Null device

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: (see TOPS-20) or NUL on CP/M and DOS (internally \DEV\NUL), nul on OS/2 and newer Windows systems[1] (internally \Device\Null on Windows NT), NIL: on Amiga operating systems,[2] and NL: on OpenVMS.[3] In Windows Powershell, the equivalent is $null.[4] It provides no data to any process that reads from it, yielding EOF immediately.[5] In IBM operating systems DOS/360 and successors[lower-alpha 1] and also in OS/360 and successors[lower-alpha 2] such files would be assigned in JCL to DD DUMMY.

For the electropop band, see Null Device.
Not to be confused with /dev/zero.

In programmer jargon, especially Unix jargon, it may also be called the bit bucket[6] or black hole.

History

According to the Berkeley UNIX man page, Version 4 Unix, which AT&T released in 1973, included a null device.[7][8]

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.[9] The fictitious person name "Dave (or Devin) Null" is sometimes similarly used (e.g., "send complaints to Dave Null").[10] 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.[11]

The null device is also a favorite subject of technical jokes,[12] such as warning users that the system's /dev/null is already 98% full. The 1995 April Fool's 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

Notes
  1. The most recent being z/VSE.
  2. The most recent being z/OS.

References
  1. "Redirecting Error Messages from Command Prompt: STDERR/STDOUT". support.microsoft.com. Retrieved 2020-03-06.
  2. Commodore-Amiga, Inc. (1986). The AmigaDOS Manual. Bantam Books. p. 12. ISBN 0-553-34294-0.
  3. "OpenVMS Programming Concepts Manual". h30266.www3.hpe.com. Archived from the original on 2020-07-02. Retrieved 2020-03-06.
  4. SteveL-MSFT. "about_Automatic_Variables - PowerShell". docs.microsoft.com. Retrieved 2020-03-06.
  5. "Single Unix Specification Section 10.1: Directory Structure and Files". The Open Group. Retrieved 2012-11-29.
  6. "bit bucket". Jargon File. Retrieved 2013-12-27.
  7. "null(4)". The NetBSD Project.
  8. "sh(1) manual from version 4 Unix".
  9. "Dev-Team Blog - Donations to /dev/null". 2015-09-07. Archived from the original on 2015-09-07. Retrieved 2018-11-15.
  10. Goodman, Danny (2004). Spam Wars: Our Last Best Chance to Defeat Spammers, Scammers, and Hackers. New York: SelectBooks. p. 170. ISBN 9781590790632. OCLC 1036874851.
  11. "Image: unixad.jpg, (1094 × 720 px)". Archived from the original on 2013-03-26. Retrieved 2015-09-02.
  12. "The FreeBSD Funnies". Freebsd.org. Retrieved 2012-11-28.
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