Cmp (Unix)

In computing, cmp is a command-line utility on Unix and Unix-like operating systems that compares two files of any type and writes the results to the standard output. By default, cmp is silent if the files are the same; if they differ, the byte and line number at which the first difference occurred is reported. The command is also available in the OS-9 shell.^[1]

cmp

Original author(s)	Dennis Ritchie (AT&T Bell Laboratories)
Developer(s)	Various open-source and commercial developers
Initial release	November 3, 1971; 53 years ago (1971-11-03)
Written in	Plan 9: C
Operating system	Unix, Unix-like, Plan 9, Inferno, OS-9, IBM i
Type	Command
License	coreutils: GPLv3+ Plan 9: MIT License

History

cmp is part of the X/Open Portability Guide since issue 2 of 1987. It was inherited into the first version of POSIX.1 and the Single Unix Specification.^[2] It first appeared in Version 1 Unix.^[3] The version of cmp bundled in GNU coreutils was written by Torbjorn Granlund and David MacKenzie.^[4] 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.^[5] The cmp command has also been ported to the IBM i operating system.^[6]

Switches

cmp may be qualified by the use of command-line switches. The switches supported by notable implementations of cmp are:

Name	Description	Unix	Plan 9	Inferno	FreeBSD	Linux	IBM i
-b, --print-bytes	Print the differing bytes. Display control bytes as a '^' followed by a letter of the alphabet and precede bytes that have the high bit set with 'M-' (which stands for "meta").	No	No	No	No	Yes	No
-h	Do not follow symbolic links.	No	No	No	Yes	No	No
-i SKIP, --ignore-initial=SKIP	Skip the first SKIP bytes of input.	No	No	No	No	Yes	No
-i SKIP1:SKIP2, --ignore-initial=SKIP1:SKIP2	Skip the first SKIP1 bytes of FILE1 and the first SKIP2 bytes of FILE2.	No	No	No	No	Yes	No
-l, --verbose	Output the (decimal) byte numbers and (octal) values of all differing bytes, instead of the default standard output. Also, output the EOF message if one file is shorter than the other.	Yes	Yes	Yes	Yes	Yes	Yes
-L	Print the line number of the first differing byte.	Yes	Yes	Yes	No	No	No
-n LIMIT, --bytes=LIMIT	Compare at most LIMIT bytes.	No	No	No	No	Yes	No
-s, --quiet, --silent	Output nothing; yield exit status only.	Yes	Yes	Yes	Yes	Yes	Yes
-t	Text mode where the files are opened in text mode and translated to the CCSID of the job before comparing byte for byte.	No	No	No	No	No	Yes
-v, --version	Output version info.	No	No	No	No	Yes	No
-x	Like -l but prints in hexadecimal and using zero as index for the first byte in the files.	No	No	No	Yes	No	No
-z	For regular files compare file sizes first, and fail the comparison if they are not equal.	No	No	No	Yes	No	No
--help	Outputs a help file.	No	No	No	No	Yes	No

Operands that are byte counts are normally decimal, but may be preceded by '0' for octal and '0x' for hexadecimal.

A byte count can be followed by a suffix to specify a multiple of that count; in this case an omitted integer is understood to be 1. A bare size letter, or one followed by 'iB', specifies a multiple using powers of 1024. A size letter followed by 'B' specifies powers of 1000 instead. For example, '-n 4M' and '-n 4MiB' are equivalent to '-n 4194304', whereas '-n 4MB' is equivalent to '-n 4000000'. This notation is upward compatible with the SI prefixes^[7] for decimal multiples and with the IEC 60027-2 prefixes for binary multiples.^[8]

Example

Example usage of cmp to find different bytes between 2 txt files

Return values

• 0 – files are identical
• 1 – files differ
• 2 – inaccessible or missing argument

See also

• Comparison of file comparison tools
• List of Unix commands