27.1. /dev

The /dev directory contains entries for the physical devices that may or may not be present in the hardware. [1] For example, the hard drive partitions containing the mounted filesystem(s) have entries in /dev, as df shows.

bash$ df Filesystem 1k-blocks Used Available Use% Mounted on /dev/hda6 495876 222748 247527 48% / /dev/hda1 50755 3887 44248 9% /boot /dev/hda8 367013 13262 334803 4% /home /dev/hda5 1714416 1123624 503704 70% /usr 

Among other things, the /dev directory contains loopback devices, such as /dev/loop0. A loopback device is a gimmick that allows an ordinary file to be accessed as if it were a block device. [2] This permits mounting an entire filesystem within a single large file. See Example 16-8 and Example 16-7.

A few of the pseudo-devices in /dev have other specialized uses, such as /dev/null, /dev/zero, /dev/urandom, /dev/sda1 (hard drive partition), /dev/udp (User Datagram Packet port), and /dev/tcp.

For instance:

To mount a USB flash drive, append the following line to /etc/fstab. [3]
/dev/sda1 /mnt/flashdrive auto noauto,user,noatime 0 0
(See also Example A-25.)

Checking whether a disk is in the CD-burner (soft-linked to /dev/hdc):
head -1 /dev/hdc # head: cannot open '/dev/hdc' for reading: No medium found # (No disc in the drive.) # head: error reading '/dev/hdc': Input/output error # (There is a disk in the drive, but it can't be read; #+ possibly it's an unrecorded CDR blank.) # Stream of characters and assorted gibberish # (There is a pre-recorded disk in the drive, #+ and this is raw output -- a stream of ASCII and binary data.) # Here we see the wisdom of using 'head' to limit the output #+ to manageable proportions, rather than 'cat' or something similar. # Now, it's just a matter of checking/parsing the output and taking #+ appropriate action.

When executing a command on a /dev/tcp/$host/$port pseudo-device file, Bash opens a TCP connection to the associated socket.

The following examples assume an active Internet connection.

Getting the time from nist.gov:

bash$ cat </dev/tcp/time.nist.gov/13 53082 04-03-18 04:26:54 68 0 0 502.3 UTC(NIST) * 

[Mark contributed the above example.]

Downloading a URL:

bash$ exec 5<>/dev/tcp/www.net.cn/80 bash$ echo -e "GET / HTTP/1.0\n" >&5 bash$ cat <&5 

[Thanks, Mark and Mihai Maties.]

Example 27-1. Using /dev/tcp for troubleshooting

#!/bin/bash # dev-tcp.sh: /dev/tcp redirection to check Internet connection. # Script by Troy Engel. # Used with permission. TCP_HOST=www.dns-diy.com # A known spam-friendly ISP. TCP_PORT=80 # Port 80 is http. # Try to connect. (Somewhat similar to a 'ping' . . .) echo "HEAD / HTTP/1.0" >/dev/tcp/${TCP_HOST}/${TCP_PORT} MYEXIT=$? : <<EXPLANATION If bash was compiled with --enable-net-redirections, it has the capability of using a special character device for both TCP and UDP redirections. These redirections are used identically as STDIN/STDOUT/STDERR. The device entries are 30,36 for /dev/tcp: mknod /dev/tcp c 30 36 >From the bash reference: /dev/tcp/host/port If host is a valid hostname or Internet address, and port is an integer port number or service name, Bash attempts to open a TCP connection to the corresponding socket. EXPLANATION if [ "X$MYEXIT" = "X0" ]; then echo "Connection successful. Exit code: $MYEXIT" else echo "Connection unsuccessful. Exit code: $MYEXIT" fi exit $MYEXIT

Example 27-2. Playing music

#!/bin/bash # music.sh # MUSIC WITHOUT EXTERNAL FILES # Author: Antonio Macchi # Used in ABS Guide with permission # /dev/dsp default = 8000 frames per second, 8 bits per frame (1 byte), #+ 1 channel (mono) duration=2000 # If 8000 bytes = 1 second, then 2000 = 1/4 second. volume=$'\xc0' # Max volume = \xff (or \x00). mute=$'\x80' # No volume = \x80 (the middle). function mknote () # $1=Note Hz in bytes (e.g. A = 440Hz :: #+ 8000 fps / 440 = 16 :: A = 16 bytes per second) { for t in `seq 0 $duration` do test $(( $t % $1 )) = 0 && echo -n $volume || echo -n $mute done } e=`mknote 49` g=`mknote 41` a=`mknote 36` b=`mknote 32` c=`mknote 30` cis=`mknote 29` d=`mknote 27` e2=`mknote 24` n=`mknote 32767` # European notation. echo -n "$g$e2$d$c$d$c$a$g$n$g$e$n$g$e2$d$c$c$b$c$cis$n$cis$d \ $n$g$e2$d$c$d$c$a$g$n$g$e$n$g$a$d$c$b$a$b$c" > /dev/dsp # dsp = Digital Signal Processor exit $? # A "bonny" example of a shell script!

Notes

[1]

The entries in /dev provide mount points for physical and virtual devices. These entries use very little drive space.

Some devices, such as /dev/null, /dev/zero, and /dev/urandom are virtual. They are not actual physical devices and exist only in software.

[2]

A block device reads and/or writes data in chunks, or blocks, in contrast to a character device, which acesses data in character units. Examples of block devices are hard drives, CDROM drives, and flash drives. Examples of character devices are keyboards, modems, sound cards.

[3]

Of course, the mount point /mnt/flashdrive must exist. If not, then, as root, mkdir /mnt/flashdrive.

To actually mount the drive, use the following command: mount /mnt/flashdrive

Newer Linux distros automount flash drives in the /media directory without user intervention.