./code
-bash: ./code: /lib/ld-linux.so.2: bad ELF interpreter: No such file or directory
-bash-4.2$ sudo yum -y install glibc.i686
...
-bash-4.2$ ./code ; echo $?
99
Ancient RedHat 7.3 to Centos 7.4 (essentially RedHat Enterprise Linux 7.4) is staying in the same "distribution" family, so will likely have better portability than going from some random "Linux from scratch" install from 2002 to some other random Linux distribution in 2018.
Something compiled for amd64 would not run on 32-bit only releases of Linux (old hardware does not know about new hardware). This is also true for new software compiled on modern systems intended to be run on ancient old things, as libraries and even system calls may not be backwards portable, so may require compilation tricks, or obtaining an old compiler and so forth, or possibly instead compiling on the old system. (This is a good reason to keep virtual machines of ancient old things around.)
Architecture does matter; amd64 (or IA-32) is vastly different from ARM or MIPS so the binary from one of those would not be expected to run on another. At the assembly level the main section of your code on IA-32 compiles via gcc -S code.c to
main:
pushl %ebp
movl %esp,%ebp
movl $99,%eax
popl %ebp
ret
which an amd64 system can deal with (on a Linux system--OpenBSD by contrast on amd64 does not support 32-bit binaries; backwards compatibility with old archs does give attackers wiggle room, e.g. CVE-2014-8866 and friends). Meanwhile on a big-endian MIPS system main instead compiles to:
main:
.frame $fp,8,$31
.mask 0x40000000,-4
.fmask 0x00000000,0
.set noreorder
.set nomacro
addiu $sp,$sp,-8
sw $fp,4($sp)
move $fp,$sp
li $2,99
move $sp,$fp
lw $fp,4($sp)
addiu $sp,$sp,8
j $31
nop
which an Intel processor will have no idea what to do with, and likewise for the Intel assembly on MIPS.
You could possibly use QEMU or some other emulator to run foreign code (perhaps very, very slowly).
However! Your code is very simple code, so will have fewer portability issues than anything else; programs typically make use of libraries that have changed over time (glibc, openssl, ...); for those one may also need to install older versions of various libraries (RedHat for example typically puts "compat" somewhere in the package name for such)
compat-glibc.x86_64 1:2.12-4.el7.centos
or possibly worry about ABI changes (Application Binary Interface) for way old things that use glibc, or more recently changes due to C++11 or other C++ releases. One could also compile static (greatly increasing the binary size on disk) to try to avoid library issues, though whether some old binary did this depends on whether the old Linux distribution was compiling most everything dynamic (RedHat: yes) or not. On the other hand, things like patchelf can rejigger dynamic (ELF, but probably not a.out format) binaries to use other libraries.
However! Being able to run a program is one thing, and actually doing something useful with it another. Old 32-bit Intel binaries may have security issues if they depend on a version of OpenSSL that has some horrible and not-backported security problem in it, or the program may not be able to negotiate at all with modern web servers (as the modern servers reject the old protocols and ciphers of the old program), or SSH protocol version 1 is no longer supported, or ...
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A layman's explanation for "Everything is a file" — what differs from Windows? | 1,402,510,190,000 |
I know that "Everything is a file" means that even devices have their filename and path in Unix and Unix-like systems, and that this allows for common tools to be used on a variety of resources regardless of their nature. But I can't contrast that to Windows, the only other OS I have worked with. I have read some articles about the concept, but I think they are somewhat uneasy to grasp for non-developers. A layman's explanation is what people need!
For example, when I want to copy a file to CF card that is attached to a card reader, I will use something like
zcat name_of_file > /dev/sdb
In Windows, I think the card reader will appear as a driver, and we will do something similar, I think. So, how does the "Everything is a file" philosophy make a difference here?
|
"Everything is a file" is a bit glib. "Everything appears somewhere in the file system" is closer to the mark, and even then, it's more an ideal than a law of system design.
For example, Unix domain sockets are not files, but they do appear in the file system. You can ls -l a domain socket to display its attributes, modify its access control via chmod, and on some Unix type systems (e.g. macOS, but not Linux) you can even cat data to/from one.
But, even though regular TCP/IP network sockets are created and manipulated with the same BSD sockets system calls as Unix domain sockets, TCP/IP sockets do not show up in the file system,¹ even though there is no especially good reason that this should be true.
Another example of non-file objects appearing in the file system is Linux's /proc file system. This feature exposes a great amount of detail about the kernel's run-time operation to user space, mostly as virtual plain text files. Many /proc entries are read-only, but a lot of /proc is also writeable, so you can change the way the system runs using any program that can modify a file. Alas, here again we have a nonideality: BSD Unixes run without /proc by default, and the System V Unixes expose a lot less via /proc than Linux does.
I can't contrast that to MS Windows
First, much of the sentiment you can find online and in books about Unix being all about file I/O and Windows being "broken" in this regard is obsolete. Windows NT fixed a lot of this.
Modern versions of Windows have a unified I/O system, just like Unix, so you can read network data from a TCP/IP socket via ReadFile() rather than the Windows Sockets specific API WSARecv(), if you want to. This exactly parallels the Unix Way, where you can read from a network socket with either the generic read(2) Unix system call or the sockets-specific recv(2) call.²
Nevertheless, Windows still fails to take this concept to the same level as Unix, even here in 2021. There are many areas of the Windows architecture that cannot be accessed through the file system, or that can't be viewed as file-like. Some examples:
Drivers.
Windows' driver subsystem is easily as rich and powerful as Unix's, but to write programs to manipulate drivers, you generally have to use the Windows Driver Kit, which means writing C or .NET code.
On Unix type OSes, you can do a lot to drivers from the command line. You've almost certainly already done this, if only by redirecting unwanted output to /dev/null.³
Inter-program communication.
Windows programs don't communicate easily with each other as Unix command line programs do, via text streams and pipes. Unix GUIs are often either built on top of command line programs or export a text command interface, so the same simple text-based communication mechanisms work with GUI programs, too.
The registry.
Unix has no direct equivalent of the Windows registry. The same information is scattered through the file system, largely in /etc, /proc and /sys.
If you don't see that drivers, pipes, and Unix's answer to the Windows registry have anything to do with "everything is a file," read on.
How does the "Everything is a file" philosophy make a difference here?
I will explain that by expanding on my three points above, in detail.
Long answer, part 1: Drives vs Device Files
Let's say your CF card reader appears as E: under Windows and /dev/sdc under Linux. What practical difference does it make?
It is not just a minor syntax difference.
On Linux, I can say dd if=/dev/zero of=/dev/sdc to overwrite the contents of /dev/sdc with zeroes.
Think about what that means for a second. Here I have a normal user space program (dd(1)) that I asked to read data in from a virtual device (/dev/zero) and write what it read out to a real physical device (/dev/sdc) via the unified Unix file system. dd doesn't know it is reading from and writing to special devices. It will work on regular files just as well, or on a mix of devices and files, as we will see below.
There is no easy way to zero the E: drive on Windows because Windows makes a distinction between files and drives, so you cannot use the same commands to manipulate them. The closest you can get is to do a disk format without the Quick Format option, which zeroes most of the drive contents, but then writes a new file system on top of it. What if I don't want a new file system? What if I really do want the disk to be filled with nothing but zeroes?
Let's be generous and accept this requirement to put a fresh new file system on E:. To do that in a program on Windows, I have to call a special formatting API.⁴ On Linux, you don't need to write a program to access the OS's "format disk" functionality: you just run the appropriate user space program for the file system type you want to create, whether that's mkfs.ext4, mkfs.xfs, or what have you. These programs will write a file system onto whatever file or /dev node you pass.
Because mkfs type programs on Unixy systems don't make artificial distinctions between devices and normal files, I can create an ext4 file system inside a normal file on my Linux box:
$ dd if=/dev/zero of=myfs bs=1k count=1k
$ mkfs.ext4 -F myfs
That creates a 1 MiB disk image called myfs in the current directory. I can then mount it as if it were any other external file system:
$ mkdir mountpoint
$ sudo mount -o loop myfs mountpoint
$ grep $USER /etc/passwd > mountpoint/my-passwd-entry
$ sudo umount mountpoint
Now I have an ext4 disk image with a file called my-passwd-entry in it which contains my user's /etc/passwd entry.
If I want, I can blast that image onto my CF card:
$ sudo dd if=myfs of=/dev/sdc1
Or, I can pack that disk image up, mail it to you, and let you write it to a medium of your choosing, such as a USB memory stick:
$ gzip myfs
$ echo "Here's the disk image I promised to send you." |
mutt -a myfs.gz -s "Password file disk image" \
[email protected]
All of this is possible on Linux⁵ because there is no artificial distinction between files, file systems, and devices. Many things on Unix systems either are files, or are accessed through the file system so they look like files, or in some other way look sufficiently file-like that they can be treated as such.
Windows' concept of the file system is a hodgepodge; it makes distinctions between directories, drives, and network resources. There are three different syntaxes, all blended together in Windows: the Unix-like ..\FOO\BAR path system, drive letters like C:, and UNC paths like \\SERVER\PATH\FILE.TXT. This is because it's an accretion of ideas from Unix, CP/M, MS-DOS, and LAN Manager rather than a single coherent design. It is why there are so many illegal characters in Windows file names.
Unix has a unified file system, with everything accessed by a single common scheme. To a program running on a Linux box, there is no functional difference between /etc/passwd, /media/CF_CARD/etc/passwd, and /mnt/server/etc/passwd. Local files, external media, and network shares all get treated the same way.⁶
Windows can achieve similar ends to my disk image example above, but you have to use special programs written by uncommonly talented programmers. This is why there are so many "virtual DVD" type programs on Windows. The lack of a core OS feature has created an artificial market for programs to fill the gap, which means you have a bunch of people competing to create the best virtual DVD type program. We don't need such programs on *ix systems, because we can just mount an ISO disk image using a loop device.
The same goes for other tools like disk wiping programs, which we also don't need on Unix systems. Want your CF card's contents irretrievably scrambled instead of just zeroed? Okay, use /dev/random as the data source instead of /dev/zero:
$ sudo dd if=/dev/random of=/dev/sdc
On Linux, we don't keep reinventing such wheels because the core OS features not only work well enough, they work so well they're used pervasively. One of several ways for booting a Linux box involves a virtual disk image created using techniques like I show above.
I feel it's only fair to point out that if Unix had integrated TCP/IP I/O into the file system from the start, we wouldn't have the netcat vs socat vs ncat vs nc mess, the cause of which was the same design weakness that led to the disk imaging and wiping tool proliferation on Windows: lack of an acceptable OS facility.
Long Answer, part 2: Pipes as Virtual Files
Despite its roots in MS-DOS, Windows never has had a rich command line tradition.
This is not to say that Windows doesn't have a command line, or that it lacks many command line programs. Windows even has a very powerful command shell these days, appropriately called PowerShell.
Yet, there are knock-on effects of this lack of a command-line tradition. You get tools like DISKPART which is almost unknown in the Windows world, because most people do disk partitioning and such through the Computer Management MMC snap-in. Then when you do need to script the creation of partitions, you find that DISKPART wasn't really made to be driven by another program. Yes, you can write a series of commands into a script file and run it via DISKPART /S scriptfile, but it's all-or-nothing. What you really want in such a situation is something more like GNU parted, which will accept single commands like parted /dev/sdb mklabel gpt. That allows your script to do error handling on a step-by-step basis.
What does all this have to do with "everything is a file"? Easy: pipes make command line program I/O into "files," of a sort. Pipes are unidirectional streams, not random-access like a regular disk file, but in many cases the difference is of no consequence. The important thing is that you can attach two independently developed programs and make them communicate via simple text. In that sense, any two programs designed with the Unix Way in mind can communicate.
In those cases where you really do need a file, it is easy to turn program output into a file:
$ some-program --some --args > myfile
$ vi myfile
But why write the output to a temporary file when the "everything is a file" philosophy gives you a better way? If all you want to do is read the output of that command into a vi editor buffer, you can do that directly from the vi "normal" mode:
:r !some-program --some --args
That inserts that program's output into the active editor buffer at the current cursor position. Under the hood, vi is using pipes to connect the output of the program to a bit of code that uses the same OS calls it would use to read from a file instead. I wouldn't be surprised if the two cases of :r — that is, with and without the ! — both used the same generic data reading loop in all common implementations of vi. I can't think of a good reason not to.
This isn't a recent feature of vi, either; it goes clear back to the ancient ed(1) text editor.
This powerful idea pops up over and over in Unix.
For a second example of this, recall my mutt email command above. The only reason I had to write that as two separate commands is that I wanted the temporary file to be named *.gz so that the email attachment would be correctly named. If I didn't care about the file's name, I could have used process substitution to avoid creating the temporary file:
$ echo "Here's the disk image I promised to send you." |
mutt -a <(gzip -c myfs) -s "Password file disk image" \
[email protected]
That turns the output of gzip -c into a FIFO (which is file-like) or a /dev/fd object (which is file-like).⁷
For yet a third way this powerful idea appears in Unix, consider gdb on Linux systems. This is the debugger used for any software written in C and C++. Programmers coming to Unix from other systems look at gdb and almost invariably gripe, "Yuck, it's so primitive!" Then they go searching for a GUI debugger, find one of several that exist, and happily continue their work…often never realizing that the GUI just runs gdb underneath, providing a pretty shell on top of it. There aren't competing low-level debuggers on most Unix systems because there is no need for programs to compete at that level. All we need is one good low-level tool that we can all base our high-level tools on, if that low-level tool communicates easily via pipes.
This means we now have a documented debugger interface which would allow drop-in replacement of gdb. It's unfortunate that the primary competitor to gdb didn't take this low-friction path, but that quibble aside, lldb is just as scriptable as gdb.
To pull the same thing off on a Windows box, the creators of the replaceable tool would have had to define some kind of formal plugin or automation API. That means it doesn't happen except for the very most popular programs, because it's a lot of work to build both a normal command line user interface and a complete programming API.
This magic happens through the grace of pervasive text-based IPC.
Although Windows' kernel has Unix-style anonymous pipes, it's rare to see normal user programs use them for IPC outside of a command shell, because Windows lacks this tradition of creating all core services in a command line version first, then building the GUI on top of it separately. This leads to being unable to do some things without the GUI, which is one reason why there are so many remote desktop systems for Windows, as compared to Linux. This is doubtless part of the reason why Linux is the operating system of the cloud, where everything's done by remote management. Command line interfaces are easier to automate than GUIs in large part because "everything is a file."
Consider SSH. You may ask, how does it work? SSH connects a network socket (which is file-like) to a pseudo tty at /dev/pty* (which is file-like). Now your remote system is connected to your local one through a connection that so seamlessly matches the Unix Way that you can pipe data through the SSH connection, if you need to.
Are you getting an idea of just how powerful this concept is now?
A piped text stream is indistinguishable from a file from a program's perspective, except that it's unidirectional. A program reads from a pipe the same way it reads from a file: through a file descriptor. FDs are absolutely core to Unix; the fact that files, pipes, and network sockets all use the same abstraction for I/O on both should tell you something.
The Windows world, lacking this tradition of simple text communications, makes do with heavyweight OOP interfaces via COM or .NET. If you need to automate such a program, you must also write a COM or .NET program. This is a fair bit more difficult than setting up a pipe on a Unix box.
Windows programs lacking these complicated programming APIs can only communicate through impoverished interfaces like the clipboard or File/Save followed by File/Open.
Long Answer, part 3: The Registry vs Configuration Files
The practical difference between the Windows registry and the Unix Way of system configuration also illustrates the benefits of the "everything is a file" philosophy.
On Unix type systems, I can look at system configuration information from the command line merely by examining files. I can change system behavior by modifying those same files. For the most part, these configuration files are just plain text files, which means I can use any tool on Unix to manipulate them that can work with plain text files.
Scripting the registry is not nearly so easy on Windows.
The easiest method is to make your changes through the Registry Editor GUI on one machine, then blindly apply those changes to other machines with regedit via *.reg files. That isn't really "scripting," since it doesn't let you do anything conditionally: it's all or nothing.
If your registry changes need any amount of logic, the next easiest option is to learn PowerShell, which amounts to learning .NET system programming. It would be like if Unix only had Perl, and you had to do all ad hoc system administration through it. Now, I'm a Perl fan, but not everyone is. Unix lets you use any tool you happen to like, as long as it can manipulate plain text files.
Footnotes:
Plan 9 fixed this design misstep, exposing network I/O via the /net virtual file system.
Bash has /dev/tcp that allows network I/O via regular file system functions. Since it is a Bash feature, rather a kernel feature, it isn't visible outside of Bash or on systems that don't use Bash at all. This shows, by counterexample, why it is such a good idea to make all data resources visible through the file system.
