CMP(1) General Commands Manual CMP(1)

cmp — compare two files

cmp [-l | -s | -x] [-bhz] [--ignore-initial=num1[:num2]] [--bytes=num] file1 file2 [skip1 [skip2]]

The cmp utility 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.

Bytes and lines are numbered beginning with one.

The following options are available:

, --print-bytes
Print each byte when a difference is found.
Do not follow symbolic links.
num1[:num2], --ignore-initial=num1[:num2]
Skip num1 bytes from file1, and optionally skip num2 bytes from file2. If num2 is not specified, then num1 is applied for both file1 and file2.
, --verbose
Print the byte number (decimal) and the differing byte values (octal) for each difference.
num, --bytes=num
Only compare up to num bytes.
, --silent, --quiet
Print nothing for differing files; return exit status only.
Like -l but prints in hexadecimal and using zero as index for the first byte in the files.
For regular files compare file sizes first, and fail the comparison if they are not equal.

The optional arguments skip1 and skip2 are the byte offsets from the beginning of file1 and file2, respectively, where the comparison will begin. The offset is decimal by default, but may be expressed as a hexadecimal or octal value by preceding it with a leading ``0x'' or ``0''.

skip1 and skip2 may also be specified with SI size suffixes.

The cmp utility exits with one of the following values:

0
The files are identical.
1
The files are different; this includes the case where one file is identical to the first part of the other. In the latter case, if the -s option has not been specified, cmp writes to standard error that EOF was reached in the shorter file (before any differences were found).
>1
An error occurred.

Assuming a file named example.txt with the following contents:

a
b
c

Compare stdin with example.txt:

$ echo -e "a\nb\nc" | cmp - example.txt

Same as above but introducing a change in the third byte of stdin. Show the byte number (decimal) and differing byte (octal):

$ echo -e "a\nR\nc" | cmp -l - example.txt
     3 122 142

Compare file sizes of example.txt and /boot/loader.conf and return 1 if they are not equal. Note that -z can only be used with regular files:

$ cmp -z example.txt /boot/loader.conf
example.txt /boot/loader.conf differ: size

Compare stdin with example.txt omitting the first 4 bytes from stdin and the first 2 bytes from example.txt:

$ echo -e "a\nR\nb\nc" | cmp - example.txt 4 2

diff(1), diff3(1)

The cmp utility is expected to be IEEE Std 1003.2 (“POSIX.2”) compatible. The -b, -h, -i, -n, -x, and -z options are extensions to the standard. skip1 and skip2 arguments are extensions to the standard.

A cmp command appeared in Version 1 AT&T UNIX.

The phrase “SI size suffixes” above refers to the traditional power of two convention, as described in expand_number(3).

September 23, 2021 macOS 15.2