The /dev directory contains device files (also
sometimes known as device special files and device
nodes) that provide access to peripheral devices such as hard disks, to resources
on peripheral devices such as disk partitions, and pseudo devices such as a random number
generator.
The /dev directory has several subdirectory hierarchies, each of which
holds device files that relate to a certain type of device. For example, the
/dev/disk/id-by-uuid directory contains device files for hard disks named
according to the universally unique identifier (UUID) for the disk. The device files in
subdirectories such as these are actually implemented as symbolic links to device files in
/dev. You can access the same device using the file in
/dev or the corresponding link to the file listed in
/dev/disk/id-by-uuid.
If you use the ls -l command to list the files under
/dev, you see that some device files are shown as being either type
b for block or type c for
character. These devices have a pair of numbers associated with them
instead of a file size. These major and minor
numbers identify the device to the system.
# ls -l /dev
total 0
crw-rw----. 1 root root 10, 56 Mar 17 08:17 autofs
drwxr-xr-x. 2 root root 640 Mar 17 08:17 block
drwxr-xr-x. 2 root root 80 Mar 17 08:16 bsg
drwxr-xr-x. 3 root root 60 Mar 17 08:16 bus
lrwxrwxrwx. 1 root root 3 Mar 17 08:17 cdrom -> sr0
drwxr-xr-x. 2 root root 2880 Mar 17 08:17 char
crw-------. 1 root root 5, 1 Mar 17 08:17 console
lrwxrwxrwx. 1 root root 11 Mar 17 08:17 core -> /proc/kcore
drwxr-xr-x. 4 root root 100 Mar 17 08:17 cpu
crw-rw----. 1 root root 10, 61 Mar 17 08:17 cpu_dma_latency
drwxr-xr-x. 6 root root 120 Mar 17 08:16 disk
brw-rw----. 1 root disk 253, 0 Mar 17 08:17 dm-0
brw-rw----. 1 root disk 253, 1 Mar 17 08:17 dm-1
...
crw-rw-rw-. 1 root root 1, 3 Mar 17 08:17 /dev/null
...
drwxr-xr-x. 2 root root 0 Mar 17 08:16 pts
...
crw-rw-rw-. 1 root root 1, 8 Mar 17 08:17 random
...
brw-rw----. 1 root disk 8, 0 Mar 17 08:17 sda
brw-rw----. 1 root disk 8, 1 Mar 17 08:17 sda1
brw-rw----. 1 root disk 8, 2 Mar 17 08:17 sda2
...
lrwxrwxrwx. 1 root root 15 Mar 17 08:17 stderr -> /proc/self/fd/2
lrwxrwxrwx. 1 root root 15 Mar 17 08:17 stdin -> /proc/self/fd/0
lrwxrwxrwx. 1 root root 15 Mar 17 08:17 stdout -> /proc/self/fd/1
...
crw--w----. 1 root tty 4, 0 Mar 17 08:17 tty0
crw--w----. 1 root tty 4, 1 Mar 17 08:17 tty1
...
crw-rw-rw-. 1 root root 1, 9 Mar 17 08:17 urandom
...
crw-rw-rw-. 1 root root 1, 5 Mar 17 08:17 zeroBlock devices support random access to data, seeking media for data, and usually allow
data to be buffered while it is being written or read. Examples of block devices include hard
disks, CD-ROM drives, flash memory, and other addressable memory devices. The kernel writes
data to or reads data from a block device in blocks of a certain number of bytes. In the
sample output, sda is the block device file that corresponds to the hard
disk, and it has a major number of 8 and a minor number of 0. sda1 and
sda2 are partitions of this disk, and they have the same major number as
sda (8), but their minor numbers are 1 and 2.
Character devices support streaming of data to or from a device, and data is not usually
buffered nor is random access permitted to data on a device. The kernel writes data to or
reads data from a character device one byte at a time. Examples of character devices include
keyboards, mice, terminals, pseudo-terminals, and tape drives. tty0 and
tty1 are character device files that correspond to terminal devices that
allow users to log in from serial terminals or terminal emulators. These files have major
number 4 and minor numbers 0 and 1.
Pseudo-terminals slave devices emulate real terminal devices to interact with software.
For example, a user might log in on a terminal device such as /dev/tty1,
which then uses the pseudo-terminal master device /dev/pts/ptmx to interact
with an underlying pseudo-terminal device. The character device files for pseudo-terminal
slaves and master are located in the /dev/pts directory:
# ls -l /dev/pts
total 0
crw--w----. 1 guest tty 136, 0 Mar 17 10:11 0
crw--w----. 1 guest tty 136, 1 Mar 17 10:53 1
crw--w----. 1 guest tty 136, 2 Mar 17 10:11 2
c---------. 1 root root 5, 2 Mar 17 08:16 ptmxSome device entries, such as stdin for the standard input, are
symbolically linked via the self subdirectory of the
proc file system. The pseudo-terminal device file to which they actually
point depends on the context of the
process.
# ls -l /proc/self/fd/[012] total 0 lrwx------. 1 root root 64 Mar 17 10:02 0 -> /dev/pts/1 lrwx------. 1 root root 64 Mar 17 10:02 1 -> /dev/pts/1 lrwx------. 1 root root 64 Mar 17 10:02 2 -> /dev/pts/1
Character devices such as null, random,
urandom, and zero are examples of pseudo-devices that
provide access to virtual functionality implemented in software rather than to physical
hardware.
/dev/null is a data sink. Data that you write to
/dev/null effectively disappears but the write operation succeeds.
Reading from /dev/null returns EOF (end-of-file).
/dev/zero is a data source of an unlimited number of zero-value
bytes.
/dev/random and /dev/urandom are data sources of
streams of pseudo-random bytes. To maintain high-entropy output,
/dev/random blocks if its entropy pool does not contains sufficient bits
of noise. /dev/urandom does not block and, as a result, the entropy of its
output might not be as consistently high as that of /dev/random. However,
neither /dev/random nor /dev/urandom are considered to
be truly random enough for the purposes of secure cryptography such as military-grade
encryption.
You can find out the size of the entropy pool and the entropy value for
/dev/random from virtual files under
/proc/sys/kernel/random:
#cat /proc/sys/kernel/random/poolsize4096 #cat /proc/sys/kernel/random/entropy_avail3467
For more information, see the null(4), pts(4), and
random(4) manual pages.