By "modern Windows," I mean Windows NT and all of its direct descendants, which includes Windows 2000, all versions of Windows Server, and all desktop-oriented versions of Windows from XP onward. I use the term to exclude the MS-DOS-based versions of Windows, being Windows 95 and its direct descendants, Windows 98 and Windows ME, plus their 16-bit predecessors.
You can see the distinction by the lack of a unified I/O system in those latter OSes. You cannot pass a TCP/IP socket to ReadFile() on Windows 95; you can only pass sockets to the Windows Sockets APIs. See Andrew Schulman's seminal article, Windows 95: What It's Not for a deeper dive into this topic.
Make no mistake, /dev/null is a real kernel device on Unix type systems, not just a special-cased file name, as is the superficially equivalent NUL in Windows.
Although Windows tries to prevent you from creating a NUL file, it is possible to bypass this protection with mere trickery, fooling Windows' file name parsing logic. If you try to access that file with cmd.exe or Explorer, Windows will refuse to open it, but you can write to it via Cygwin, since it opens files using similar methods to the example program, and you can delete it via similar trickery.
By contrast, Unix will happily let you rm /dev/null, as long as you have write access to /dev, and let you recreate a new file in its place, all without trickery, because that dev node is just another file. While that dev node is missing, the kernel's null device still exists; it's just inaccessible until you recreate the dev node via mknod.
You can even create additional null device dev nodes elsewhere: it doesn't matter if you call it /home/grandma/Recycle Bin, as long as it's a dev node for the null device, it will work exactly the same as /dev/null.
There are actually two high-level "format disk" APIs in Windows: SHFormatDrive() and Win32_Volume.Format().
There are two for a very…well…Windows sort of reason. The first one asks Windows Explorer to display its normal "Format Disk" dialog box, which means it works on any modern version of Windows, but only while a user is interactively logged in. The other you can call in the background without user input, but it wasn't added to Windows until Windows Server 2003. That's right, core OS behavior was hidden behind a GUI until 2003, in a world where Unix shipped mkfs from day 1.
The /etc/mkfs in my copy of Unix V5 from 1974 is a 4136 byte statically-linked PDP-11 executable. (Unix didn't get dynamic linkage until the late 1980s, so it's not like there's a big library somewhere else doing all the real work.) Its source code — included in the V5 system image as /usr/source/s2/mkfs.c — is an entirely self-contained 457-line C program. There aren't even any #include statements!
This means you can not only examine what mkfs does at a high level, you can experiment with it using the same tool set Unix was created with, just like you're Ken Thompson, four decades ago. Try that with Windows. The closest you can come today is to download the MS-DOS source code, first released in 2014, which you find amounts to just a pile of assembly sources. It will only build with obsolete tools you probably won't have on-hand, and in the end you get your very own copy of MS-DOS 2.0, an OS far less powerful than 1974's Unix V5, despite its being released nearly a decade later.
(Why talk about Unix V5? Because it is the earliest complete Unix system still available. Earlier versions are apparently lost to time. There was a project that pieced together a V1/V2 era Unix, but it appears to be missing mkfs, despite the existence of the V1 manual page linked above proving it must have existed somewhere, somewhen. Either those putting this project together couldn't find an extant copy of mkfs to include, or I suck at finding files without find(1), which also doesn't exist in that system. :))
Now, you might be thinking, "Can't I just call format.com? Isn't that the same on Windows as calling mkfs on Unix?" Alas, no, it isn't the same, for a bunch of reasons:
First, format.com wasn't designed to be scripted. It prompts you to "press ENTER when ready", which means you need to send an Enter key to its input, or it'll just hang.
Then, if you want anything more than a success/failure status code, you have to open its standard output for reading, which is far more complicated on Windows than it has to be. (On Unix, everything in that linked article can be accomplished with a simple popen(3) call.)
Having gone through all this complication, the output of format.com is harder to parse for computer programs than the output of mkfs, being intended primarily for human consumption.
If you trace what format.com does, you find that it does a bunch of complicated calls to DeviceIoControl(), ufat.dll, and such. It is not simply opening a device file and writing a new file system onto that device. This is the sort of design you get from a company that employs 221000 people worldwide and needs to keep employing them.
Contrast what happens when your core OS tools are written by volunteers in their spare time: they come up with expedient, minimal solutions to their problems that pay simplicity dividends to the rest of us.
When talking about loop devices, I talk only about Linux rather than Unix in general because loop devices aren't portable between Unix type systems. There are similar mechanisms in macOS, BSD, etc., but the syntax varies somewhat.
Back in the days when disk drives were the size of washing machines and cost more than the department head's luxury car, big computer labs would share a larger proportion of their collective disk space as compared to modern computing environments. The ability to transparently graft a remote disk into the local file system made such distributed systems far easier to use. This is where we get /usr/share, for instance.
Contrast Windows, where drive letters offer you few choices for symbolic expression; does P: refer to the "public" space on BigServer or to the "packages" directory on the software mirror server? The UNC alternative requires you to remember which server your remote files are on, which gets difficult in a large organization with hundreds or thousands of file servers.
Windows didn't get symlinks until 2007, when Vista introduced NTFS symbolic links, and they weren't made usable until a decade later. Windows' symbolic links are more powerful than Unix's symbolic links — a feature of Unix since since 1977 — in that they can also point to a remote file share, not just to a local path. Unix did that differently, via NFS in 1984, which builds on top of Unix's preexisting mount point feature, which it has had since the beginning.
So, depending on how you look at it, Windows trailed Unix by roughly 2, 3, or 4 decades. You may object, "But it has Unix-style symlinks now!" Yet this misses the point, since it means there is no decades-old tradition of using them on Windows, so people are unaware of them in a world where Unix systems use them pervasively. It's impossible to use a Unix system for any significant length of time without learning about symlinks.
It doesn't help that Windows' MKLINK program is backwards, and you still can't create them from Windows Explorer, whereas the Unix equivalents to Windows Explorer typically do let you create symlinks.
Bash chooses the method based on the system's capabilities since /dev/fd isn't available everywhere.
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Why is rm allowed to delete a file under ownership of a different user? | 1,402,510,190,000 |
From the post Why can rm remove read-only files? I understand that rm just needs write permission on directory to remove the file. But I find it hard to digest the behaviour where we can easily delete a file who owner and group different.
I tried the following
mtk : my username
abc : created a new user
$ ls -l file
-rw-rw-r-- 1 mtk mtk 0 Aug 31 15:40 file
$ sudo chown abc file
$ sudo chgrp abc file
$ ls -l file
-rw-rw-r-- 1 abc abc 0 Aug 31 15:40 file
$ rm file
$ ls -l file
<deleted>
I was thinking this shouldn't have been allowed. A user should be able to delete only files under his ownership? Can someone shed light on why this is permitted? and what is the way to avoid this? I can think only restricting the write permission of the parent directory to dis-allow surprised deletes of file.
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The reason why this is permitted is related to what removing a file actually does. Conceptually, rm's job is to remove a name entry from a directory. The fact that the file may then become unreachable if that was the file's only name and that the inode and space occupied by the file can therefore be recovered at that point is almost incidental. The name of the system call that the rm command invokes, which is unlink, is even suggestive of this fact.
And, removing a name entry from a directory is fundamentally an operation on that directory, so that directory is the thing that you need to have permission to write.
The following scenario may make it feel more comfortable? Suppose there are directories:
/home/me # owned and writable only by me
/home/you # owned and writable only by you
And there is a file which is owned by me and which has two hard links:
/home/me/myfile
/home/you/myfile
Never mind how that hard link /home/you/myfile got there in the first place. Maybe root put it there.
The idea of this example is that you should be allowed to remove the hard link /home/you/myfile. After all, it's cluttering up your directory. You should be able to control what does and doesn't exist inside /home/you. And when you do remove /home/you/myfile, notice that you haven't actually deleted the file. You've only removed one link to it.
Note that if the sticky bit is set on the directory containing a file (shows up as t in ls), then you do need to be the owner of the file in order to be allowed to delete it (unless you own the directory). The sticky bit is usually set on /tmp.
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On Unix systems, why do we have to explicitly `open()` and `close()` files to be able to `read()` or `write()` them? | 1,402,510,190,000 |
Why do open() and close() exist in the Unix filesystem design?
Couldn't the OS just detect the first time read() or write() was called and do whatever open() would normally do?
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Dennis Ritchie mentions in «The Evolution of the Unix Time-sharing System» that open and close along with read, write and creat were present in the system right from the start.
I guess a system without open and close wouldn't be inconceivable, however I believe it would complicate the design.
You generally want to make multiple read and write calls, not just one, and that was probably especially true on those old computers with very limited RAM that UNIX originated on. Having a handle that maintains your current file position simplifies this. If read or write were to return the handle, they'd have to return a pair -- a handle and their own return status. The handle part of the pair would be useless for all other calls, which would make that arrangement awkward. Leaving the state of the cursor to the kernel allows it to improve efficiency not only by buffering. There's also some cost associated with path lookup -- having a handle allows you to pay it only once. Furthermore, some files in the UNIX worldview don't even have a filesystem path (or didn't -- now they do with things like /proc/self/fd).
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What are software and hardware interrupts, and how are they processed? | 1,402,510,190,000 |
I am not sure if I understand the concept of hardware and software interrupts.
If I understand correctly, the purpose of a hardware interrupt is to get some attention of the CPU, part of implementing CPU multitasking.
Then what issues a hardware interrupt? Is it the hardware driver process?
If yes, where is the hardware driver process running? If it is running on the CPU, then it won't have to get attention of the CPU by hardware interrupt, right? So is it running elsewhere?
Does a hardware interrupt interrupt the CPU directly, or does it first contact the kernel process and the kernel process then contacts/interrupts the CPU?
On the other hand, I think the purpose of a software interrupt is for a process currently running on a CPU to request some resources.
What are the resources? Are they all in the form of running processes? For example, do CPU driver process and memory driver processes represent CPU and memory resources? Do the driver process of the I/O devices represent I/O resources? Are other running processes that the process would like to communicate with also resources?
If yes, does a software interrupt contact the processes (which represent the resources) indirectly via the kernel process? Is it right that unlike a hardware interrupt, a software interrupt never directly interrupts the CPU, but instead, it interrupts/contacts the kernel process?
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A hardware interrupt is not really part of CPU multitasking, but may drive it.
Hardware interrupts are issued by hardware devices like disk, network cards, keyboards, clocks, etc. Each device or set of devices will have its own IRQ (Interrupt ReQuest) line. Based on the IRQ the CPU will dispatch the request to the appropriate hardware driver. (Hardware drivers are usually subroutines within the kernel rather than a separate process.)
The driver which handles the interrupt is run on the CPU. The CPU is interrupted from what it was doing to handle the interrupt, so nothing additional is required to get the CPU's attention. In multiprocessor systems, an interrupt will usually only interrupt one of the CPUs. (As a special cases mainframes have hardware channels which can deal with multiple interrupts without support from the main CPU.)
The hardware interrupt interrupts the CPU directly. This will cause the relevant code in the kernel process to be triggered. For processes that take some time to process, the interrupt code may allow itself to be interrupted by other hardware interrupts.
In the case of timer interrupt, the kernel scheduler code may suspend the process that was running and allow another process to run. It is the presence of the scheduler code which enables multitasking.
Software interrupts are processed much like hardware interrupts. However, they can only be generated by processes which are currently running.
Typically software interrupts are requests for I/O (Input or Output). These will call kernel routines which will schedule the I/O to occur. For some devices the I/O will be done immediately, but disk I/O is usually queued and done at a later time. Depending on the I/O being done, the process may be suspended until the I/O completes, causing the kernel scheduler to select another process to run. I/O may occur between processes and the processing is usually scheduled in the same manner as disk I/O.
The software interrupt only talks to the kernel. It is the responsibility of the kernel to schedule any other processes which need to run. This could be another process at the end of a pipe. Some kernels permit some parts of a device driver to exist in user space, and the kernel will schedule this process to run when needed.
It is correct that a software interrupt doesn't directly interrupt the CPU. Only code that is currently running code can generate a software interrupt. The interrupt is a request for the kernel to do something (usually I/O) for running process. A special software interrupt is a Yield call, which requests the kernel scheduler to check to see if some other process can run.
Response to comment:
For I/O requests, the kernel delegates the work to the appropriate kernel driver. The routine may queue the I/O for later processing (common for disk I/O), or execute it immediately if possible. The queue is handled by the driver, often when responding to hardware interrupts. When one I/O completes, the next item in the queue is sent to the device.
Yes, software interrupts avoid the hardware signalling step. The process generating the software request must be a currently running process, so they don't interrupt the CPU. However, they do interrupt the flow of the calling code.
If hardware needs to get the CPU to do something, it causes the CPU to interrupt its attention to the code it is running. The CPU will push its current state on a stack so that it can later return to what it was doing. The interrupt could stop: a running program; the kernel code handling another interrupt; or the idle process.
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Object-oriented shell for *nix | 1,402,510,190,000 |
Preface: I love bash and have no intention of starting any sort of argument or holy-war, and hopefully this is not an extremely naive question.
This question is somewhat related to this post on superuser, but I don't think the OP really knew what he was asking for. I use bash on FreeBSD, linux, OS X, and cygwin on Windows. I've also had extensive experience recently with PowerShell on Windows.
Is there a shell for *nix, already available or in the works, that is compatible with bash but adds a layer of object-oriented scripting into the mix? The only thing I know of that comes close is the python console, but as far as I can tell it doesn't provide access to the standard shell environment. For example, I can't just cd ~ and ls, then chmod +x file inside the python console. I would have to use python to perform those tasks rather than the standard unix binaries, or call the binaries using python code.
Does such a shell exist?
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I can think of three desirable features in a shell:
Interactive usability: common commands should be quick to type; completion; ...
Programming: data structures; concurrency (jobs, pipe, ...); ...
System access: working with files, processes, windows, databases, system configuration, ...
Unix shells tend to concentrate on the interactive aspect and subcontract most of the system access and some of the programming to external tools, such as:
bc for simple math
openssl for cryptography
sed, awk and others for text processing
nc for basic TCP/IP networking
ftp for FTP
mail, Mail, mailx, etc. for basic e-mail
cron for scheduled tasks
wmctrl for basic X window manipulation
dcop for KDE ≤3.x libraries
dbus tools (dbus-* or qdbus) for various system information and configuration tasks (including modern desktop environments such as KDE ≥4)
Many, many things can be done by invoking a command with the right arguments or piped input. This is a very powerful approach — better have one tool per task that does it well, than a single program that does everything but badly — but it does have its limitations.
A major limitation of unix shells, and I suspect this is what you're after with your “object-oriented scripting” requirement, is that they are not good at retaining information from one command to the next, or combining commands in ways fancier than a pipeline. In particular, inter-program communication is text-based, so applications can only be combined if they serialize their data in a compatible way. This is both a blessing and a curse: the everything-is-text approach makes it easy to accomplish simple tasks quickly, but raises the barrier for more complex tasks.
Interactive usability also runs rather against program maintainability. Interactive programs should be short, require little quoting, not bother you with variable declarations or typing, etc. Maintainable programs should be readable (so not have many abbreviations), should be readable (so you don't have to wonder whether a bare word is a string, a function name, a variable name, etc.), should have consistency checks such as variable declarations and typing, etc.
In summary, a shell is a difficult compromise to reach. Ok, this ends the rant section, on to the examples.
The Perl Shell (psh) “combines the interactive nature of a Unix shell with the power of Perl”. Simple commands (even pipelines) can be entered in shell syntax; everything else is Perl. The project hasn't been in development for a long time. It's usable, but hasn't reached the point where I'd consider using it over pure Perl (for scripting) or pure shell (interactively or for scripting).
IPython is an improved interactive Python console, particularly targetted at numerical and parallel computing. This is a relatively young project.
irb (interactive ruby) is the Ruby equivalent of the Python console.
scsh is a scheme implementation (i.e. a decent programming language) with the kind of system bindings traditionally found in unix shells (strings, processes, files). It doesn't aim to be usable as an interactive shell however.
zsh is an improved interactive shell. Its strong point is interactivity (command line edition, completion, common tasks accomplished with terse but cryptic syntax). Its programming features aren't that great (on par with ksh), but it comes with a number of libraries for terminal control, regexps, networking, etc.
fish is a clean start at a unix-style shell. It doesn't have better programming or system access features. Because it breaks compatibility with sh, it has more room to evolve better features, but that hasn't happened.
Addendum: another part of the unix toolbox is treating many things as files:
Most hardware devices are accessible as files.
Under Linux, /sys provides more hardware and system control.
On many unix variants, process control can be done through the /proc filesystem.
FUSE makes it easy to write new filesystems. There are already existing filesystems for converting file formats on the fly, accessing files over various network protocols, looking inside archives, etc.
Maybe the future of unix shells is not better system access through commands (and better control structures to combine commands) but better system access through filesystems (which combine somewhat differently — I don't think we've worked out what the key idioms (like the shell pipe) are yet).
