29.1. /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 17-8 and Example 17-7.

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-23.)

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.

A socket is a communications node associated with a specific I/O port. (This is analogous to a hardware socket, or receptacle, for a connecting cable.) It permits data transfer between hardware devices on the same machine, between machines on the same network, between machines across different networks, and, of course, between machines at different locations on the Internet.

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 this example.]

Generalizing the above into a script:

#!/bin/bash
# This script must run with root permissions.

URL="time.nist.gov/13"

Time=$(cat </dev/tcp/"$URL")
UTC=$(echo "$Time" | awk '{print$3}')   # Third field is UTC (GMT) time.
# Exercise: modify this for different time zones.

echo "UTC Time = "$UTC""

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 29-1. 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=news-15.net       # 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 29-2. 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.