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27.1. /dev

The /dev directory contains entries for the physical devices that may or may not be present in the hardware. [1] Appropriately enough, these are called device files. As an 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 manually 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.


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