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How does a Linux terminal work? | 1,402,510,190,000 |
If you fire up a terminal and call an executable (assuming one that's line oriented for simplicity) you get a reply to the command from the executable. How does this get printed to you (the user)? Does the terminal do something like pexpect? (poll waiting for output) or what? How does it get notified of output to be printed out? And how does a terminal start a program? (Is it something akin to python's os.fork()? ) I'm puzzled how a terminal works, I've been playing with some terminal emulator and I still don't get how all this magic works. I'm looking at the source of konsole (kde) and yakuake (possibly uses konsole) an I can't get where all that magic happens.
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Originally you had just dumb terminals - at first actually teletypewriters (similar to an electric typewriter, but with a roll of paper) (hence /dev/tty - TeleTYpers), but later screen+keyboard-combos - which just sent a key-code to the computer and the computer sent back a command that wrote the letter on the terminal (i.e. the terminal was without local echo, the computer had to order the terminal to write what the user typed on the terminal) - this is one of the reason why so many important Unix-commands are so short. Most terminals were connected by serial-lines, but (at least) one was directly connected to the computer (often the same room) - this was the console. Only a select few users were trusted to work on "the console" (this was often the only "terminal" available in single-user mode).
Later there also were some graphical terminals (so-called "xterminals", not to be confused with the xterm-program) with screen & graphical screen-card, keyboard, mouse and a simple processor; which could just run an X-server. They did not do any computations themselves, so the X-clients ran on the computer they were connected to. Some had hard disks, but they could also boot over the network. They were popular in the early 1990s, before PCs became so cheap and powerful.
Later still, there were "smart" or "intelligent" terminals. Smart terminals have the ability to process user input (line-editing at the shell prompt like inserting characters, removing words with Ctrl-W, removing letters with Ctrl-H or Backspace) without help from the computer. The earlier dumb terminals, on the other hand, could not perform such onsite line-editing. On a dumb terminal, when the user presses a key, the terminal sends/delegates the resulting key-code to the computer to handle. After handling it, the computer sends the result back to the dumb terminal to display (e.g. pressing Ctrl-W would send a key-code to the computer, the computer would interpret that to mean "delete the last word", so the computer would handle that text change, then simply give the dumb terminal the output it should display).
A "terminal emulator" – the "terminal-window" you open with programs such as xterm or konsole – tries to mimic the functionality of such smarter terminals. Also programs such as PuTTY (Windows) emulate these smart terminal emulators.
With the PC, where "the console" (keyboard+screen) and "the computer" is more of a single unit, you got "virtual terminals" (on Linux, keys Alt+F1 through Alt+F6) instead, but these too mimic old-style terminals. Of course, with Unix/Linux becoming more of a desktop operating system often used by a single user, you now do most of your work "at the console", where users before used terminals connected by serial-lines.
It's of course the shell that starts programs. And it uses the fork system-call (C language) to make a copy of itself with a environment-settings, then the exec system-call is used to turn this copy into the command you wanted to run. The shell suspends (unless the command is run in the background) until the command completes. As the command inherits the settings for stdin, stdout and stderr from the shell, the command will write to the terminal's screen and receive input from the terminal's keyboard.
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How does a unix or linux system work? [closed] | 1,402,510,190,000 |
I would like to know how the OS works in a nutshell:
The basic components it's built upon
How those components work together
What makes unix UNIX
What makes it so different from other OSs like Windows
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A UNIX system consists of several parts, or layers as I'd like to call them.
To start a system, a program called the boot loader lives at the first sector of a hard disk partition. It is started by the system, and in turn it locates the Operating System kernel, and load it.
Layering
The Kernel. This is the central program which is started by the boot loader. It does the basic hardware interaction for the system (disk, memory, video, sound) and offers a virtual environment in which it can start programs. The kernel also ships all drivers which deal with all the little differences between hardware devices. To the outside world (the higher layers), each class of devices appear to behave exactly in the same consistent way - which in turn, the programs can build upon.
Background subsystems. There are just regular programs, which just stay out of your way. They handle things like remote login, provide a cental message bus, and do actions based on hardware/network events. For example, bluetooth discovery, wifi management, etc.. Any network services (file server, print server, web server) also live at this level. In UNIX systems, these are all just normal programs.
The command line tools. These are all little programs which can be started to do things like text editing, downloading files, or administrating the system. At this point, a UNIX system is fully usable for system adminstrators. In Windows, this layer doesn't really exist anymore.
The graphical user interface. These are also just programs, the only difference is they draw windows at the screen instead of writing text. This makes the system easier to use for regular users.
Any service or event will go from the bottom all up to the top.
Libraries - the common platform
Programs do a lot of common things like displaying a window, drawing stuff at the screen or downloading a file. These things are the same for multiple programs, hence that code are put in separate "library" files (.so files - meaning shared object). The library can be shared across all programs.
For every imaginable thing, there is a library. There is one for reading/writing PNG files. There is one for JPEG files, for reading XML, for encryption, for video playback, and so on.
On Linux, the common libraries for application developers are Qt and Gtk. These libraries use lower-level libraries internally for their specific needs, while exposing their functionality in a nice consistent and concise way for application developers to create applications even faster.
Libraries provide the application platform, on which programmers can build end user applications for an Operating System. The more high quality libraries a system provides, the fewer code a programmer has to write to make a beautiful program.
Some libraries can be used across different operating systems (for instance, Qt is), some are really specifically tied into one operating system. This will restrict your program to be able to run at that platform only.
Inter process communication
A third corner piece of an operating system, is the way programs can communicate with each other. These are Inter Process Communication (IPC) machanisms. These exist in several flavors, e.g. a piece of shared memory, or a small channel is set up between two programs to exchange data. There is also a central message bus on which each program can post a message, and receive a response. This is used for global communication, where it's unknown which program can respond.
From libraries to Operating Systems
With libraries, IPC and the kernel in place, programmers can build all kinds of applications for system services, user administration, configuration, administration, office work, entertainment, etc.. This forms the complete suite which novice users recognize as the "operating system".
In UNIX/Linux systems, all services are just programs. All system admin tools are just programs. They all do their job, and they can be chained together. I've summarized a lot of major programs at http://codingdomain.com/linux/sysadmin/
Distinguishable parts with Windows
UNIX is mainly a system of programs, files and restricted permissions. A lot of complexities are avoided, making it a powerful system while it looks like it has an easy job doing it.
In detail, these are principles which can be found across UNIX/Linux systems:
There are uniform ways to access information. ("Everything is just a file"). You can open a file, network socket, IPC channel, kernel parameters and block device as a file. Hence the appearance of the virtual filesystems in /dev, /sys and /proc. The only API you ever need is open, read and close.
The underlying system is transparent. Every program operates under the same rules. Unlike Windows, there is no artificial difference between a "console program", "gui program" or "background service". They are all just programs, that happen to do different things. They can also all be observed, analyzed and debugged in the same way.
Settings are readable, editable, and can be annotated with comments. They typically have an INI-style format, but may use a custom format for the needs of that application. Because they are just files, they can be copied to other systems, archived or being backuped with standard tools.
No large "do it all in once" applications. The mantra is "do one thing, do it well". Command line tools can be chained and together be powerful. Separate services (e.g. SMTP, IMAP and POP, and login) are separate subprograms, avoiding complex intertwined code and security issues. Complex desktop environments delegate hard work to individual programs.
fork(). New programs are started by an existing program cloning itself. The clone sets up everything (e.g. file handles), and optionally replaces itself with the new program code. This makes it really easy to apply the same security settings and restrictions to new programs, share memory or setup an IPC mechanism. The cost of starting a process is also very low.
The file system is one tree, in which other disk partitions and network shares can be mounted. There is again, an universal way of accessing data. Common system locations (e.g. /usr can easily be mounted as network share.
The system is built for low user privileges. After login, every user (except root) is confined their own resources, running applications and files only. Network services reduce their privileges as soon as possible. There is a single clear way to get more privileges, or ask someone to execute a privileged job on their behalf. Every other call is limited by the restrictions and limitations of the program.
Every program stores settings in a hidden file/folder of the user home directory. No program ever attempts to write a global setting file.
A favor towards openly described communication mechanisms over secret mechanisms or specific 1-to-1 mechanisms. Other vendors and software developers are encouraged to follow the same specification, so things can easily be connected, swapped out and yet stay loosely coupled.
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Interruption of system calls when a signal is caught | 1,402,510,190,000 |
From reading the man pages on the read() and write() calls it appears that these calls get interrupted by signals regardless of whether they have to block or not.
In particular, assume
a process establishes a handler for some signal.
a device is opened (say, a terminal) with the O_NONBLOCK not set (i.e. operating in blocking mode)
the process then makes a read() system call to read from the device and as a result executes a kernel control path in kernel-space.
while the precess is executing its read() in kernel-space, the signal for which the handler was installed earlier is delivered to that process and its signal handler is invoked.
Reading the man pages and the appropriate sections in SUSv3 'System Interfaces volume (XSH)', one finds that:
i. If a read() is interrupted by a signal before it reads any data (i.e. it had to block because no data was available), it returns -1 with errno set to [EINTR].
ii. If a read() is interrupted by a signal after it has successfully read some data (i.e. it was possible to start servicing the request immediately), it returns the number of bytes read.
Question A):
Am I correct to assume that in either case (block/no block) the delivery and handling of the signal is not entirely transparent to the read()?
Case i. seems understandable since the blocking read() would normally place the process in the TASK_INTERRUPTIBLE state so that when a signal is delivered, the kernel places the process into TASK_RUNNING state.
However when the read() doesn't need to block (case ii.) and is processing the request in kernel-space, I would have thought that the arrival of a signal and its handling would be transparent much like the arrival and proper handling of a HW interrupt would be. In particular I would have assumed that upon delivery of the signal, the process would be temporarily placed into user mode to execute its signal handler from which it would return eventually to finish off processing the interrupted read() (in kernel-space) so that the read() runs its course to completion after which the process returns back to the point just after the call to read() (in user-space), with all of the available bytes read as a result.
But ii. seems to imply that the read() is interrupted, since data is available immediately, but it returns returns only some of the data (instead of all).
This brings me to my second (and final) question:
Question B):
If my assumption under A) is correct, why does the read() get interrupted, even though it does not need to block because there is data available to satisfy the request immediately?
In other words, why is the read() not resumed after executing the signal handler, eventually resulting in all of the available data (which was available after all) to be returned?
|
Summary: you're correct that receiving a signal is not transparent, neither in case i (interrupted without having read anything) nor in case ii (interrupted after a partial read). To do otherwise in case i would require making fundamental changes both to the architecture of the operating system and the architecture of applications.
The OS implementation view
Consider what happens if a system call is interrupted by a signal. The signal handler will execute user-mode code. But the syscall handler is kernel code and does not trust any user-mode code. So let's explore the choices for the syscall handler:
Terminate the system call; report how much was done to the user code. It's up to the application code to restart the system call in some way, if desired. That's how unix works.
Save the state of the system call, and allow the user code to resume the call. This is problematic for several reasons:
While the user code is running, something could happen to invalidate the saved state. For example, if reading from a file, the file might be truncated. So the kernel code would need a lot of logic to handle these cases.
The saved state can't be allowed to keep any lock, because there's no guarantee that the user code will ever resume the syscall, and then the lock would be held forever.
The kernel must expose new interfaces to resume or cancel ongoing syscalls, in addition to the normal interface to start a syscall. This is a lot of complication for a rare case.
The saved state would need to use resources (memory, at least); those resources would need to be allocated and held by the kernel but be counted against the process's allotment. This isn't insurmountable, but it is a complication.
Note that the signal handler might make system calls that themselves get interrupted; so you can't just have a static resource allotment that covers all possible syscalls.
And what if the resources cannot be allocated? Then the syscall would have to fail anyway. Which means the application would need to have code to handle this case, so this design would not simplify the application code.
Remain in progress (but suspended), create a new thread for the signal handler. This, again, is problematic:
Early unix implementations had a single thread per process.
The signal handler would risk overstepping on the syscall's shoes. This is an issue anyway, but in the current unix design, it's contained.
Resources would need to be allocated for the new thread; see above.
The main difference with an interrupt is that the interrupt code is trusted, and highly constrained. It's usually not allowed to allocate resources, or run forever, or take locks and not release them, or do any other kind of nasty things; since the interrupt handler is written by the OS implementer himself, he knows that it won't do anything bad. On the other hand, application code can do anything.
The application design view
When an application is interrupted in the middle of a system call, should the syscall continue to completion? Not always. For example, consider a program like a shell that's reading a line from the terminal, and the user presses Ctrl+C, triggering SIGINT. The read must not complete, that's what the signal is all about. Note that this example shows that the read syscall must be interruptible even if no byte has been read yet.
So there must be a way for the application to tell the kernel to cancel the system call. Under the unix design, that happens automatically: the signal makes the syscall return. Other designs would require a way for the application to resume or cancel the syscall at its leasure.
The read system call is the way it is because it's the primitive that makes sense, given the general design of the operating system. What it means is, roughly, “read as much as you can, up to a limit (the buffer size), but stop if something else happens”. To actually read a full buffer involves running read in a loop until as many bytes as possible have been read; this is a higher-level function, fread(3). Unlike read(2) which is a system call, fread is a library function, implemented in user space on top of read. It's suitable for an application that reads for a file or dies trying; it's not suitable for a command line interpreter or for a networked program that must throttle connections cleanly, nor for a networked program that has concurrent connections and doesn't use threads.
The example of read in a loop is provided in Robert Love's Linux System Programming:
ssize_t ret;
while (len != 0 && (ret = read (fd, buf, len)) != 0) {
if (ret == -1) {
if (errno == EINTR)
continue;
perror ("read");
break;
}
len -= ret;
buf += ret;
}
It takes care of case i and case ii and few more.
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dpkg: error: cannot remove architecture 'i386' currently in use by the database | 1,402,510,190,000 |
I used this command to add i386 arch:
sudo dpkg --add-architecture i386
And then immediately after without installing any packages I tried to remove the i386 arch like so:
sudo dpkg --remove-architecture i386
And i got the error:
dpkg: error: cannot remove architecture 'i386' currently in use by the database
Solutions I have seen so far involve removing i386 packages, I haven't installed any, the ones that are installed are vital to the functioning of the OS. What do I do?
EDIT, PLEASE READ THE FOLLOWING TO AVOID DESTROYING YOUR OS:
Turns out that 64-bit Linux OSes already include the i386 arch, so the command sudo dpkg --add-architecture i386 didn't really do anything.
|
From your list, it looks like you just had the 32-bit packages used for Wine. Wine needs a bunch of 32-bit libraries to run 32-bit Windows applications. You won't be able to remove the i386 architecture unless you uninstall the 32-bit Wine. But there's no point in doing this: there's nothing wrong with having the i386 architecture enabled.
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Why there are `/lib` and `/lib64` but only `/bin`? | 1,402,510,190,000 |
In my laptop:
$ cat /etc/issue
Ubuntu 18.04 LTS \n \l
There are two different folders for libraries x86 and x86_64:
~$ ls -1 /
bin
lib
lib64
sbin
...
Why for binaries exists only one directory?
P.S. I'm also interested in Android but I hope that answer should be the same.
|
First, why there are separate /lib and /lib64:
The Filesystem Hierarchy Standard
mentions that separate /lib and /lib64 exist because:
10.1. There may be one or more variants of the /lib directory on systems which support more than one binary format requiring
separate libraries. (...) This is commonly used for 64-bit or 32-bit
support on systems which support multiple binary formats, but require
libraries of the same name. In this case, /lib32 and /lib64 might be
the library directories, and /lib a symlink to one of them.
On my Slackware 14.2 for example there are /lib and /lib64
directories for 32-bit and 64-bit libraries respectively even though
/lib is not as a symlink as the FHS snippet would suggest:
$ ls -l /lib/libc.so.6
lrwxrwxrwx 1 root root 12 Aug 11 2016 /lib/libc.so.6 -> libc-2.23.so
$ ls -l /lib64/libc.so.6
lrwxrwxrwx 1 root root 12 Aug 11 2016 /lib64/libc.so.6 -> libc-2.23.so
There are two libc.so.6 libraries in /lib and /lib64.
