cmp (Unix)

From Wikipedia, the free encyclopedia
Original author(s)Dennis Ritchie
(AT&T Bell Laboratories)
Developer(s)Various open-source and commercial developers
Initial releaseNovember 3, 1971; 52 years ago (1971-11-03)
Written inPlan 9: C
Operating systemUnix, Unix-like, Plan 9, Inferno, OS-9, IBM i
Licensecoreutils: GPLv3+
Plan 9: MIT License

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 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]


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
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,
Skip the first SKIP bytes of input. No No No No Yes No
Skip the first SKIP1 bytes of FILE1 and the first SKIP2 bytes of FILE2. No No No No Yes No
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
Compare at most LIMIT bytes. No No No No Yes No
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
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]






> cmp  0 00

0 00 differ: char 21, line 2

> cmp -l 0 00 # note: byte location is in decimal, contents is in octal
 21  64  60
 22  64  60
 23  64  60

Return values[edit]

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

See also[edit]


  1. ^ Paul S. Dayan (1992). The OS-9 Guru - 1 : The Facts. Galactic Industrial Limited. ISBN 0-9519228-0-7.
  2. ^ printf – Shell and Utilities Reference, The Single UNIX Specification, Version 4 from The Open Group
  3. ^ cmp(1) – FreeBSD General Commands Manual
  4. ^ "cmp(1): compare two files byte by byte - Linux man page".
  5. ^ "Native Win32 ports of some GNU utilities".
  6. ^ IBM. "IBM System i Version 7.2 Programming Qshell" (PDF). IBM. Retrieved 2020-09-05.
  7. ^ "Welcome - BIPM".
  8. ^ "Definitions of the SI units: The binary prefixes". Retrieved 21 April 2018.

External links[edit]