/dev

Advanced Bash-Scripting Guide:
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# 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.

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 an elegant 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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