Each dynamically built
ELF binary
contains a hardcoded path to the interpreter, in this case either
/lib/ld-linux.so.2 or /lib64/ld-linux-x86-64.so.2:
$ file main
main: ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV), dynamically linked, interpreter /lib/ld-linux.so.2, not stripped
$ readelf -a main | grep 'Requesting program interpreter'
[Requesting program interpreter: /lib/ld-linux.so.2]
$ file ./main64
./main64: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, not stripped
$ readelf -a main64 | grep 'Requesting program interpreter'
[Requesting program interpreter: /lib64/ld-linux-x86-64.so.2]
The job of the interpreter is to load necessary shared libraries. You
can ask a GNU interpreter what libraries it would load without even
running a binary using LD_TRACE_LOADED_OBJECTS=1 or a ldd wrapper:
$ LD_TRACE_LOADED_OBJECTS=1 ./main
linux-gate.so.1 (0xf77a9000)
libc.so.6 => /lib/libc.so.6 (0xf760e000)
/lib/ld-linux.so.2 (0xf77aa000)
$ LD_TRACE_LOADED_OBJECTS=1 ./main64
linux-vdso.so.1 (0x00007ffd535b3000)
libc.so.6 => /lib64/libc.so.6 (0x00007f56830b3000)
/lib64/ld-linux-x86-64.so.2 (0x00007f568347c000)
As you can see a given interpreter knows exactly where to look for
libraries - 32-bit version looks for libraries in /lib and 64-bit
version looks for libraries in /lib64.
FHS standard says the following about /bin:
/bin contains commands that may be used by both the system
administrator and by users, but which are required when no other
filesystems are mounted (e.g. in single user mode). It may also
contain commands which are used indirectly by scripts.
IMO the reason why there are no separate /bin and /bin64 is that if we had
the file with the same name in both of these directories we couldn't call one of them
indirectly because we'd have to put /bin or /bin64 first in
$PATH.
However, notice that the above is just the convention - the Linux
kernel does not really care if you have separate /bin and /bin64.
If you want them, you can create them and setup your system accordingly.
You also mentioned Android - note that except for running a modified
Linux kernel it has nothing to do with GNU systems such as
Ubuntu - no glibc, no bash (by default, you can of course compile and deploy it manually), and also directory structure is
completely different.
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How are directories implemented in Unix filesystems? | 1,402,510,190,000 |
My question is how directories are implemented? I can believe a data structure like a variable e.g. table, array or similar. Since UNIX is Open Source I can look in the source what the program does when it created a new directory. Can you tell me where to look or elaborate on the topic? That a directory "is" a file I could understand and is a directory really a file? I'm not sure that it is true that files are stored "in" files while still in way you could say the word file about nearly anything and I'm not sure what absolutely not is a file since you could call even a variable a file. For example a link is certainly not a file and a link is like a directory but then this violates that a directory is a file?
|
The internal structure of directories is dependent on the filesystem in use. If you want to know precisely what happens, have a look at filesystem implementations.
Basically, in most filesystems, a directory is an associative array between filenames (keys) and inodes numbers (values). Something like this¹:
1167010 .
1158721 ..
1167626 subdir
132651 barfile
132650 bazfile
This list is coded in some – more or less – efficient way inside a chain of (usually) 4KB blocks. Notice that the content of regular files is stored similarly. In the case of directories, there is no point in knowing which size is actually used inside these blocks. That's why the sizes of directories reported by du are multiples of 4KB.
Inodes are there to tie blocks together, forming a single entity, namely a 'file' in the general sense. They are identified by a number which is some kind of address and each one is usually stored as a single, special block.
Management of all this happens in kernel mode. Software just asks for the creation of a directory with a function named int mkdir(const char *pathname, mode_t mode); leading to a system call, and all the rest is performed behind the scenes.
About links structure:
A hard link is not a file, it's just a new directory entry (i.e. a name – inode number association) referring to a preexisting inode entity². This means that the same inode can be accessed from different pathnames. In particular, since metadatas (permissions, ownership, timestamps…) are stored within the inode, these are unique and independent of the pathname chosen to access the file.
A symbolic link is a file and it's distinct from its target. This means that it has its own inode. It used to be handled just like a regular file: the target path was stored in a data block. But now, for efficiency reasons in recent ext filesystems, paths shorter than 60 bytes long are stored within the inode itself (using the fields which would normally be used to store the pointers to data blocks).
—
1. this was obtained using ls -ai1 testdir.
2. whose type must be different than 'directory' nowadays.
|
How to understand pipes | 1,402,510,190,000 |
When I just used pipe in bash, I didn't think more about this. But when I read some C code example using system call pipe() together with fork(), I wonder how to understand pipes, including both anonymous pipes and named pipes.
It is often heard that "everything in Linux/Unix is a file". I wonder if a pipe is actually a file so that one part it connects writes to the pipe file, and the other part reads from the pipe file? If yes, where is the pipe file for an anonymous pipe created? In /tmp, /dev, or ...?
However, from examples of named pipes, I also learned that using pipes has space and time performance advantage over explicitly using temporary files, probably because there are no files involved in implementation of pipes. Also pipes seem not store data as files do. So I doubt a pipe is actually a file.
|
About your performance question, pipes are more efficient than files because no disk IO is needed. So cmd1 | cmd2 is more efficient than cmd1 > tmpfile; cmd2 < tmpfile (this might not be true if tmpfile is backed on a RAM disk or other memory device as named pipe; but if it is a named pipe, cmd1 should be run in the background as its output can block if the pipe becomes full). If you need the result of cmd1 and still need to send its output to cmd2, you should cmd1 | tee tmpfile | cmd2 which will allow cmd1 and cmd2 to run in parallel avoiding disk read operations from cmd2.
Named pipes are useful if many processes read/write to the same pipe. They can also be useful when a program is not designed to use stdin/stdout for its IO needing to use files. I put files in italic because named pipes are not exactly files in a storage point of view as they reside in memory and have a fixed buffer size, even if they have a filesystem entry (for reference purpose). Other things in UNIX have filesystem entries without being files: just think of /dev/null or others entries in /dev or /proc.
As pipes (named and unnamed) have a fixed buffer size, read/write operations to them can block, causing the reading/writing process to go in IOWait state. Also, when do you receive an EOF when reading from a memory buffer ? Rules on this behavior are well defined and can be found in the man.
One thing you cannot do with pipes (named and unnamed) is seek back in the data. As they are implemented using a memory buffer, this is understandable.
About "everything in Linux/Unix is a file", I do not agree. Named pipes have filesystem entries, but are not exactly file. Unnamed pipes do not have filesystem entries (except maybe in /proc). However, most IO operations on UNIX are done using read/write function that need a file descriptor, including unnamed pipe (and socket). I do not think that we can say that "everything in Linux/Unix is a file", but we can surely say that "most IO in Linux/Unix is done using a file descriptor".
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What are the minimum root filesystem applications that are required to fully boot linux? | 1,402,510,190,000 |
It's a question about user space applications, but hear me out!
Three "applications", so to speak, are required to boot a functional distribution of Linux:
Bootloader - For embedded typically that's U-Boot, although not a hard requirement.
Kernel - That's pretty straightforward.
Root Filesystem - Can't boot to a shell without it. Contains the filesystem the kernel boots to, and where init is called form.
My question is in regard to #3. If someone wanted to build an extremely minimal rootfs (for this question let's say no GUI, shell only), what files/programs are required to boot to a shell?
|
That entirely depends on what services you want to have on your device.
Programs
You can make Linux boot directly into a shell. It isn't very useful in production — who'd just want to have a shell sitting there — but it's useful as an intervention mechanism when you have an interactive bootloader: pass init=/bin/sh to the kernel command line. All Linux systems (and all unix systems) have a Bourne/POSIX-style shell in /bin/sh.
You'll need a set of shell utilities. BusyBox is a very common choice; it contains a shell and common utilities for file and text manipulation (cp, grep, …), networking setup (ping, ifconfig, …), process manipulation (ps, nice, …), and various other system tools (fdisk, mount, syslogd, …). BusyBox is extremely configurable: you can select which tools you want and even individual features at compile time, to get the right size/functionality compromise for your application. Apart from sh, the bare minimum that you can't really do anything without is mount, umount and halt, but it would be atypical to not have also cat, cp, mv, rm, mkdir, rmdir, ps, sync and a few more. BusyBox installs as a single binary called busybox, with a symbolic link for each utility.
The first process on a normal unix system is called init. Its job is to start other services. BusyBox contains an init system. In addition to the init binary (usually located in /sbin), you'll need its configuration files (usually called /etc/inittab — some modern init replacement do away with that file but you won't find them on a small embedded system) that indicate what services to start and when. For BusyBox, /etc/inittab is optional; if it's missing, you get a root shell on the console and the script /etc/init.d/rcS (default location) is executed at boot time.
That's all you need, beyond of course the programs that make your device do something useful. For example, on my home router running an OpenWrt variant, the only programs are BusyBox, nvram (to read and change settings in NVRAM), and networking utilities.
Unless all your executables are statically linked, you will need the dynamic loader (ld.so, which may be called by different names depending on the choice of libc and on the processor architectures) and all the dynamic libraries (/lib/lib*.so, perhaps some of these in /usr/lib) required by these executables.
Directory structure
The Filesystem Hierarchy Standard describes the common directory structure of Linux systems. It is geared towards desktop and server installations: a lot of it can be omitted on an embedded system. Here is a typical minimum.
/bin: executable programs (some may be in /usr/bin instead).
/dev: device nodes (see below)
/etc: configuration files
/lib: shared libraries, including the dynamic loader (unless all executables are statically linked)
/proc: mount point for the proc filesystem
/sbin: executable programs. The distinction with /bin is that /sbin is for programs that are only useful to the system administrator, but this distinction isn't meaningful on embedded devices. You can make /sbin a symbolic link to /bin.
/mnt: handy to have on read-only root filesystems as a scratch mount point during maintenance
/sys: mount point for the sysfs filesystem
/tmp: location for temporary files (often a tmpfs mount)
/usr: contains subdirectories bin, lib and sbin. /usr exists for extra files that are not on the root filesystem. If you don't have that, you can make /usr a symbolic link to the root directory.
Device files
Here are some typical entries in a minimal /dev:
console
full (writing to it always reports “no space left on device”)
log (a socket that programs use to send log entries), if you have a syslogd daemon (such as BusyBox's) reading from it
null (acts like a file that's always empty)
ptmx and a pts directory, if you want to use pseudo-terminals (i.e. any terminal other than the console) — e.g. if the device is networked and you want to telnet or ssh in
random (returns random bytes, risks blocking)
tty (always designates the program's terminal)
urandom (returns random bytes, never blocks but may be non-random on a freshly-booted device)
zero (contains an infinite sequence of null bytes)
Beyond that you'll need entries for your hardware (except network interfaces, these don't get entries in /dev): serial ports, storage, etc.
For embedded devices, you would normally create the device entries directly on the root filesystem. High-end systems have a script called MAKEDEV to create /dev entries, but on an embedded system the script is often not bundled into the image. If some hardware can be hotplugged (e.g. if the device has a USB host port), then /dev should be managed by udev (you may still have a minimal set on the root filesystem).
Boot-time actions
Beyond the root filesystem, you need to mount a few more for normal operation:
procfs on /proc (pretty much indispensible)
sysfs on /sys (pretty much indispensible)
tmpfs filesystem on /tmp (to allow programs to create temporary files that will be in RAM, rather than on the root filesystem which may be in flash or read-only)
tmpfs, devfs or devtmpfs on /dev if dynamic (see udev in “Device files” above)
devpts on /dev/pts if you want to use [pseudo-terminals (see the remark about pts above)
You can make an /etc/fstab file and call mount -a, or run mount manually.
Start a syslog daemon (as well as klogd for kernel logs, if the syslogd program doesn't take care of it), if you have any place to write logs to.
After this, the device is ready to start application-specific services.
How to make a root filesystem
This is a long and diverse story, so all I'll do here is give a few pointers.
The root filesystem may be kept in RAM (loaded from a (usually compressed) image in ROM or flash), or on a disk-based filesystem (stored in ROM or flash), or loaded from the network (often over TFTP) if applicable. If the root filesystem is in RAM, make it the initramfs — a RAM filesystem whose content is created at boot time.
Many frameworks exist for assembling root images for embedded systems. There are a few pointers in the BusyBox FAQ. Buildroot is a popular one, allowing you to build a whole root image with a setup similar to the Linux kernel and BusyBox. OpenEmbedded is another such framework.
Wikipedia has an (incomplete) list of popular embedded Linux distributions. An example of embedded Linux you may have near you is the OpenWrt family of operating systems for network appliances (popular on tinkerers' home routers). If you want to learn by experience, you can try Linux from Scratch, but it's geared towards desktop systems for hobbyists rather than towards embedded devices.
A note on Linux vs Linux kernel
The only behavior that's baked into the Linux kernel is that the first program that's launched at boot time. (I won't get into initrd and initramfs subtleties here.) This program, traditionally called init, has process ID 1 and has certain privileges (immunity to KILL signals) and responsibilities (reaping orphans). You can run a system with a Linux kernel and start whatever you want as the first process, but then what you have is an operating system based on the Linux kernel, and not what is normally called “Linux” — Linux, in the common sense of the term, is a Unix-like operating system whose kernel is the Linux kernel. For example, Android is an operating system which is not Unix-like but based on the Linux kernel.
|
How are system commands like ls created? | 1,402,510,190,000 |
I have some doubts regarding *nix.
I don't know which type of executable file is ls, whether it is .sh
or .ksh or any other kind of system executable if it is, what
is that?
when I tried to see what is the source code of ls command looks like, it shows something unreadable, what method does *nix use to create these types of unreadable files and can I make my files similar to these files (like ls - unreadable).
|
You can determine the nature of an executable in Unix using the file command and the type command.
type
You use type to determine an executable's location on disk like so:
$ type -a ls
ls is /usr/bin/ls
ls is /bin/ls
So I now know that ls is located here on my system in 2 locations:/usr/bin/ls & /bin/ls. Looking at those executables I can see they're identical:
$ ls -l /usr/bin/ls /bin/ls
-rwxr-xr-x. 1 root root 120232 Jan 20 05:11 /bin/ls
-rwxr-xr-x. 1 root root 120232 Jan 20 05:11 /usr/bin/ls
NOTE: You can confirm they're identical beyond their sizes by using cmp or diff.
with diff
$ diff -s /usr/bin/ls /bin/ls
Files /usr/bin/ls and /bin/ls are identical
with cmp
$ cmp /usr/bin/ls /bin/ls
$
Using file
If I query them using the file command:
$ file /usr/bin/ls /bin/ls
/usr/bin/ls: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked (uses shared libs), for GNU/Linux 2.6.32, BuildID[sha1]=0x303f40e1c9349c4ec83e1f99c511640d48e3670f, stripped
/bin/ls: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked (uses shared libs), for GNU/Linux 2.6.32, BuildID[sha1]=0x303f40e1c9349c4ec83e1f99c511640d48e3670f, stripped
So these would be actual physical programs that have been compiled from C/C++. If they were shell scripts they'd typically present like this to file:
$ file somescript.bash
somescript.bash: POSIX shell script, ASCII text executable
What's ELF?
ELF is a file format, it is the output of a compiler such as gcc, which is used to compile C/C++ programs such as ls.
In computing, the Executable and Linkable Format (ELF, formerly called Extensible Linking Format) is a common standard file format for executables, object code, shared libraries, and core dumps.
It typically will have one of the following extensions in the filename: none, .o, .so, .elf, .prx, .puff, .bin
|
How does the set-user-ID mechanism work in Unix? | 1,402,510,190,000 |
Can someone please explain the set-user-ID mechanism in Unix ? What was the rationale behind this design decision? How is it different from effective user id mechanism ?
|
You might know the normal read, write and execute permissions for files in unix.
However, in many applications, this type of permission structure--e.g. giving a given user either full permission to read a given file, or no permission at all to read the file--is too coarse. For this reason, Unix includes another permission bit, the set-user-ID bit. If this bit is set for an executable file, then whenever a user other than the owner executes the file, that user acquires all the file read/write/execute privileges of the owner in accessing any of the owner's other files!
To set the set-user-ID bit for a file, type
chmod u+s filename
Make sure that you have set group-other execute permission too; it would be nice to have group-other read permission as well. All of this can be done with the single statement
chmod 4755 filename
It is also referred to as Saved UID. A file that is launched that has a Set-UID bit on, the saved UID will be the UID of the owner of the file. Otherwise, saved UID will be the Real UID.
What is effective uid ?
This UID is used to evaluate privileges of the process to perform a particular action. EUID can be changed either to Real UID, or Superuser UID if EUID!=0. If EUID=0, it can be changed to anything.
Example
An example of such program is passwd. If you list it in full, you will see that it has Set-UID bit and the owner is "root". When a normal user, say "mtk", runs passwd, it starts with:
Real-UID = mtk
Effective-UID = mtk
Saved-UID = root
Reference link 1
Reference link 2
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Are different Linux/Unix kernels interchangeable? | 1,402,510,190,000 |
Can I take a Linux kernel and use it with, say, FreeBSD and vice versa (FreeBSD kernel in, say, a Debian)? Is there a universal answer? What are the limitations? What are the obstructions?
|
No, kernels from different implementations of Unix-style operating systems are not interchangeable, notably because they all present different interfaces to the rest of the system (user space) — their system calls (including ioctl specifics), the various virtual file systems they use...
