Cksum

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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.^[1] 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 OS (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 not however 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.

References

↑ GNU cksum manual page available with most Unix-like distributions; invoked via man cksum

External links

cksum: write file checksums and sizes – Commands & Utilities Reference, The Single UNIX® Specification, Issue 7 from The Open Group
cksum/ - complete source code, including Makefile, for an implementation written in C

v t e Unix command-line interface programs and shell builtins

File system	cat cd chmod chown chgrp cksum cmp cp dd du df file fsck fuser ln ls mkdir mount mv pax pwd rm rmdir size split tee touch type umask

Processes	at bg chroot cron fg kill killall nice pgrep pidof pkill ps pstree time top

User environment	clear env exit finger history id logname mesg passwd su sudo uptime talk tput uname w wall who whoami write

Text processing	awk banner basename comm csplit cut diff dirname ed ex fmt fold head iconv join less more paste sed sort spell strings tail tr uniq vi wc xargs

Shell builtins	alias echo expr printf sleep test true and false unset wait yes

Networking	dig host ifconfig inetd netcat netstat nslookup ping rdate rlogin route ssh traceroute

Searching	find grep locate whatis whereis

Documentation	apropos help man

Miscellaneous	bc dc cal date lp lpr

List of Unix utilities

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