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 13-7 and Example 13-6.
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]
/dev/sda1 /mnt/flashdrive auto noauto,user,noatime 0 0 |
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 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.slashdot.org/80 bash$ echo -e "GET / HTTP/1.0\n" >&5 bash$ cat <&5 |
[Thanks, Mark and Mihai Maties.]
Example 28-1. 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.slashdot.org
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 |
| [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 |
| [4] | A socket is a communications node associated with a specific I/O port. 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. |