What is interchangeable to some extent, at the source level, is the combination of the kernel and the C library, or rather, the user-level APIs that the kernel and libraries expose (essentially, the view at the layer described by POSIX, without considering whether it is actually POSIX). Examples of this include Debian GNU/kFreeBSD, which builds a Debian system on top of a FreeBSD kernel, and Debian GNU/Hurd, which builds a Debian system on top of the Hurd.
This isn’t quite at the level of kernel interchangeability, but there have been attempts to standardise a common application binary interface, to allow binaries to be used on various systems without needing recompilation. One example is the Intel Binary Compatibility Standard, which allows binaries conforming to it to run on any Unix system implementing it, including older versions of Linux with the iBCS 2 layer. I used this in the late 90s to run WordPerfect on Linux.
See also How to build a FreeBSD chroot inside of Linux.
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Interpret the output of lstopo | 1,402,510,190,000 |
I have a output from lstopo --output-format txt -v --no-io > lstopo.txt for a 8-core node in a cluster, which is https://dl.dropboxusercontent.com/u/13029929/lstopo.txt
The file is a text drawing of the node. It is too wide for both the terminal and gedit on Ubuntu of my laptop, and some of its right is moved by my laptop to the left and overlap the left part of the drawing. I wonder how I can view the file properly? ( Added: I realize that I can view the drawing properly by uploading to dropbox and opening in Firefox, which zoom out the drawing properly. But open the local file in Firefox will mis-display the dash lines "-", and I wonder why? Other than Firefox, any software can also work on it?)
what does "PU P#" mean in each core "Core P#"? Why are their numbers not the same?
Does "L1i" mean a L1 instruction cache, and "L1d" a L1 data cache?
Why do L2 and L3 caches not have distinction between instruction cache and data cache? Is this common for computers?
What does "Socket P#" mean? Is the "socket" used for connection between the L3 caches and the main memory?
What does "NUMANode P# (16GB)" mean? Is it a main memory chip?
Does the drawing show that there are four cores sharing a main memory chip , and the other four cores sharing another main memory chip?
Is there not a main memory shared by all the 8 cores in the node? So is the node just like a distributed system with two 4-core computers without shared memory between them? How can the two 4-core groups communicate with each other?
Does "Machine (32GB)" mean the sum of the sizes of the two main memory chips mentioned in 6?
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Here are the answers to your questions:
I'd view it as a graphical image rather than an ASCII image.
$ lstopo --output-format png -v --no-io > cpu.png
NOTE: You can view the generated file cpu.png
"PU P#" = Processing Unit Processor #. These are processing elements within the cores of the CPU. On my laptop (Intel i5) I have 2 cores that each have 2 processing elements, for a total of 4. But in actuality I have only 2 physical cores.
L#i = Instruction Cache, L#d = Data Cache. L1 = a Level 1 cache.
In the Intel architectures the instruction & data get mixed as you move down from L1 → L2 → L3.
"Socket P#" is that there are 2 physical sockets on the motherboard, there are 2 physically discrete CPUs in this setup.
In multiple CPU architectures the RAM is usually split so that a portion of it is assigned to each core. If CPU0 needs data from CPU1's RAM, then it needs to "request" this data through CPU1. There are a number of reasons why this is done, too many to elaborate here. Read up on NUMA style memory architectures if you're really curious.
The drawing is showing 4 cores (with 1 Processing Unit in each) that are in 2 physical CPU packages. Each physical CPU has "isolated" access to 16 GB of RAM.
No, there is no shared memory among all the CPUs. The 2 CPUs have to interact with the other's RAM through the CPU. Again see the NUMA Wikipage for more on the Non Uniform Memory Architecture.
Yes, the system has a total of 32 GB of RAM. But only 1/2 of the RAM is accessible by either physical CPU directly.
What's a socket?
A socket is the term used to describe the actual package that a CPU is contained inside of, for mounting on the motherboard. There are many different styles and configurations; check out the Wikipedia page on CPU Sockets.
This picture also kind of illustrates the relationships between the "cores", the CPUs, and the "sockets".
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Difference between system calls and library functions | 1,402,510,190,000 |
I have been through the answer of this question but do not quite understand the difference between system calls and library functions. Conceptually, what is the difference between the two?
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Conceptually, a library function is part of your process.
At run-time, your executable code and the code of any libraries (such as libc.so) it depends on, get linked into a single process. So, when you call a function in such a library, it executes as part of your process, with the same resources and privileges. It's the same idea as calling a function you wrote yourself (with possible exceptions like PLT and/or trampoline functions, which you can look up if you care).
Conceptually, a system call is a special interface used to make a call from your code (which is generally unprivileged) to the kernel (which has the right to escalate privileges as necessary).
For example, see the Linux man brk.
When a C program calls malloc to allocate memory, it is calling a library function in glibc.
If there is already enough space for the allocation inside the process, it can do any necessary heap management and return the memory to the caller.
If not, glibc needs to request more memory from the kernel: it (probably) calls the brk glibc function, which in turn calls the brk syscall. Only once control has passed to the kernel, via the syscall, can the global virtual memory state be modified to reserve more memory, and map it into your process' address space.
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What is the relationship between system calls, message passing, and interrupts? | 1,402,510,190,000 |
I am reading the Wikipedia article for process management. My focus is on Linux. I cannot figure out the relation and differences between system call, message passing and interrupt, in their concepts and purposes. Are they all for processes to make requests to kernel for resources and services?
Some quotes from the article and some other:
There are two possible ways for an OS to regain control of the
processor during a program’s execution in order for the OS to
perform
de-allocation or allocation:
The process issues a system call (sometimes called a
software
interrupt); for example, an I/O request occurs requesting to
access a
file on hard disk.
A hardware interrupt occurs; for example, a key was pressed
on
the keyboard, or a timer runs out (used in pre-emptive
multitasking).
There are two techniques by which a program executing in user mode
can
request the kernel's services:
* System call
* Message passing
an interrupt is an asynchronous signal indicating the need for
attention or a synchronous event in software indicating the need
for a
change in execution.
A hardware interrupt causes the processor to save its state of
execution and begin execution of an interrupt handler. Software
interrupts are usually implemented as instructions in the
instruction
set, which cause a context switch to an interrupt handler similar
to a
hardware interrupt.
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All modern operating systems support multitasking. This means that the system is able to execute multiple processes at the same time; either in pseudo-parallel (when only one CPU is available) or nowadays with multi-core CPUs being common in parallel (one task/core).
Let's take the simpler case of only one CPU being available. This means that if you execute at the same time two different processes (let's say a web browser and a music player) the system is not really able to execute them at the same time. What happens is that the CPU is switching from one process to the other all the time; but this is happening extremely fast, thus you never notice it.
Now let's assume that while those two processes are executing, you press the reset button (bad boy). The CPU will immediately stop whatever is doing and reboot the system. Congratulations: you generated an interrupt.
The case is similar when you are programming and want to ask for a service from the CPU. The difference is that in this case you execute software code -- usually library procedures that are executing system calls (for example fopen for opening a file).
Thus, 1 describes two different ways of getting attention from the CPU.
Most modern operating systems support two execution modes: user mode and kernel mode. By default an operating system runs in user mode. User mode is very limited. For example, all I/O is forbidden; thus, you are not allowed to open a file from your hard disk. Of course this never happens in real, because when you open a file the operating system switches from user to kernel mode transparently. In kernel mode you have total control of the hardware.
If you are wondering why those two modes exist, the simplest answer is for protection. Microkernel-based operating systems (for example MINIX 3) have most of their services running in user mode, which makes them less harmful. Monolithic kernels (like Linux) have almost all their services running in kernel mode. Thus a driver that crashes in MINIX 3 is unlikely to bring down the whole system, while this is not unusual in Linux.
System calls are the primitive used in monolithic kernels (shared data model) for switching from user to kernel mode. Message passing is the primitive used in microkernels (client/server model). To be more precise, in a message passing system programmers also use system calls to get attention from the CPU. Message passing is visible only to the operating system developers. Monolithic kernels using system calls are faster but less reliable, while microkernels using message passing are slower but have better fault isolation.
Thus, 2 mentions two different ways of switching from user to kernel mode.
To revise, the most common way of creating a software interrupt, aka trap, is by executing a system call. Interrupts on the other hand are generated purely by hardware.
When we interrupt the CPU (either by software or by hardware) it needs to save somewhere its current state -- the process that it executes and at which point it did stop -- otherwise it will not be able to resume the process when switching back. That is called a context switch and it makes sense: Before you switch off your computer to do something else, you first need to make sure that you saved all your programs/documents, etc so that you can resume from the point where you stopped the next time you'll turn it on :)
Thus, 3 explains what needs to be done after executing a trap or an interrupt and how similar the two cases are.
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Why is architecture listed thrice in uname -a? | 1,402,510,190,000 |
$ uname -a
Linux 3.13.0-29-generic #53-Ubuntu SMP Wed Jun 4 21:00:20 UTC 2014 x86_64 x86_64 x86_64 GNU/Linux
Running ubuntu 12.04.1 LTS. Why does it have the architecture (x86_64) listed thrice?
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I checked uname manual (man uname) and it says the following for the "-a" option:
print all information, in the following order, except omit -p and -i if unknown
In Ubuntu, I guess, options "-m", "-p" and "-i" (machine, processor and hardware-platform) are returning the machine architecture. For example, if you use the command
uname -mpi
You will see:
x86_64 x86_64 x86_64
On the other hand, if you choose all the option:
uname -snrvmpio
You will get the same result as:
uname -a
Output:
Linux <hostname> 3.13.0-29-generic #53-Ubuntu SMP Wed Jun 4 21:00:20 UTC 2014 x86_64 x86_64 x86_64 GNU/Linux
I also executed "uname" with options "-m", "-p" and "-i" on an ARCHLINUX distro and I got a different answer:
x86_64 unknown unknown
In fact, when I asked for "uname -a" on the ARCHLINUX distro the answer was:
Linux <hostname> xxxxxx-ARCH #1 SMP PREEMPT Mon Feb 14 20:40:47 CEST 2015 x86_64 GNU/Linux
While when executed "uname -snrvmpio" on the ARCHLINUX distro I got:
Linux <hostname> xxxxxx-ARCH #1 SMP PREEMPT Mon Feb 14 20:40:47 CEST 2015 x86_64 unknown unknown GNU/Linux
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First FreeBSD install. Is there anything I should know about differences between Linux and BSD? | 1,402,510,190,000 |
I want to install FreeBSD today on a spare HDD I have lying around. I'd like to give it a trial run, learn a few things, and if it suits me I'll replace my current Ubuntu 10.10 'server/NAS/encoding box' with it. Curiosity is the main reason. I also want to see most of the major bugs ironed out of GNOME 3/Unity before I jump aboard the next Ubuntu iteration.
I have no experience with the BSDs (except for OS X) but I have installed and used quite a few Linux distros over the years. I have a fairly good understanding of how to get Linux up and running, including some of the roll-your-own distros such as Arch. But I'm not an expert by any stretch of the imagination. Basically, I'd say I'm better than my grandma is.
So is there anything that I should keep in mind when installing FreeBSD for the first time? In particular, are there any major differences between installing and setting up FreeBSD and a Linux distro? Furthermore, should I be using a i386 release? I read somewhere in the documentation that i386 is recommended but I'm not sure if that's out-of-date information.
|
You will notice differences certainly. Most noticable will be differences in the standard userland utilities. FreeBSD does not use GNU ls, GNU cp, and so on. For example, if you're attached to a colorized ls, you may want to alias ls to "ls -G". It does use GNU grep, though. The default shell is a much simpler and less bloated shell than GNU Bash, which is the default on most Linux distributions. If you are attached to bash, that may be one of the first packages you will want to install. The ports system has been the standard way to install software on the various BSDs. Ports downloads the source code, builds it, and then installs it. It's nearly entirely automatic. To install bash, for example, do this as root:
cd /usr/ports/shells/bash && make install && make clean
If you don't do a make clean at the end, you will leave the built source code lying in the ports tree. Many ports have pre-built packages that can be downloaded if you prefer not to waste time building it and don't need to customize it. To install bash as a package, this should do it:
pkg_add -r bash
You can find most any common program in ports including Gnome 3, sudo, rsync, or what ever else you need. A great website for navigating ports is FreshPorts. You also should get familiar with the FreeBSD Handbook.
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Do GUIX and NixOS differ architecturally? | 1,402,510,190,000 |
(This is not a "which distribution is better" question!)
GNU GUIX and NixOS are two Linux distributions based on the NixOS package manager.
I realize that GUIX seems to use Guile for defining packages/dependencies or other meta-data uses; and I'm guess everything in GUIX is GPL'ed, while perhaps not everything in NixOS is... but those seem more like superficial differences.
What I'm hoping to understand is whether these two distributions have architectural differences of any significance.
|
Basically, there aren't any architectural differences between the two distributions, except for the way they handle the init system:
Guix System uses GNU Sheperd while NixOS uses System D.
To the best of my understanding, Guix/Guix System is a re-implementation of the framework seen in Nix/NixOS, utilizing GNU tooling. In other words, it is like NixOS but with a different user experience:
The entirety of its codebase is developed using Guile and Lisp, in contrast to Nix and Bash.
It employs GNU Shepherd in lieu of System D.
Guix does not package non free software while nixpkgs do.
Guix provides support for the GNU Herd kernel.
I tried Guix out about a year ago and found some limitations back then:
Impossibility to install the root filesystem on LVM.
Building a package requires to recompile all Guix modules.
It is noteworthy that nixpkgs is one of the largest package repositories, whereas Guix repositories are constrained by the limited number of maintainers and the "libre software only" limitation.
The Nix project is also more mature, enjoying a ten-year head start and a much larger community.
Furthermore, since Nix is a package manager, it can be installed on any distribution, including Guix System. This means that you can install packages from nixpkgs using Nix on a Guix System.
As pointed out in the comments by MegaTux, Guix is also a standalone package manager (that is shipped with the Guix System distribution) and can be installed on any distribution.
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login and su internals | 1,402,510,190,000 |
I am trying to understand how user permissions work in Linux. The kernel boots and starts init as root, right? Init then runs startup scripts and runs getty (agetty), again as root. Agetty just reads user name and runs login, still as root, I think. Nothing interesting yet. But what does login do? I wasn't able to find anything better than "it attempts to log in". Suppose login finds that password matches (and we trying to log in as usual user), how does it change user id? I thought that there should be system call for that but I wasn't able to find it (maybe I'm just blind?)
Also, about su. su has the 'setuid' bit set so when we run it, it always runs as root. But when we tell it to log in as usual user, it again needs to change user id. Do I understand correctly that the same "magic" happens in su and login when they need to change user? If so, why have two different programs? Is there any additional sorts of serious business happening when running login?
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There are several parts to what login programs do. Login programs differ in how they interact with the user who's trying to log in. Here are a few examples:
login: reads input on a text terminal
su: invoked by an already logged-in users, gets most of the data from its command-line arguments, plus authentication data (password) from a terminal
gksu: similar to su, but reads authentication data in X
rlogind: obtains input over a TCP connection through the rlogin protocol
sshd: obtains input over a TCP connection through the SSH protocol
X display managers (xdm, gdm, kdm, …): similar to login, but read input on an X display
These programs operate in similar ways.
The first part is authentication: the program reads some input from the user and decides whether the user is authorized to log in. The traditional method is to read a user name and password, and check that the user is mentioned in the system's user database and that the password that the user typed is the one in the database. But there are many other possibilities (one-time passwords, biometric authentication, authorization transfer, …).
Once it has been established that the user is authorized to log in and in what account, the login program establishes the user's authorization, for example what groups the user will belong to in this session.
The login program may also check account restrictions. For example, it may enforce a login time, or a maximum number of logged-in users, or refuse certain users on certain connections.
Finally the login program sets up the user's session. There are several substeps:
Set the process permissions to what was decided in the authorization: user, groups, limits, … You can see a simple example of this substep here (it only handles user and groups). The basic idea is that the login program is still running as root at this point, so it has maximum privileges; it first removes all privileges other than being the root user, and finally calls setuid to drop that last but not least privilege.
Possibly mount the user's home directory, display a “you have mail” message, etc.
Invoke some program as the user, typically the user's shell (for login and su, or sshd if no command was specified; an X display manager invokes an X session manager or window manager).
Most unices nowadays use PAM (Pluggable Authentication Modules) to provide a uniform way of managing login services. PAM divides its functionality into 4 parts: “auth” encompasses both authentication (1 above) and authorization (2 above); “account” and “session” are as 3 and 4 above; and there's also “password”, which is not used for logins but to update authentication tokens (e.g. passwords).
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How can I build a rpm for i386 target on a x86-64 machine? | 1,402,510,190,000 |
I am building an rpm using rpmbuild command as:
rpmbuild -bb --root <DIRECTORY> --target i386 --define "_topdir <DIRECTORY>" <specfile>.spec
When I use my SLED 10 SP3 x86 machine, it runs successfully. But on my SLES 10 SP3 x64 Virtual Machine, it gives following error:
error: No compatible architectures found for build
Initially I was not using --target option, still it was running on x86 machine, but same error was there in x64 machine.
