Null device
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Template:Special device files
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 and Unix-like systems, NUL: or NUL on CP/M and DOS (internally \DEV\NUL), nul on newer Windows systems (internally \Device\Null on Windows NT), NIL: on Amiga operating systems, and NL: on OpenVMS. In Windows Powershell, the equivalent is $null. It provides no data to any process that reads from it, yielding EOF immediately.[1] 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[2] or black hole.
History
According to the Berkeley UNIX man page, Version 4 Unix, which AT&T released in 1973, included a null device.[3][4]
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.
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.[5] The fictitious person name "Dave (or Devin) Null" is sometimes similarly used (e.g., "send complaints to Dave Null").[6] In 1996, Dev Null was an animated virtual reality character created by Leo Laporte for MSNBC's computer and technology TV series The Site. Dev/null is also the name of a vampire hacker in the computer game Vampire: The Masquerade – Redemption. A 2002 advertisement for the Titanium PowerBook G4 reads The Titanium Powerbook G4 Sends other UNIX boxes to /dev/null.[7]
The null device is also a favorite subject of technical jokes,[8] 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
References
- ^ "Single Unix Specification Section 10.1: Directory Structure and Files". The Open Group. Retrieved 2012-11-29.
- ^ "bit bucket". Jargon File. Retrieved 2013-12-27.
- ^ "null(4)". The NetBSD Project.
- ^ "sh(1) manual from version 4 Unix".
- ^ "Dev-Team Blog - Donations to /dev/null". 2015-09-07. Archived from the original on 2015-09-07. Retrieved 2018-11-15.
- ^ Goodman, Danny (2004). Spam wars : our last best chance to defeat spammers, scammers, and hackers. New York: SelectBooks. p. 170. ISBN 9781590790632. OCLC 1036874851.
- ^ "Image: unixad.jpg, (1094 × 720 px)". Archived from the original on March 26, 2013. Retrieved 2015-09-02.
- ^ "The FreeBSD Funnies". Freebsd.org. Retrieved 2012-11-28.
