Contains device driver files, and things related to device drivers like symlinks that give nice names to device drives.
Files are documented under man 4.
To understand what is going on, make a minimal device driver yourself. This is not documented here. Prerequisites you will learn when you do that:
- what do the type, major and minor numbers mean
- you can device arbitrary effects to standard file system calls like
openandcloseon device files. They are usually not represented on disk in any way.
Like proc and sysfs, the dev filesystem is a virtual filesystem that can be mounted any number of times:
mkdir newdev
sudo mount -t devtmpfs none newdev
The actual filesystem name is devtmpfs, and /dev is just the usual mount point.
Most distributions also mount devpts inside /dev in addition to devtmpfs:
man null
Type, major, minor: character, 1, 3
Discards whatever input is given to it by a write syscall.
Very useful to discard undesired stdout / stderr:
echo a > /dev/null
Generates no output.
man zero
Major, minor: 1, 5
Returns as many zeros as asked for by a read syscall.
You cannot cat this because cat reads until the file is over, but this special file is never over.
Application: reset entire partitions to 0.
Example:
dd if=/dev/zero count=1 status=none | od -Ax -tx1
Output:
000000 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
*
000200
Meaning if you don't speak od language (now is a good time to learn): 512 bytes with value 0.
man full
If you write to it, the write always returns ENOSPC error.
Example:
echo a > /dev/full
If you read from it, returns as many null chars as were asked for by read, like /dev/zero.
The kernel implements a random number generator which draws entropy from non-predictable events, typically device events such as mice movements or disk reads for example.
Just like for /dev/zero, it is useless to cat those files, since they don't have and end, and cat tries to read to the end of the file before printing.
man random
Major, minor: 1, 8
/dev/random returns random numbers one by one whenever enough entropy is generated. It is slower than urandom, but has greater entropy.
x86 considered using the RDRAND instruction introduced in 2011 as part of the entropy pool, but this has generated some controversy as it would rely on a process which is not observable and could lead to cryptographic back-doors.
man urandom
Major, minor: 1, 9
/dev/urandom returns random numbers continuously. It is faster than random, but has less entropy. It should however be good enough for most applications.
Example: get 16 random bytes:
dd bs=16 if=/dev/urandom count=1 status=none | od -Ax -tx1
dd bs=16 if=/dev/urandom count=1 status=none | od -Ax -tx1
Sample output:
000000 51 e6 4d 6d 98 5f 3e 48 c9 9a 04 6f d7 f2 57 c6
000010
000000 7d 87 2c a3 32 9d d0 78 18 9d 5f ab c5 7a d2 ea
000010
Since the 16 bytes are random, the lines are extremely likely to be different.
The current terminal. Also works on xterm windows.
Try:
echo a >/dev/tty
a appears on the terminal screen even if stdout was redirected somewhere else, because a was sent directly to the tty.
Application: show some output that is also going to be redirected. E.g.:
echo a | tee /dev/tty | grep a
This both forwards the pipe, and prints it to the terminal.
See also: /proc/self/fd/[0-2]
- https://unix.stackexchange.com/questions/22545/how-to-connect-to-a-serial-port-as-simple-as-using-ssh
- https://stackoverflow.com/questions/6947413/how-to-open-read-and-write-from-serial-port-in-c
Similar to tty, but may only work on actual ttys such as Ubuntu Ctrl + Alt + F2, and not on xterm windows.
Discussion: http://unix.stackexchange.com/questions/60641/linux-difference-between-dev-console-dev-tty-and-dev-tty0
Device files of this type represent block devices such as hard disks or flash memory.
Type, major, minor: block, 8, X, one per partition:
brw-rw---- 1 root disk 8, 0 Aug 23 03:45 /dev/sda
brw-rw---- 1 root disk 8, 1 Aug 23 04:43 /dev/sda1
brw-rw---- 1 root disk 8, 2 Aug 23 03:45 /dev/sda2
brw-rw---- 1 root disk 8, 3 Aug 23 03:45 /dev/sda3
brw-rw---- 1 root disk 8, 4 Aug 23 03:45 /dev/sda4
brw-rw---- 1 root disk 8, 5 Aug 23 03:45 /dev/sda5
brw-rw---- 1 root disk 8, 6 Aug 23 03:45 /dev/sda6
brw-rw---- 1 root disk 8, 7 Aug 23 03:45 /dev/sda7
brw-rw---- 1 root disk 8, 8 Aug 23 03:45 /dev/sda8
The first device is sda, the second sdb, and so on.