Please help me to resolve this error
|
From the Fedora documentation for rpm, spec files, and rpmbuild:
The --target option sets the target architecture at build time. Chapter 3,
Using RPM covers how you can use the --ignoreos and --ignorearch options
when installing RPMs to ignore the operating system and architecture that
is flagged within the RPM. Of course, this works only if you are installing
on a compatible architecture.
On the surface level, the --target option overrides some of the macros in
the spec file, %_target, %_target_arch, and %_target_os. This flags the RPM
for the new target platform.
Under the covers, setting the architecture macros is not enough. You really
cannot create a PowerPC executable, for example, on an Intel-architecture
machine, unless you have a PowerPC cross compiler, a compiler that can make
PowerPC executables.
http://docs.fedoraproject.org/en-US/Fedora_Draft_Documentation/0.1/html/RPM_Guide/ch-rpmbuild.html
So, as it says, make sure you have the additional compilers installed (for example gcc.i686 & gcc.x86_64).
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Process in user mode switch to kernel mode. Then the process will have root privileges? | 1,402,510,190,000 |
According to http://www.linfo.org/kernel_mode.html in paragraph 7:
When a user process runs a portion of the kernel code via a system call, the process temporarily becomes a kernel process and is in kernel mode. While in kernel mode, the process will have root (i.e., administrative) privileges and access to key system resources. The entire kernel, which is not a process but a controller of processes, executes only in kernel mode. When the kernel has satisfied the request by a process, it returns the process to user mode.
It is quit unclear to me about the line,
While in kernel mode, the process will have root (i.e., administrative) privileges and access to key system resources.
How come a userspace process running not as root will have root privilege? How does it differ from userspace process running as root?
|
(I'll try to be brief.)
In theory, there are two dimensions of privileges:
The computer's instruction set architecture (ISA), which protects certain information and/or functions of the machine.
The operating system (OS) creating an eco-system for applications and communication. At its core is the kernel, a program that can run on the ISA with no dependencies of any kind.
Today's operating systems perform a lot of very different tasks so that we can use computers as we do today. In a very(, very, very) simplified view you can imagine the kernel as the only program that is executed by the computer. Applications, processes and users are all artefacts of the eco-system created by the OS and especially the kernel.
When we talk about user(space) privileges with respect to the operating system, we talk about privileges managed, granted and enforced by the operating system. For instance, file permissions restricting fetching data from a specific directory is enforced by the kernel. It looks at the some IDs assodicated with the file, interpretes some bits which represents privileges and then either fetches the data or refuses to do so.
The privileges hierarchy within the ISA provides the tools the kernel uses for its purposes. The specific details vary a lot, but in general there is the kernel mode, in which programs executed by the CPU are very free to perform I/O and use the instructions offered by the ISA and the user mode where I/O and instructions are constrained.
For instance, when reading the instruction to write data to a specific memory addres, a CPU in kernel mode could simply write data to a specific memory address, while in user mode it first performs a few checks to see if the memory address is in a range of allowed address to which data may be written. If it is determined that the address may not be written to, usually, the ISA will switch into kernel mode and start executing another instruction stream, which is a part of the kernel and it will do the right thing(TM).
That is one example for an enforcement strategy to ensure that one program does not interfere with another program ... so that the javascript on the webpage you are currently visiting cannot make your online banking application perform dubious transactions ...
Notice, in kernel mode nothing else is triggered to enforce the right thing, it is assumed the program running in kernel mode is doing the right thing. That's why in kernel mode nothing can force a program to adhere to the abstract rules and concepts of the OS's eco-system. That's why programs running in kernel mode are comparibly powerful as the root user.
Technically kernel mode is much more powerful than just being the root-user on your OS.
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Why can the waitpid system call only be used with child processes? | 1,402,510,190,000 |
The man page wait(2) states that the waitpid system call returns the ECHILD error if the specified process is not a child of the calling process. Why is this? Would waiting on a non-child process create some sort of security issue? Is there a technical reason why implementing waiting on a non-child process would be difficult or impossible?
|
Because of how waitpid works. On a POSIX system, a signal (SIGCHLD) is delivered to a parent process when one of its child processes dies. At a high level, all waitpid is doing is blocking until a SIGCHLD signal is delivered for the process (or one of the processes) specified. You can't wait on arbitrary processes, because the SIGCHLD signal would never be delivered for them.
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How do file permissions/attributes work? Kernel-level, FS-level or both? | 1,402,510,190,000 |
A question that occurred to me earlier: are file permissions/attributes OS- (and therefore kernel-) dependent or are they filesystem-dependent?
It seems to me that the second alternative is the more logical one, yet I never heard of reiserfs file permissions, for example: only "Unix file permissions". On the other hand, to quote from a Wikipedia article:
As new versions of Windows came out, Microsoft has added to the inventory of available attributes on the NTFS file system
which seems to suggest that Windows file attributes are somehow tied to the filesystem.
Can someone please enlighten me?
|
Both the kernel and the filesystem play a role. Permissions are stored in the filesystem, so there needs to be a place to store the information in the filesystem format. Permissions are enforced and communicated to applications by the kernel, so the kernel must implement rules to determine what the information stored in the filesystem means.
“Unix file permissions” refer to a traditional permission system which involves three actions (read, write, execute) controlled via three role types (user, group, other). The job of the filesystem is to store 3×3=9 bits of information. The job of the kernel is to interpret these bits as permissions; in particular, when a process attempts an operation on a file, the kernel must determine, given the user and groups that the process is running as, the permission bits of the file, and the requested operation, whether to allow the operation. (“Unix file permissions” also usually includes setuid and setgid bits, which aren't strictly speaking permissions.)
Modern unix systems may support other forms of permissions. Most modern unix systems (Solaris, Linux, *BSD) support access control lists which allow assigning read/write/excecute permissions for more than one user and more than one group for each file. The filesystem must have room to store this extra information, and the kernel must include code to look up and use this information. Ext2, reiserfs, btrfs, zfs, and most other modern unix filesystem formats define a place to store such ACLs. Mac OS X supports a different set of ACL which include non-traditional permissions such “append” and “create subdirectory”; the HFS+ filesystem format supports them. If you mount an HFS+ volume on Linux, these ACLs won't be enforced since the Linux kernel doesn't support them.
Conversely, there are operating systems and filesystems that don't support access control. For example, FAT and variants were designed for single-user operating systems and removable media and its permissions are limited to read/read-write and hidden/visible. These are the permissions enforced by DOS. If you mount an ext2 filesystem on DOS, it won't enforce the ext2 permissions. Conversely, if you access a FAT filesystem on Linux, all files will have the same permissions.
Successive versions of Windows have added support for more permission types. The NTFS filesystem was extended to store those extra permissions. If you access a filesystem with the newer permissions on an older operating system, the OS won't know about these newer permissions and so won't enforce them. Conversely, if you access an older filesystem with a newer operating system, it won't have contain of the new permissions and it is up to the OS to provide sensible fallbacks.
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How to know which commands are executed when I do something in GUI | 1,402,510,190,000 |
This is for academic purpose. I want to know which commands are executed when we do something in GUI, for example creating a folder. I want to show that both the mkdir shell command and create folder option from GUI does the same thing.
|
You can observe what the process does with the strace command. Strace shows the system calls performed by a process. Everything¹ a process that affects its environment is done through system calls. For example, creating a directory can only be done by ultimately calling the mkdir system call. The mkdir shell command is a thin wrapper around the system call of the same name.
To see what mkdir is doing, run
strace mkdir foo
You'll see a lot of calls other than mkdir (76 in total for a successful mkdir on my system), starting with execve which loads the process binary image, then calls to load the libraries and data files used by the program, calls to allocate memory, calls to observe the system state, … Finally the command calls mkdir and wraps down, finishing with exit_group.
To observe what a GUI program is doing, start it and only observe it during one action. Find out the process ID of the program (with ps x, htop or any other process viewer), then run
strace -o file_manager.mkdir.strace -p1234
This puts the trace from process 1234 in the file file_manager.mkdir.strace. Press Ctrl+C to stop strace without stopping the program. Note that something like entering the name of the directory may involve thousands or tens of thousands of system calls: handling mouse movements, focus changes and so on is a lot more complex at that level than creating a directory.
You can select what system calls are recorded in the strace output by passing the -e option. For example, to omit read, write and select:
strace -e \!read,write,select …
To only record mkdir calls:
strace -e mkdir …
¹ Ok, almost everything. Shared memory only involves a system call for the initial setup.
|
Why can an aarch64 ELF executable be run on an x86_64 machine? | 1,402,510,190,000 |
I compiled a simple "Hello World" C program on Raspberry Pi 3, which was then transferred to an AMD64 laptop. Out of curiosity, I executed it, and it runs even though I did not expect it to:
$ uname -a
Linux 15ud490-gx76k 6.5.0-25-generic #25~22.04.1-Ubuntu SMP PREEMPT_DYNAMIC Tue Feb 20 16:09:15 UTC 2 x86_64 x86_64 x86_64 GNU/Linux
$ file hello64
hello64: ELF 64-bit LSB executable, ARM aarch64, version 1 (GNU/Linux), statically linked, BuildID[sha1]=486ee1cde035cd704b49c037a32fb77239b6a1c2, for GNU/Linux 3.7.0, not stripped
$ ./hello64
Hello World!
Like that, how can it execute?
QEMU User Emulation is installed, but I don't know whether it is playing a part in this or not.
|
QEMU user emulation is exactly why your binary runs: on your system, one of the QEMU-related packages you’ve installed ensures that QEMU is registered as a handler for all the architectures it can emulate, and the kernel then passes binaries to it. As long as you have the required libraries, if any, the binary will run; since your binary is statically linked, it has no external dependencies.
See Why can my statically compiled ARM binary of BusyBox run on my x86_64 PC? and How is Mono magical? for details.
|
Rings and run levels | 1,402,510,190,000 |
The question stated below might not be technically correct(misconception) so it would be appreciable if misconception is also addressed.
Which ring level do the different *nix run levels operate in?
Ring tag not available.
|
Unix runlevels are orthogonal (in the sense "unrelated", "independent of" - see comments) to protection rings.
Runlevels are basically a run time configurations/states of the operating system as a whole, they describe what services are available ("to the user") - like SSH access, MTA, file server, GUI.
Rings are a hardware aided concept which allows finer grained control over the hardware (as mentioned in the wikipedia page you link to). For example code running in higher Ring may not be able to execute some CPU instructions.
Linux on the x86 architecture usually uses Ring0 for kernel (including device drivers) and Ring3 for userspace applications (regerdless of whether they are run by root or another ordinary or privileged user).
Hence you can't really say that a runlevel is running in some specific Ring - there are always1 userspace applications (at least PID 1 - the init) running in Ring3 and the kernel (Ring0).
1As always, the "always" really means "almost always", since you can run "normal" programs in Ring0, but you are unlikely to see that in real life (unless you work on HPC).
|
How does the Linux login work? [duplicate] | 1,402,510,190,000 |
I am wondering how the login actually works. It certainly is not part of the kernel, because I can set the login to use ldap for example, or keep using /etc/passwd; but the kernel certainly is able to use information from it to perform authentication and authorization activities.
There is also a systemd daemon, called logind which seems to start up the whole login mechanism.
Is there any design document I can look at, or can someone describe it here?
|
The login binary is pretty straightforward (in principle). It's just a program that runs as root user (started, indirectly through getty or an X display manager, from init, the first user-space process). It performs authentication of the logging-in user, and if that is successful, changes user (using one of the setuid() family of system calls), sets appropriate environment variables, umask, etc, and exec()s a login shell.
It may be instructive to read the source code, but if you do so, you'll find it easiest (assuming the standard shadow-utils login that Debian installs) to read it assuming USE_PAM is not set, at least until you are comfortable with its operation, or you'll find too much distraction.
|
Meaning of hardware platform in uname command ouput | 1,402,510,190,000 |
man uname
-m, --machine print the machine hardware name
-i, --hardware-platform print the hardware platform or "unknown"
What exactly is meant by hardware platform here and how is it different from the "machine hardware name"? I found some related questions on SE but there seems to be some contradictions among the accepted answers. Where can I find accurate information about this nomenclature?
|
A bit more info in info uname:
`-i'
`--hardware-platform'
Print the hardware platform name (sometimes called the hardware
implementation). Print `unknown' if the kernel does not make this
information easily available, as is the case with Linux kernels.
`-m'
`--machine'
Print the machine hardware name (sometimes called the hardware
class or hardware type).
Basically classification types - you can have different hw implementations (-i) but with/in the same hw class (-m).
Used, for example, to differentiate between kernel modules shared by the same hw class and modules specific to a certain hw implementation.
|
Concept of memory mapping in Unix like systems | 1,402,510,190,000 |
Can some one explain in an easy to understand way the concept of memory mappings (achieved by mmap() system call) in Unix like systems ? When do we require this functionality ?
|
Consider: two processes can have the same file open for reading & writing at the same time, so some kind of communication is possible between the two.
When process A writes to the file, it first populates a buffer inside its own process-specific memory with some data, then calls write which copies that buffer into another buffer owned by the kernel (in practise, this will be a page cache entry, which the kernel will mark as dirty and eventually write back to disk).
Now process B reads from same point in the same the file; read copies the data from the same place in the page cache, into a buffer in B's memory.
Note that two copies are required: first the data is copied from A into the "shared" memory, and then copied again from the "shared" memory into B.
A could use mmap to make the page cache memory available directly in its own address space. Now it can format its data directly into the same "shared" memory, instead of populating an intermediate buffer, and avoiding a copy.
Similarly, B could mmap the page directly into its address space. Now it can directly access whatever A put in the "shared" memory, again without having to copy it into a separate buffer.
(Obviously some kind of synchronization is required if you really want to use this scheme for IPC, but that's out of scope).
Now consider the case where A is replaced by the driver for whatever device this file is stored on. By accessing the file with mmap, B still avoids a redundant copy (the DMA or whatever into the page cache is unavoidable, but it doesn't need to be copied again into B's buffer).
There are also some drawbacks, of course. For example:
if your device and OS support asynchronous file I/O, you can avoid blocking reads/writes using that ... but reading or writing a mmapped page can cause a blocking page fault which you can't handle directly (although you can try to avoid it using mincore etc.)
it won't stop you trying to read off the end of a file, or help you append to it, in a nice way (you need to check the length or explicitly truncate the file larger)
|
Metaphor for the concept of shell? | 1,402,510,190,000 |
I'm finding myself helping out some classmates in my computer science class, because I have prior development experience, and I'm having a hard time explaining certain things like the shell. What's a good metaphor for the shell in the context of the Terminal on Mac, contrasted with a remote shell via SSH?
|
Put simply, a terminal is an I/O environment for programs to operate in, and a shell is a command processor that allows for the input of commands to cause actions (usually both interactively and non-interactively (scripted)). The shell is run within the terminal as a program.
There is little difference between a local and remote shell, other than that they are local and remote (and a remote shell generally is connected to a pty, although local shells can be too).
|
Can there be multiple kernels executing at the same time? | 1,402,510,190,000 |
I know that Linux OS's are typically multi-programmed, which means that multiple processes can be active at the same time. Can there be multiple kernels executing at the same time?
|
Sort of. Check out User-mode Linux.
|
What is a windowing system? | 1,402,510,190,000 |
Can someone provide me with a very clear and practical example of a "windowing system"? I was reading on Linux, and although I've always known that it's a kernel, I didn't really know what a kernel is because I haven't taken an OS class yet. My understanding of it is that it's basically the layer between hardware and software. Would that be correct? Now the Linux distros everyone uses is combination of GNU/Linux/X Window System. I think I got the Linux kernel part, but what is a windowing system and what is GNU? Wikipedia says GNU is an OS, but then that would mean Linux distros are composed of another OS. Can someone clear this up for me?
|
GNU (Gnu is Not Unix) is an Operative System, created by Richard M. Stallman.
You can use this operative system with different kernel: such as Linux kernel, Hurd kernel, Darwin kernel, etc.
The X Window System (common on Unix like system) is just the basic layer for a GUI environment.
Every Linux distribution is a GNU operative system with a Linux kernel and an X Window System; on top of X Windows, you have the window manager (GUI) such as Xfce, Gnome, or KDE that lets you easily use your system.
|
What's the difference between /etc/rc.d/rc*.d and /etc/rc*.d | 1,402,510,190,000 |
I know that rc*.d directories are used at startup, or reboot, or so on time, for starting or stopping programs. Can anybody explain me what's the difference between the rc*.d folders placed under the /etc/ path and the other placed under the /etc/rc.d/ path.
Also, what's the difference between /etc/init.d and /etc/rc.d/init.d?
Thanks.
