cksum is a command in Unix and Unix-like operating systems that generates a checksum value for a file or stream of data.

The command is available as a separate package for Microsoft Windows as part of the UnxUtils collection of native Win32 ports of common GNU Unix-like utilities.

[3] Latest GNU Coreutils cksum provides additional checksum algorithms via -a option, as an extension beyond POSIX.

On Tru64 operating systems, the cksum command returns a different CRC value, unless the environment variable CMD_ENV is set to xpg4.

[citation needed] cksum uses the generator polynomial 0x04C11DB7 and appends to the message its length in little endian representation.