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 (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. 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.