Chapter 27. /dev and /proc

A Linux or UNIX machine typically has the /dev and /proc special-purpose directories.

27.1. /dev

The /dev directory contains entries for the physical devices that may or may not be present in the hardware. [1] The hard drive partitions containing the mounted filesystem(s) have entries in /dev, as a simple 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 also 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 enables 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, /dev/udp, and /dev/tcp.

For instance:

To 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-24.)

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. [4]

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

   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=www.dns-diy.com   # 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

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 a hard drive and CD ROM drive. An example of a character device is a keyboard.

[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.

[4]

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.

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