Null device

For the electropop band, see Null Device.

Special device files
/dev/full /dev/null /dev/random and /dev/urandom /dev/zero

In some operating systems, the null device is a device file that discards all data written to it but reports that the write operation succeeded. This device is called /dev/null on Unix or Unix-like systems, NUL: or NUL on DOS and CP/M, \Device\Null on Windows NT, nul on newer Windows systems, NIL: on Amiga operating systems, and the NL: on OpenVMS. In Windows Powershell, the equivalent is $null. It provides no data to any process that reads from it, yielding EOF immediately. In IBM DOS/360, OS/360 (MFT, MVT), OS/390 and z/OS operating systems, such files would be assigned in JCL to DD DUMMY.

In programmer jargon, especially Unix jargon, it may also be called the bit bucket or black hole.

Usage

The null device is typically used for disposing of unwanted output streams of a process, or as a convenient empty file for input streams. This is usually done by redirection.

The /dev/null device is a special file, not a directory, so one cannot move a whole file or directory into it with the Unix mv command. The rm command is the proper way to delete files in Unix.

References in computer culture

This entity is a common inspiration for technical jargon expressions and metaphors by Unix programmers, e.g. "please send complaints to /dev/null", "my mail got archived in /dev/null", and "redirect to /dev/null"—being jocular ways of saying, respectively: "don't bother sending complaints", "my mail was deleted", and "go away". The iPhone Dev Team commonly uses the phrase "send donations to /dev/null", meaning they do not accept donations. The fictitious person name "Dave (or Devin) Null" is sometimes similarly used (e.g., "send complaints to Dave Null"). In 1996, Dev Null was an animated virtual reality character created by Leo Laporte for MSNBC's computer and technology TV series, The Site. A 2002 advertisement for the Titanium PowerBook G4 reads [The Titanium Powerbook G4] Sends other UNIX boxes to /dev/null.

The null device is also a favorite subject of technical jokes, such as warning users that the system's /dev/null is already 98% full. The April Fool's, 1995 issue of the German magazine c't reported on an enhanced /dev/null chip that would efficiently dispose of the incoming data by converting it to a flicker on an internal glowing LED.

See also

• Filesystem Hierarchy Standard
• Memory hole
• rm (Unix)
• Standard streams
• Unix philosophy
• Write-only memory