N.B. I'm running CentOS 6.2.
|
Nothing. Different Linux distributions, and the LSB, had different standards, so both are present on CentOS to make it easier to run software from different versions. One is just a symbolic link to the other.
http://www.centos.org/docs/5/html/5.1/Installation_Guide/s2-boot-init-shutdown-init.html gives details on the boot process, but ultimately all the init scripts are almost-but-not-completely identical on the different Linux systems.
|
Cannot remove architecture i386 | 1,402,510,190,000 |
I'm using 64 bit Kali Linux, previously installed i386 architecture and now I want to remove it, because it downloads about 30Mb data for 32bit package every time apt update.
I tried dpkg --remove-architecture i386, it failed with
dpkg: error: cannot remove architecture 'i386' currently in use by the database
Google says the i386 packages should be removed first, but some package like "gcc-12-base:i386, libc6:i386, libcrypt1:i386, libgcc-s1:i386" cannot be removed, how to solve it?
|
You need to remove them simultaneously, and force their removal in spite of their “protected” status:
dpkg --purge --force-remove-protected {gcc-12-base,libc6,libcrypt1,libgcc-s1}:i386
|
How to determine bitness of hardware and OS? | 1,432,207,891,000 |
Output of uname -a on my RHEL 5.4 machine is:
Linux <machine name> 2.6.18-164.el5 #1 SMP Tue Aug 18 15:51:48 EDT 2009 x86_64 x86_64 x86_64 GNU/Linux
Does it mean that hardware is 64 bit (going by perhaps first x86_64) and OS is also 64-bit going by last x86_64?
Also, what are these so many instances of x86_64?
Can I install 64-bit vm over 32-bit OS and vice versa?
|
The hardware, the kernel and the user space programs may have different word sizes¹.
You can see whether the CPU is 64-bit, 32-bit, or capable of both by checking the flags line in /proc/cpuinfo. You have to know the possible flags on your architecture family. For example, on i386/amd64 platforms, the lm flag identifies amd64-capable CPUs (CPUs that don't have that flag are i386-only).
grep -q '^flags *:.*\blm\b' /proc/cpuinfo # Assuming a PC
You can see whether the kernel is 32-bit or 64-bit by querying the architecture with uname -m. For example, i[3456]86 are 32-bit while x86_64 is 64-bit. Note that on several architectures, a 64-bit kernel can run 32-bit userland programs, so even if the uname -m shows a 64-bit kernel, there is no guarantee that 64-bit libraries will be available.
[ "$(uname -m)" = "x86_64" ] # Assuming a PC
You can see what is available in userland by querying the LSB support with the lsb_release command. More precisely, lsb_release -s prints a :-separated list of supported LSB features. Each feature has the form module-version-architecture. For example, availability of an ix86 C library is indicated by core-2.0-ia32, while core-2.0-amd64 is the analog for amd64. Not every distribution declares all the available LSB modules though, so more may be available than is detectable in this way.
You can see what architecture programs on the system are built for with a command like file /bin/ls. Note that it's possible to have a mixed system; even if ls is a 64-bit program, your system may have libraries installed to run 32-bit programs, and (less commonly) vice versa.
You can find out the preferred word size for development (assuming a C compiler is available) by compiling a 5-line C program that prints sizeof(void*) or sizeof(size_t). You can obtain the same information in a slightly less reliable way² by running the command getconf LONG_BIT.
#include <stdio.h>
int main() {
printf("%d\n", (int)sizeof(void*));
return 0;
}
As for virtual machines, whether you can run a 64-bit VM on a 32-bit system or vice versa depends on your virtual machine technology. See in particular
How can I install a 64bit Linux virtual machine on a 32bit Linux?
¹ “Word size” is the usual name for what you call bitness.
² It can be unreliable if someone installed an alternate C compiler with a different target architecture but kept the system default getconf.
|
`uname -m` valid values | 1,432,207,891,000 |
On my computer, uname -m prints x86_64 as output. What is the list of possible values that this command could output? I intend to use this command from a dynamic runtime to check the CPU architecture.
|
I’m not aware of a definitive list of possible values; however there is a list of values for all Debian architectures, which gives good coverage of the possible values on Linux: aarch64, alpha, arc, arm, i?86, ia64, m68k, mips, mips64, parisc, ppc, ppc64, ppc64le, ppcle, riscv64, s390, s390x, sh, sparc, sparc64, x86_64 (there are other possible values, but they’re not supported by Debian; I’m ignoring the Hurd here). Another source of information is the $UNAME_MACHINE matches in config.guess; this isn’t limited to Linux.
Note that uname -m reflects the current process’ personality, and the running kernel’s architecture; not necessarily the CPU architecture. See Meaning of hardware platform in uname command ouput for details.
|
Are system calls the only way to interact with the Linux kernel from user land? | 1,432,207,891,000 |
Are there any other interfaces, e.g. the /proc filesystem?
|
The Linux kernel syscall API is the the primary API (though hidden under libc, and rarely used directly by programmers), and most standard IPC mechanisms are heavily biased toward the everything is a file approach, which eliminates them here as they ultimately require read/write (and more) calls.
However, on most platforms (if you exclude all the system calls to get you there) there is a way: VDSO. This is a mechanism where the kernel maps one (or more) slightly magic pages into each process (usually in the form of an ELF .so). You can see this as linux-vdso.so or similar with ldd or in /proc/PID/maps. This is effectively memory-mapped IPC between the kernel and a user process (albeit one-way in its current implementation).
It's used to speed up syscalls in general and was originally implemented (linux-gate.so) to address x86 performance issues, but it may also contain kernel data and access functions. Calls like getcpu() and gettimeofday() may use these rather than making an actual syscall and a kernel context switch. The availability of these optimised calls is detected and enabled by the glibc startup code (subject to platform availability). Current implementations contain a (read-only) page of shared kernel variables known as the "VVAR" page which can be read directly.
You can check this by inspecting the output of strace -e trace=clock_gettime date to see if your date command makes any clock_gettime() syscalls, with a working VDSO it will not (the time will be read from the VVARS page by a function in the VDSO page, see arch/x86/vdso/vclock_gettime.c).
There's a useful technical summary here: http://blog.tinola.com/?e=5 a more detailed tutorial: http://www.linuxjournal.com/content/creating-vdso-colonels-other-chicken , and the man page: http://man7.org/linux/man-pages/man7/vdso.7.html
|
When I move a file to a different directory on the same partition, does the file's data actually move on disk? | 1,432,207,891,000 |
I could see it going both ways. If the filesystem stores it's directory structure and list of files in each directory, and then points to the disk location of each of the files, it shouldn't require the file's data to actually be moved on disk in order to 'move' a file. On the other hand, I could see the 'move' being implemented by copying the file, checking the copy, and then deleting the original if the copy checks out. Does the answer depend on the type of filesystem?
|
Yes, this depends on the type of filesystem. But all the modern filsystems I know of use a pointer scheme of some kind. The linux/unix-filesystems (like ext2, ext3, ext4, ...) do this with INODES.
You can use ls -i on a file to see which inode-number is referenced by the filename (residing as meta-information in the directory-entry). If you use mv on these filesystems the resulting action will be a new pointer within the filesystem or a cp/ rm if you cross FS-borders.
|
how is a keyboard shortcut given to the correct program? | 1,432,207,891,000 |
in Ubuntu (or for that matter most other linux distros), I could use the shortcut ctrl+t to open a new tab (in firefox or similar), or I could use alt+tab to make unity switch highlighted window, or I could use alt+ctrl+F<1-6> to get to another tty. What part of linux handles and resolves these shortcuts? What if several programs/processes share the same shortcut, how is priority resolved?
(For the latter I'm assuming that this is only relevant for programs on different 'levels', e.g. firefox and the session script might compete, but firefox and chrome would never compete because they should not both be responding at the same time)
|
What part of linux handles and resolves these shortcuts?
For the most part, individual applications or a window manager(WM)/desktop environment(DE). There are a few caught and handled by the kernel, such as VT switching with Cntl-Alt-F[N].
The actual event propagates:
From the kernel
To the Xorg server
To the WM/DE
To the application
If caught and handled at any point therein, it will probably not continue to the next level down.
If you run a (non-GUI) application inside a GUI terminal, the GUI terminal will have precedence over it.
What if several programs/processes share the same shortcut, how is priority resolved?
The WM/DE will take priority over the application.
|
What is the default or most commonly used multiprocessing model in Linux? Symmetric or Asymmetric? | 1,432,207,891,000 |
What the multiprocessing model for Linux? Is there a default or most commonly used model? Is it similar or very different from say BSD or even the MS Windows kernel?
If SMP is used normally, can assymetric be used instead if desired?
|
From Wikipedia:
Asymmetric multiprocessing (AMP) was a software stopgap for handling
multiple CPUs before symmetric multiprocessing (SMP) was available.
Linux uses SMP.
|
What are the very fundamental differences in architecture between Unix and Linux? [duplicate] | 1,432,207,891,000 |
I watched a short intro to Unix from the 70s (https://www.youtube.com/watch?v=7FjX7r5icV8 3D animation starts at 1:56), at the end the general tripartite architecture of Unix was displayed as a 3D animation. Because I have seen already diagrams of the ovarall Linux architecture, I became confused.
Both diagrams, Unix and Linux, share the Kernel, but then Unix is wrapped by the Shell and the Shell by the Utilities. Linux instead is only wrapped by the Userspace, and the Shell does not wrap anything but is just one of many processes within the Userspace.
How do Unix and Linux differ on a very basic level, what do they have in common ?
Why is Unix tripartite and Linux two-layered ?
Is a Shell a complete different concept within Unix than in Linux ?
|
Because the distinction remains a little vague to me, this may not be a very clear answer. I'll just try to expose my point of view, more than actual, technical facts.
First of all, it is probably relevant to note that Linux is a UNIX-like system. This means that while most concepts and implementations have been inspired, sometimes taken, from UNIX, there was originally no common code base between the two systems. Actually, Linux was mostly inspired from MINIX, another UNIX-like system, the licensing of which Linus Torvalds found too restrictive.
Why is Unix tripartite and Linux two-layered ? Is a Shell a complete different concept within Unix than in Linux ?
To me, both are two-layered. The shell does not have any kind of privileged relationship with the kernel, nor should it. The first, privileged layer, is the kernel, where everything is possible. The second, unprivileged layer, is userland, in which various programs run, including the shell, and standard utilities such as ls. All these programs may communicate with the kernel through the UNIX or Linux set of system calls (these lists are probably not exhaustive).
In my opinion, this is the only layer distinction which really needs to be mentioned when it comes to either UNIX or Linux. Now, while the kernel sees no difference between a shell and another program, the user certainly does in the way he interacts with each. If a difference has to be made between the shell and other programs, then this difference definitely comes from the user, but remains unknown to the system.
This is much more striking in your video than it would be for users of today's systems. Have a look at their terminals: this is amazingly minimal, and we would probably never think of using such things nowadays (even though, I'll admit I'd love to). The thing is: back then, the shell was the first (and only) thing you got when your system had booted and you had logged in. This was the thing you had to go through if you wanted to run any other program. This is probably where the difference is: while the shell is no different from any other program in the kernel's eye, it is a gateway to other programs for the user, and this gateway was much more visible in the 70s, in "core UNIX's" prime.
Of course, this distinction is a lot less significant nowadays, probably because of two things:
Terminal emulation. You can actually get several shells at the same time, and switch between them. This means that you have something before the shell that gives you control over it.
Graphical interfaces. You can now start processes from GUIs, window managers, desktop environments, ... without ever seeing a terminal. We even have graphical programs designed to wrap around shell instances and make them more pleasant to use.
Now, I'm not very good at diagrams, but I guess I would put it this way:
Where I would say that:
Dashed lines represent user interaction.
Dotted lines represent shell-to-process interaction (spawning processes, manipulating I/O flows between them, ...).
Plain lines represent system interaction.
If you remove everything but the elements involving system interaction, you end up with two things: the kernel, and user programs. There are two layers, connected by system calls.
Now if, as a user, you see the shell not just as another program, but as a gateway to others, you add user interaction and shell-to-process interaction. Here comes the third layer, yet nothing has changed for the kernel.
|
Which arch linux should I download? | 1,432,207,891,000 |
I'm going to install Arch linux yet I have to choose between several architectures my computer has.
I have an aluminium macbook pro, with a 2.3 GHz Intel Core i5 processor. . The intel webpage tells me this is a dual core processor.
running uname -a in the shell returns:
Darwin Romeos-MacBook-Pro.local 11.3.0 Darwin Kernel Version 11.3.0:
Thu Jan 12 18:47:41 PST 2012; root:xnu-1699.24.23~1/RELEASE_X86_64 x86_64
Which makes me believe I have an x86_64 machine, yet when executing arch in the shell it returns i386.
I'm a bit confused about what to pick:
i686 CPU
x86-64 CPU
Dual Architecture
What would you recommend?
|
Intel Core 2 (i5) is a 64-bit processor supporting Intel 64, Intel 64 is Intel's implementation of x86-64
|
Get Linux architecture from /proc filesystem | 1,432,207,891,000 |
I'm writing a program in Java and I need to determine the architecture for which Linux was compiled.
I need something like uname -m, but without running any program, but instead from the /proc pseduo-fs.
What is a reliable source to read from?
|
As you can have a 32-bit Linux installed in a 64-bit machine, the safer way seems to be verifying CPU capabilities. For Intel and compatible processors:
grep -o -w 'lm' /proc/cpuinfo
http://www.unixtutorial.org/2009/05/how-to-confirm-if-your-cpu-is-32bit-or-64bit/
What you're looking for is the following flag: lm. It stands for
X86_FEATURE_LM, the Long Mode (64bit) support. If you can find the
"lm" flag among your CPU flags, this means you're looking at a 64bit
capable processor.
|
Accessing PHPMyAdmin as installed by its distro package-index from the domain of each website | 1,432,207,891,000 |
I have a remote machine with LAMP and PHPMyAdmin (PMA). Let's assume this distro is Debian/Ubuntu.
If I install PMA via apt install phpmyadmin (which will make it to be installed under /usr/share/phpmyadmin/ I think) then I wouldn't be able to navigate to PMA based on domains of my websites hosted on that lamp (the following will error):
example-1.com/phpmyadmin
example-2.com/phpmyadmin
If I remember correctly, I'll have to navigate via say MY_IP_ADDRESS/usr/share/phpmyadmin/ to access PMA successfully.
But if I'll install PMA directly on the document root via the following way I would indeed be able to navigate to PMA based on domains (as shown above):
pma="[pP][hH][pP][mM][yY][aA][dD][mM][iI][nN]"
cd /var/www/html/
rm -rf ${pma}*
wget https://www.phpmyadmin.net/downloads/phpMyAdmin-latest-all-languages.zip
unzip ${pma}*.zip
mv ${pma}*/ phpmyadmin/
rm ${pma}*.zip
unset pma
cd
On the one hand, installing PMA with apt install phpmyadmin is simple and convenient. On the other hand but doesn't let me navigate to it based on domains. On the other hand, I do want to navigate to it just based on domains.
If I'm not wrong, a symlink can be helpful. Am I in the right direction (I can't test now)?
|
In fact, Debian installs the majority of PMA into /usr/share/phpmyadmin which is the LSB standard correct location for it. But that's a detail that's not terribly relevant to the premise of your question.
What Debian's PMA package also does is drop a config file in /etc/apache2/conf-available/phpmyadmin.conf that sets up the specifics PMA needs to run properly. You can look into it on your own time if you want the details, but what it boils down to is that from that point on PMA can and will work with every site you configure that has working PHP available, simply by adding the following line to the <VirtualHost> directive:
Alias /phpmyadmin /usr/share/phpmyadmin
At that point PMA should work for that site without any further actions required.
(Also, drat. Ninja'd.)
|
Are “kernel mode” and “user mode” hardware features or software features? | 1,432,207,891,000 |
I read that there are two modes called “kernel mode” and “user mode” to handle execution of processes. (Understanding the Linux Kernel, 3rd Edition.) Is that a hardware switch (kernel/user) that is controlled by Linux, or software feature provided by the Linux kernel?
|
Kernel mode and user mode are a hardware feature, specifically a feature of the processor. Processors designed for mid-to-high-end systems (PC, feature phone, smartphone, all but the simplest network appliances, …) include this feature. Kernel mode can go by different names: supervisor mode, privileged mode, etc. On x86 (the processor type in PCs), it is called “ring 0”, and user mode is called “ring 3”.
The processor has a bit of storage in a register that indicates whether it is in kernel mode or user mode. (This can be more than one bit on processors that have more than two such modes.) Some operations can only be carried out while in kernel mode, in particular changing the virtual memory configuration by modifying the registers that control the MMU. Furthermore, there are only very few ways to switch from user mode to kernel mode, and they all require jumping to addresses controlled by the kernel code. This allows the code running in kernel mode to control the memory that code running in user mode can access.
