# 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.

## 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. It first appeared in Version 1 Unix. The version of cmp bundled in GNU coreutils was written by Torbjorn Granlund and David MacKenzie. 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. The cmp command has also been ported to the IBM i operating system.

## 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 for decimal multiples and with the IEC 60027-2 prefixes for binary multiples.

## Return values

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