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: or NUL on CP/M and DOS, nul on newer Windows systems, 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. In IBM DOS/360, OS/360, 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.

History

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

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.