Unix-like operating systems (and most other operating systems with process isolation) are divided in two parts:
The kernel runs in kernel mode. The kernel can do everything.
Processes run in user mode. Processes can't access hardware and can't access the memory of other processes (except as explicitly shared).
The operating system thus leverages the hardware features (privileged mode, MMU) to enforce isolation between processes.
Microkernel-based operating systems have a finer-grained architecture, with less code running in kernel mode.
When user mode code needs to perform actions that it can't do directly (such as access a file, access a peripheral, communicate with another process, …), it makes a system call: a jump into a predefined place in kernel code.
When a hardware peripheral needs to request attention from the CPU, it switches the CPU to kernel mode and jumps to a predefined place in kernel code. This is called an interrupt.
Further reading
Wikipedia
What is the difference between user-level threads and kernel-level threads?
Hardware protection needed for operating system kernel
|
How to check Linux kernel? | 1,432,207,891,000 |
I want to install a package, and it has different versions for different OSes. The description in the package site is like this
X86-64 Linux 3.0 Kernel
I looked it up and found people saying to use
uname -r
uname -m
I tried it and got this:
3.2.0-24-generic
x86_64
Does this tell me the Linux I'm using is x86_64 and 3.2.0 Kernel? What does -24-generic mean?
|
3.2.0 is the version of the source code used to compile this kernel. These can be four numbers long (e.g. 2.6.32.55) indicating a patch level on that version. However, this four digit system was only used for version 2.6 kernels starting at 2.6.8. I.e., it is not used with 3.x kernels, which are the 3 numbers, release-major-minor. Note the subtle difference from the three number major-minor-patch level system commonly used with software.
-24-generic indicates a patch level and configuration used by the distro, 24 being their patch level, and generic being the configuration used in compiling. This patch level does not necessarily reset/change for different kernel source versions; the distro either applies the patches unchanged (so, e.g., 3.2.1-24-generic) or they increment the patch level (3.2.1-25-generic).
The most significant aspects are the source version number and the configuration style. The later is important because it indicates significant differences in the way the kernel was actually configured for build.
This doesn't reveal which architecture the kernel was built for -- e.g., x86_64 -- but the uname -m output does.
|
How I can emulate a big endian platform on a x86? | 1,432,207,891,000 |
I need to get a big endian platform to develop with gcc and g++, what is a solution for that? I know that the SPARC is one of those big endian architectures, but I have no idea what OSs can run on it and how to emulate a SPARC machine under Linux; I also should note that I need any big endian that I can emulate on an X86 but with g++ available on it.
|
Why Sparc specifically? ARM or MIPS is easier to emulate or to get in hardware, both are bi-endian, and both are supported by Linux in either endianness.
There doesn't seem to be a well-maintained ARM big-endian port, your best bet for ARM seems to be the old Debian NSLU2 port. For MIPS you have the MIPS port.
QEMU can emulate all of these CPUs.
|
When does a shell get executed during the linux startup process | 1,432,207,891,000 |
I do not understand when does a shell, lets say bash, get executed, which program runs bash initially first.
|
The boot sequence of linux/unix has many stages, and there are many references and answers on this site that explain the detail. But to summarise;
Eventually the kernel is loaded with drivers so that the disk and devices can be used, it then starts process with a pid (process id) of 1.
Traditionally this program was a program called init but today there are several newer programs (systemd or upstart). It depends on your distribution and version, which one is used.
Starting up is a tiered process.
There is a concept of escalating run levels (1,2,3,4,5,6 ...) and the start up program will flip between these levels automatically or staged (so that the user can gain control).
being the initial step (single user mode),
multi user mode,
multi user with networking
GUI mode ...
.. 6., ...
These run levels are not fixed in stone either, they depend on the distribution and start up program being used (init, systemd, ...) and convention.
The levels also depend on how the staged start-up/shutdown pattern has been designed. (think, linux is used in routers, android phones, servers and desktops all with different requirements).
In order to transgress from one run-level to another various other programs (services), like bind (for DNS), networking, routing, webservers, ... are started or stopped, and bash may be used then to run a particular script which starts or stops a service.
Eventually you need to login, either at a console or at a graphical interface, and you may be prompt for your username and password.
Let's take a simple route, and say you are at a non-graphical console, and the login program is prompting you to authenticate. When you pass, it will read which shell is configured for the entered username from /etc/passwd and start it, with input and output set to your console and then you have the prompt and can start doing your work. So in this scenario,
init starts -> login which starts -> bash
So every process is a child of the first process, (it might be more accurate to say, every process has pid 1 as an ancestor). In the above example, login will exec the shell, replacing login process with bash, the process id doesn't change. When you look with ps it looks like bash was started by init because it's parent pid is 1, but there was a chain of events.
There's nothing really stopping pid 1 from just starting bash at the console (if pid 1 can work out what the console is at that point) and this is down to how the start-up sequence is designed. (I had to do that once, but it is not normal practice).
|
How to find out what is the Instruction Set Architecture (ISA) of a CPU? | 1,432,207,891,000 |
In the Debian download CD/DVD images page they have different ISO's for the different instruction set architectures. How do I know what is the ISA of a CPU before I buy one? I know about using the commands
cat /proc/cpuinfo
and
lscpu
but these are only good after getting the CPU and running these commands on a Linux based OS. How do I find out this information before getting the CPU?
For example the CPU:
Intel(r) core(tm) i5-6300hq cpu @ 2.30ghz
In the official intel website they show the ISA is "64 bits". But nothing specific as mentioned in the debian website:
amd64 / arm64 / armel / armhf / i386 / mips64el /mipsel / ppc64el / s390x / multi-arch
Can someone tell me how they would go about finding this information?
|
If you don't have the cpu, I presume you are buying one or something.
If that is the case, then you can find out everything about the prospective cpu you are going to buy by looking up the data by the model number of the cpu you are looking at.
You can guess the architecture by the manufacturer, as most manufacturers (e.g., Intel) only produce a small number of architectures (for intel, currently, AMD64 aka x86-64, but i386 and IA-64 in the past).
Typically the model number of the cpu will allow you to look up even more detailed information. Wikipedia typically has well collected data in tables on this, but you can also typically find this on the manufacturers' websites.
For your specific example i5-6300hq, a google search finds a reference to it in the wikipedia page https://en.wikipedia.org/wiki/List_of_Intel_Core_i5_processors (with a specific table entry for your example further down) which in turn calls this an "Intel Core" processor, which links to https://en.wikipedia.org/wiki/Intel_Core
In the side bar on this page, it lists x86-64, linked to https://en.wikipedia.org/wiki/X86-64 and the first line of that page lists AMD64.
Each of these pages has abundant details on what each classification means and how it relates to similar cpus, including the outdated i386 and IA-64.
|
Difference between architecture and platform in linux kernel | 1,432,207,891,000 |
I want to know the difference between architecture and platform in Linux kernel. When I had downloaded the latest kernel tarball, observed that a directory named with arch, it contains different names of processors & inside to any one processor directory again there is a directory called platform.
For example:-
/arch/powerpc is a directory under arch in Linux kernel & /arch/powerpc/platforms is a directory under powerpc.
So, what does this actually mean?
Can anyone explain this in detail, referring from hardware perspective to software perspective, please?
|
The architecture is the processor type. There are only a relatively small number of architectures. All processor types that execute the same user code are classified as the same architecture, even though there may be several different ways to compile the kernel; for example x86 and powerpc are a single architecture but the kernel can be compiled using the 32-bit instruction set or the 64-bit instruction set (and a 32-bit kernel can execute only 32-bit programs, while a 64-bit kernel can execute both 32-bit and 64-bit programs).
The platform describes everything else about the hardware that Linux cares about. This includes variations on the way booting works, on how some peripherals such as a memory controller, a power management coprocessor, cryptographic accelerators and so on work, etc. Whether features are classified according to a platform or are separate drivers or compilation options depends partly on how fundamental the feature is (i.e. how difficult it is to isolate the code that uses it) and partly on how the person who coded support for it decided to do it.
|
How could Linux use 'sda' device file when it hasn't been installed? | 1,432,207,891,000 |
I am installing CentOS Linux distribution.
At the partition step, CentOS tells me that it has detected a sda HD in my machine and I should create partitions and assign mount points for this disk.
But I found the logic a little twisted. I understand that Linux treat everything as file and sda is usually the device file representing my first SATA hard disk. But since no Linux is installed yet, there should be no file system yet. So how could there be any device file like sda?
Someone tells me that “Linux installer is also a Linux OS and hence there's a in-memory file system. My hard drive is just one tiny element of the file system”. Why doing like this? Does Windows or other OS do the same thing?
|
What /dev/sda means
There are four levels: raw disk, raw partition of that disk, formatted filesystem on a partition, and actual files stored within a filesystem.
/dev/sda means an entire disk, not a filesystem. Something with a number at the end is a partition of a disk: dev/sda1 is the first partition of the /dev/sda disk, and it's not even necessarily formatted yet! The filesystems each go on their own partitions by formatting each partition with its filesystem.
So, what will generally happen is that you'll partition /dev/sda, format /dev/sda1 with a filesystem, mount /dev/sda1's filesystem to somewhere, and then begin working with files on that filesystem.
Why have a unified filesystem
Linux (and UNIX in general) has the concept of the virtual filesystem. It combines all your real disks into one unified file system.
This can be quite useful. You might, for example, want to put your operating system and its programs on one really fast real disk and all the users' personal files on another fairly slow but huge disk because you want the OS to be fast but you want an affordable means of handling the files of thousands of users.
Unlike the usual method in Windows, which by default breaks each disk up into a separate letter and where using D:\Users might break some programs that hard code the path C:\Users, this can be done with ease and fluency. You format one partition in each disk, you mount the OS one to / and the user one to /home, and it acts like a system that put everything on one real disk, except you get that speed and affordability tradeoff you wanted.
|
How do i change the output of "uname -m" | 1,432,207,891,000 |
I am trying to execute this shell script - https://raw.githubusercontent.com/oneindex/script/master/gclone.sh
This shell script checks for uname -m output and doesn't like it ( i.e. aarch64 ).
xd003@localhost:~$ uname -m
aarch64
xd003@localhost:~$
I want to change the uname -m output from aarch64 to arm64 so that it bypasses this check in the shell script and execute properly.
|
Since it happens to be a bash script (despite the .sh extension), you can always do (within bash):
uname()
if [ "$#" -eq 1 ] && [ "$1" = -m ]; then
echo arm64
else
command uname "$@"
fi
export -f uname
gclone.sh
That is, replace uname with an exported function that outputs what you want when passed a -m argument.
|
Building packages: command which yields 'amd64' (like uname) | 1,432,207,891,000 |
Suppose I have a makefile that builds my package, and I only want the package to build if the package file is not present:
package: foo_0.0.0_amd64.deb
cd foo-0.0.0 && debuild -uc -us
So I am new to the debian build process, but I am anticipating that I'll either find a way to build for different architectures, or I'll be on a different architecture natively and that file name will change. So, I set it as a variable:
major=0
minor=0
update=0
release=amd64
package: foo_${major}.${minor}.${update}_${release}.deb
I have a machine where uname -r yields #.##.#-#-amd64. What is the bulletproof way to fetch that amd64 in unix/linux?
|
On a Debian-based system, the bullet-proof way of determining the architecture, as appropriate for use in a package’s file name, is
dpkg --print-architecture
Note that architecture-independent packages use all there, and you’d have to know that in advance.
|
Can't install rust-doc on Debian Stretch | 1,432,207,891,000 |
I'm running Debian Stretch. According to the Debian website, I should be able to install the package rust-doc, yet I can't:
wizzwizz4@myLaptop:~$ sudo apt install rust-doc
Reading package lists... Done
Building dependency tree
Reading state information... Done
E: Unable to locate package rust-doc
Everything seems to say it doesn't exist. But it does. Do I have to do something special to install all-arch packages, or something?
The output of apt policy is normal.
|
There’s nothing wrong with your setup, the problem here is the package pool and the web site. The rust-doc package was disabled with the 1.24.1+dfsg1-1~deb9u1 upload:
Disable -doc package, requires packages not found in stretch and
docs are available online anyway
As a result the package is no longer included in the indexes and isn’t available from apt’s perspective. The package which can still be downloaded from the web site is the old 1.14.0 release. I’ve informed the site team about the discrepancy.
You’ll be able to install the package again normally once Debian 10 is released and you upgrade to that.
|
What does this mkfs.ext4 operand mean? | 1,432,207,891,000 |
I am using GParted (0.28.1, Fedora 25) to format a external drive and noticed that the command displayed is:
mkfs.ext4 -F -O ^64bit -L "INSTALL" /dev/sdd1
When making disks in the past from command line I have just used mkfs.ext4 DEVICE which seemed to work well for various architectures. However the above includes the option -O ^64bit, which I guess removes some default 64bit feature of the filesystem so it works with 32bit. Does it do this and is normally necessary to pass it on modern Linux OSs (to enable compatibility with 32bit etc systems), and what cost could it have other than probably reducing the volume size limit?
|
The default options for mke2fs including those for ext4 can be found in /etc/mke2fs.conf. They could be different depending on the distro you're using. I'd take a look at that file on any distro you're curious about to see if the -O ^64bit param would be necessary. According to the man page the '^' is indeed the prefix used to disable a feature. The effect of not using 64bit ext4 is that you'll be limited to ~ 15T volumes. Where as you can have 1EiB volumes if you use the 64Bit flag. HOWEVER, 16T is the recommended max volume size for ext4 anyway.
|
Are application layer protocols part of library routines? | 1,432,207,891,000 |
Where do application layer protocols reside? Are they part of library routines of language e.g. C, C++, Java?
As goldilocks says in his answer, this is about the implementation of application layer protocols.
|
Where do application layer protocols reside?
Protocols are an abstraction, so they don't really "reside" anywhere beyond specifications and other documentation.
If you mean, where are they implemented, there's a few common patterns:
They may be implemented first in native C as libraries which can be wrapped by for use in other languages (since most other languages are themselves implemented in C and have a C interface). E.g., encryption protocols are generally like this.
They may be implemented from scratch as libraries or modules for use in a specific language, using just that language (and/or the language it is implemented in). E.g., high level networking protocols.
They may be implemented from scratch by a given application.
These are all pure userland implementations, but some protocols -- e.g., low level networking -- may be implemented in the kernel. This may include a corresponding native C userland library (as with networking and filesystems) or the kernel (including independent kernel modules) may provide a language agnostic interface via procfs, /dev, etc.
|
Compiling from source: What are the options for config script "build"? | 1,432,207,891,000 |
When I am running a line like:
./configure --build=x86_64-redhat-linux-gnu --host=x86_64-redhat-linux-gnu {*shortened*} \
--with-imap-ssl=/usr/include/openssl/ --enable-ftp --enable-mbstring --enable-zip
I understand what the "x86_64-redhat-linux-gnu" means descriptively, but I have questions?
1) Is there a list somewhere of all the choices? Either in each configure script or on the internet.
2) Does making the answer more specific or more generic have much of an effect on the outcome?
Thank you.
|
The --build and -host options are to configure scripts are standard configure options, and you very rarely need to specify them unless you are doing a cross-build (that is, building a package on one system to run on a different system). The values of these options are called "triples" because they have the form cpu-vendor-os. (Sometimes, as in your case, os is actually kernel-os but it's still called a triple.)
The base configure script is quite capable of deducing the host triple, and you should let it do that unless you have some really good evidence that the results are incorrect. The script which does that is called config.guess, and you'll find it somewhere in the build bundle (it might be in a build-aux subdirectory). If you're doing a cross-build and you need to know the host triple, the first thing to try is to run config-guess on the host system.
The values supplied (or guessed) for --host and --build are passed through another script called config.sub, which will normalize the values. (According to the autoconf docs, if config.sub is not present, you can assume that the build doesn't care about the host triple.) The developers of a specific software package might customize the config.sub script for the particular needs of their build, and there are a lot of different versions of the standard config.sub script, so you shouldn't expect config.sub from one software package to work on another software package, or even on a different version of the same software package.
Despite all the above, autoconf'ed software packages really should not need to know the names of the host os and vendor, except for identifying default filesystem layout so that they provide the correct default file locations.
You can read through config.sub to get an idea of the range of options which will be recognized, but it is not so easy to figure out how the values are used, or even if the values are used. The first field -- the cpu -- is the most likely to be used.
You can get a list of all the options by typing:
./configure --help
or, better,
./configure --help | less
since there are always a lot of options.
Other than the standard options (--build, --host and --target as above, and the options which override file locations), the specific options allowed by each configure script are different. Since they also tend to change from version to version of the software package, you should always check the configure script itself rather than relying on external documentation.
Unfortunately, the contents of the configure script's help are not always 100% complete, because they rely on the package developers to maintain them. Sometimes unusual or developer-only options are not part of the ./configure --help output, but that is usually an indication that the option should not be used in a normal install.
|
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