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 OS/2 and 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 operating systems DOS/360 and successors and also in OS/360 and successors 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

is described as an empty regular file in Version 4 Unix.
The Version 5 Unix manual describes a device with modern semantics.

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. For example, tar -c -f /dev/null "example directory" can be used to dry-run the TAR file archiving utility to see if any errors would occur but without writing any file.
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.
cat /dev/null may be replaced with :