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]

1 /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):

1 head -1 /dev/hdc 2  3  4 # head: cannot open '/dev/hdc' for reading: No medium found 5 # (No disc in the drive.) 6  7 # head: error reading '/dev/hdc': Input/output error 8 # (There is a disk in the drive, but it can't be read; 9 #+ possibly it's an unrecorded CDR blank.) 10  11 # Stream of characters and assorted gibberish 12 # (There is a pre-recorded disk in the drive, 13 #+ and this is raw output -- a stream of ASCII and binary data.) 14 # Here we see the wisdom of using 'head' to limit the output 15 #+ to manageable proportions, rather than 'cat' or something similar. 16  17  18 # Now, it's just a matter of checking/parsing the output and taking 19 #+ 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 this example.]

Generalizing the above into a script:

1 #!/bin/bash 2 # This script must run with root permissions. 3  4 URL="time.nist.gov/13" 5  6 Time=$(cat </dev/tcp/"$URL") 7 UTC=$(echo "$Time" | awk '{print$3}') # Third field is UTC (GMT) time. 8 # Exercise: modify this for different time zones. 9  10 echo "UTC Time = "$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.]

   1 #!/bin/bash
   2 # dev-tcp.sh: /dev/tcp redirection to check Internet connection.
   3 
   4 # Script by Troy Engel.
   5 # Used with permission.
   6  
   7 TCP_HOST=news-15.net       # A known spam-friendly ISP.
   8 TCP_PORT=80                # Port 80 is http.
   9   
  10 # Try to connect. (Somewhat similar to a 'ping' . . .) 
  11 echo "HEAD / HTTP/1.0" >/dev/tcp/${TCP_HOST}/${TCP_PORT}
  12 MYEXIT=$?
  13 
  14 : <<EXPLANATION
  15 If bash was compiled with --enable-net-redirections, it has the capability of
  16 using a special character device for both TCP and UDP redirections. These
  17 redirections are used identically as STDIN/STDOUT/STDERR. The device entries
  18 are 30,36 for /dev/tcp:
  19 
  20   mknod /dev/tcp c 30 36
  21 
  22 >From the bash reference:
  23 /dev/tcp/host/port
  24     If host is a valid hostname or Internet address, and port is an integer
  25 port number or service name, Bash attempts to open a TCP connection to the
  26 corresponding socket.
  27 EXPLANATION
  28 
  29    
  30 if [ "X$MYEXIT" = "X0" ]; then
  31   echo "Connection successful. Exit code: $MYEXIT"
  32 else
  33   echo "Connection unsuccessful. Exit code: $MYEXIT"
  34 fi
  35 
  36 exit $MYEXIT

   1 #!/bin/bash
   2 # music.sh
   3 
   4 # Music without external files
   5 
   6 # Author: Antonio Macchi
   7 # Used in ABS Guide with permission.
   8 
   9 
  10 #  /dev/dsp default = 8000 frames per second, 8 bits per frame (1 byte),
  11 #+ 1 channel (mono)
  12 
  13 duration=2000       # If 8000 bytes = 1 second, then 2000 = 1/4 second.
  14 volume=$'\xc0'      # Max volume = \xff (or \x00).
  15 mute=$'\x80'        # No volume = \x80 (the middle).
  16 
  17 function mknote ()  # $1=Note Hz in bytes (e.g. A = 440Hz ::
  18 {                   #+ 8000 fps / 440 = 16 :: A = 16 bytes per second)
  19   for t in `seq 0 $duration`
  20   do
  21     test $(( $t % $1 )) = 0 && echo -n $volume || echo -n $mute
  22   done
  23 }
  24 
  25 e=`mknote 49`
  26 g=`mknote 41`
  27 a=`mknote 36`
  28 b=`mknote 32`
  29 c=`mknote 30`
  30 cis=`mknote 29`
  31 d=`mknote 27`
  32 e2=`mknote 24`
  33 n=`mknote 32767`
  34 # European notation.
  35 
  36 echo -n "$g$e2$d$c$d$c$a$g$n$g$e$n$g$e2$d$c$c$b$c$cis$n$cis$d \
  37 $n$g$e2$d$c$d$c$a$g$n$g$e$n$g$a$d$c$b$a$b$c" > /dev/dsp
  38 # dsp = Digital Signal Processor
  39 
  40 exit      # A "bonny" example of an elegant shell script!