cksum

cksum
Example usage of `cksum` to calculate CRC checksum and byte counts of a file
Operating system	Unix, Unix-like
Platform	Cross-platform
Type	Command

cksum is a command in Unix and 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 32-bit cyclic redundancy check (CRC) checksum and byte count.^[1] The CRC output by cksum is different from the CRC-32 used in zip, PNG and zlib.^[2]

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 sha256sum.

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.^[1]

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Interoperability

The standard cksum command, as found on most Unix and Unix-like operating systems (including Linux, *BSD,^[4]^[5]^[6] macOS, and Solaris^[7]) uses a CRC algorithm based on the ethernet standard frame check^[8] and is therefore interoperable between implementations. This is in contrast to the sum command, which is not as interoperable and not compatible with the CRC-32 calculation. On Tru64 operating systems, the cksum command returns a different CRC value, unless the environment variable CMD_ENV is set to xpg4.^{[citation needed]}

Algorithm

cksum uses the generator polynomial 0x04C11DB7 and appends to the message its length in little endian representation. That length has null bytes trimmed on the right end.^[8]

Syntax

cksum [FILE]...
cksum [OPTION]

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

^ ^a ^b ^c "cksum: Print CRC checksum and byte counts". gnu.org. Retrieved 2015-07-05.
^ "Catalogue of parametrised CRC algorithms". reveng.sourceforge.io.
^ "Native Win32 ports of some GNU utilities". unxutils.sourceforge.net.
^ "cksum(1)". www.freebsd.org. April 28, 1995. Retrieved 2023-01-07.
^ "cksum(1) - NetBSD Manual Pages". man.netbsd.org. December 14, 2019. Retrieved 2023-01-07.
^ "cksum(1) - OpenBSD manual pages". man.openbsd.org. August 4, 2022. Retrieved 2023-01-07.
^ "cksum - man pages section 1: User Commands". docs.oracle.com. Retrieved 2023-01-07.
^ ^a ^b "cksum". pubs.opengroup.org.

External links

The Wikibook Guide to Unix has a page on the topic of: Commands

cksum – Shell and Utilities Reference, The Single UNIX Specification, Version 4 from The Open Group
cksum – GNU Core Utilities
cksum(1) – FreeBSD General Commands Manual
cksum(1) – Linux User Manual – User Commands

v t e Unix command-line interface programs and shell builtins
File system	cat chattr chmod chown chgrp cksum cmp cp dd du df file fuser ln ls mkdir mv pax pwd rm rmdir split tee touch type umask
Processes	at bg crontab fg kill nice ps time
User environment	env exit logname mesg talk tput uname who write
Text processing	awk basename comm csplit cut diff dirname ed ex fold head iconv join m4 more nl paste patch printf read sed sort strings tail tr troff uniq vi wc xargs
Shell builtins	alias cd echo test unset wait
Searching	find grep
Documentation	man
Software development	ar ctags lex make nm strip yacc
Miscellaneous	bc cal expr lp od sleep true and false
Categories Standard Unix programs Unix SUS2008 utilities List

v t e GNU Core Utilities command-line interface programs
File system	chcon chmod chown chgrp cksum cp dd df dir dircolors install ln ls mkdir mkfifo mknod mktemp mv realpath rm rmdir shred sync touch truncate vdir
Text utilities	b2sum base32 base64 cat cksum comm csplit cut expand fmt fold head join md5sum nl numfmt od paste ptx pr sha1sum shuf sort split sum tac tail tr tsort unexpand uniq wc
Shell utilities	arch basename chroot date dirname du echo env expr factor false groups hostid id link logname nice nohup nproc pathchk pinky printenv printf pwd readlink runcon seq sleep stat stdbuf stty tee test timeout true tty uname unlink uptime users who whoami yes

This page was last edited on 26 February 2024, at 00:16

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