Also, partitions inside those devices have device files for them too.
The first primary partition inside sda will be called sda1, the second main partition sda2, and so on.
Logical partitions are numbered from sda5 onwards.
Warning: usage of block devices can be very dangerous and lead to data loss!
Example: copy a block device on /dev/sda/ to the one one /dev/dsb:
#sudo dd bs=4M if=/dev/zero of=/dev/sdb
This could be used to make a full system backup.
Example: write pseudorandom sequences into /dev/sda/ to hide data:
#sudo dd bs=4M if=/dev/urandom of=/dev/sda
You can have some fun with mouses. Search for the mice or mouse device files and cat them:
sudo hd /dev/input/mice
And then:
sudo hd /dev/input/mouse0
Now note that when you move the mouse, cat spits something out to the screen!
mice is the combination of all mice, and each other mouseX is a single mouse device.
This subsystem is called evdev: https://en.wikipedia.org/wiki/Evdev
Files created by shm_open calls. This function is part of POSIX, but creation of the files under /dev is not.
Symlinks to partition identifiers.
Allows you to get identifier info.
If id no present, link not there.
TODO what is the name of the subsystem that implements this? Where is it documented?
Example:
ls -l /dev/disk/
Sample output:
total 0
drwxr-xr-x 2 root root 420 Aug 23 2015 by-id
drwxr-xr-x 2 root root 100 Aug 23 2015 by-label
drwxr-xr-x 2 root root 180 Aug 23 2015 by-uuid
Then:
ls -l /dev/disk/by-id/
Sample output:
total 0
lrwxrwxrwx 1 root root 10 Aug 23 03:45 322246df-fed3-42f7-ba4d-4d72d6cce95f -> ../../sda5
lrwxrwxrwx 1 root root 10 Aug 23 03:45 5CE89967E8994066 -> ../../sda3
lrwxrwxrwx 1 root root 10 Aug 23 03:45 84f8087d-fc74-45b8-a6a2-180856b48982 -> ../../sda6
lrwxrwxrwx 1 root root 10 Aug 23 04:43 A6DA9BF3DA9BBE4D -> ../../sda1
lrwxrwxrwx 1 root root 10 Aug 23 03:45 D0EC9F59EC9F38A4 -> ../../sda2
lrwxrwxrwx 1 root root 10 Aug 23 03:45 e45497f8-6295-41da-ad8c-90dbbac264e8 -> ../../sda8
lrwxrwxrwx 1 root root 10 Aug 23 03:45 ff1b6e50-82ff-4696-823f-774b0f803298 -> ../../sda7
Symlinks to /proc/self/fd/{0,2}.
No portability: http://unix.stackexchange.com/questions/36403/portability-of-dev-stdouit
Bash also implements it as a built-in magic path.
Useful when annoying programs don't follow the usual convention of - for stdout / stdin in the place of files, e.g.: http://www.serverfault.com/questions/329150/is-there-a-way-to-make-objdump-read-from-stdin-instead-of-file
But it does not work all the time, since those programs may not implement - because they rely on seek calls which are not supported on pipes.
man rtc
Real time clock.
TODO example.
man mem
Allows you to read physical memory.
TODO: minimal example.
TODO what is it exactly?
I think this can be used to dump and write to the screen.
Used by fbi to show images to screen, also used by X I guess?
Looks like the X-server is implemented through it!
- https://www.kernel.org/doc/Documentation/fb/api.txt
- https://www.kernel.org/doc/Documentation/fb/framebuffer.txt
- http://stackoverflow.com/questions/4996777/paint-pixels-to-screen-via-linux-framebuffer
- http://superuser.com/questions/495948/can-i-display-a-jpg-or-png-to-the-framebuffer-dev-fb
- http://unix.stackexchange.com/questions/25063/capturing-area-of-the-screen-without-a-desktop-environment
- http://serverfault.com/questions/125524/linux-cli-screenshot-without-x
man fbdev
https://en.wikipedia.org/wiki/Direct_Rendering_Manager
TODO: minimal example.
Supersedes fbdev.