cksum

cksum is a command in Unix-like operating systems that generates a checksum value for a file or stream of data. The cksum command reads each file given in its arguments, or standard input if no arguments are provided, and outputs the file's CRC checksum and byte count.

The cksum command can be used to verify that files transferred by unreliable means arrived intact. However, the CRC checksum calculated by the cksum command is not cryptographically secure: While it guards against accidental corruption (it is unlikely that the corrupted data will have the same checksum as the intended data), it is not difficult for an attacker to deliberately corrupt the file in a specific way that its checksum is unchanged. Unix-like systems typically include other commands for cryptographically secure checksums, such as sha1sum.

Interoperability

The standard cksum command, as found on most UNIX-like operating systems (including GNU/Linux, *BSD, Mac OS X, and Solaris) uses a CRC algorithm based on the ethernet standard frame check and is therefore interoperable between implementations. It is however not compatible with the CRC-32 calculation. This is in contrast to the sum command, which is not as interoperable. On Tru64 operating systems, the cksum command returns a different CRC value, unless the environment variable CMD_ENV is set to “xpg4”.

Syntax

cksum [ File ... ]

Usage example

$ cksum test.txt
4038471504 75 test.txt

where "4038471504" represents the checksum value and "75" represents the file size of test.txt.

See also

• sum command
• Cyclic redundancy check
• GNU Core Utilities
• md5sum
• sha1sum
• Comparison of file verification software

External links

• cksum: write file checksums and sizes – Commands & Utilities Reference, The Single UNIX® Specification, Issue 7 from The Open Group