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4.5.8 Device files

Device files refer to physical or virtual devices on your system, such as your hard disk, video card, screen, or keyboard. An example of a virtual device is the console, represented by /dev/console.

There are two types of devices:

• character device

  • This can be accessed one character at a time, that is, the smallest unit of data which can be written to or read from the device is a character (byte).

• block device

  • This must be accessed in larger units called blocks, which contain a number of characters. Your hard disk is a block device.

You can read and write device files, though the file may well contain binary data which may be an incomprehensible-to-humans gibberish. Writing data directly to these files is sometimes useful for the troubleshooting of hardware connections. For example, you can dump a text file to the printer device /dev/lp0 or send modem commands to the appropriate serial port /dev/ttyS0. But, unless this is done carefully, it may cause a major disaster. So be cautious.

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4.5.8.1 /dev/null etc.

/dev/null is a special device file that discards anything you write to it. If you don't want something, throw it in /dev/null. It's essentially a bottomless pit. If you read /dev/null, you'll get an end-of-file (EOF) character immediately.

/dev/zero is similar, only if you read from it you get the \0 character (not the same as the number zero ASCII). See Dummy files, Section 8.6.34.

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4.5.8.2 Device node number

The device node number are displayed by executing ls as:

     $ ls -l /dev/hda /dev/ttyS0 /dev/zero
     brw-rw----    1 root     disk       3,   0 Mar 14  2002 /dev/hda
     crw-rw----    1 root     dialout    4,  64 Nov 15 09:51 /dev/ttyS0
     crw-rw-rw-    1 root     root       1,   5 Aug 31 03:03 /dev/zero

Here,

• /dev/hda has the major device number 3 and the minor device number 0. This is read/write accessible by the user who belongs to disk group,

• /dev/ttyS0 has the major device number 4 and the minor device number 64. This is read/write accessible by the user who belongs to dialout group, and

• /dev/zero has the major device number 1 and the minor device number 5. This is read/write accessible by anyone.

In the older system, the installation process creates the device nodes using /sbin/MAKEDEV command. See MAKEDEV(8).

In the newer system, the filesystem under in the /dev is automatically populated by the device filesystem similar to the /proc filesystem.

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Debian GNU/Linux Reference Guide
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