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12 Invoking cmp

The cmp command compares two files, and if they differ, tells the first byte and line number where they differ or reports that one file is a prefix of the other. Bytes and lines are numbered starting with 1. The arguments of cmp are as follows:

cmp optionsfrom-file [to-file [from-skip [to-skip]]]

The file name - is always the standard input. cmp also uses the standard input if one file name is omitted. The from-skip and to-skip operands specify how many bytes to ignore at the start of each file; they are equivalent to the --ignore-initial=from-skip:to-skip option.

By default, cmp outputs nothing if the two files have the same contents. If the two files have bytes that differ, cmp reports the location of the first difference to standard output:

from-file to-file differ: char byte-number, line line-number

If one file is a prefix of the other, cmp reports the shorter file’s name to standard error, followed by a blank and extra information about the shorter file:

cmp: EOF on shorter-file extra-info

The message formats can differ outside the POSIX locale. POSIX allows but does not require the EOF diagnostic’s file name to be followed by a blank and additional information.

An exit status of 0 means no differences were found, 1 means some differences were found, and 2 means trouble.

12.1 Options to cmp

Below is a summary of all of the options that GNU cmp accepts. Most options have two equivalent names, one of which is a single letter preceded by ‘-’, and the other of which is a long name preceded by ‘--’. Multiple single letter options (unless they take an argument) can be combined into a single command line word: -bl is equivalent to -b -l.


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”).


Output a summary of usage and then exit.

-i skip

Ignore any differences in the first skip bytes of the input files. Treat files with fewer than skip bytes as if they are empty. If skip is of the form from-skip:to-skip, skip the first from-skip bytes of the first input file and the first to-skip bytes of the second.


Output the (decimal) byte numbers and (octal) values of all differing bytes, instead of the default standard output. Each output line contains a differing byte’s number relative to the start of the input, followed by the differing byte values. Byte numbers start at 1. Also, output the EOF message if one file is shorter than the other.

-n count

Compare at most count input bytes.


Do not print anything; only return an exit status indicating whether the files differ.


Output version information and then exit.

In the above table, 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.

The following suffixes are defined. Large sizes like 1Y may be rejected by your computer due to limitations of its arithmetic.


kilobyte: 10^3 = 1000.


kibibyte: 2^10 = 1024. ‘K’ is special: the SI prefix is ‘k’ and the IEC 60027-2 prefix is ‘Ki’, but tradition and POSIX use ‘k’ to mean ‘KiB’.


megabyte: 10^6 = 1,000,000.


mebibyte: 2^20 = 1,048,576.


gigabyte: 10^9 = 1,000,000,000.


gibibyte: 2^30 = 1,073,741,824.


terabyte: 10^12 = 1,000,000,000,000.


tebibyte: 2^40 = 1,099,511,627,776.


petabyte: 10^15 = 1,000,000,000,000,000.


pebibyte: 2^50 = 1,125,899,906,842,624.


exabyte: 10^18 = 1,000,000,000,000,000,000.


exbibyte: 2^60 = 1,152,921,504,606,846,976.


zettabyte: 10^21 = 1,000,000,000,000,000,000,000


2^70 = 1,180,591,620,717,411,303,424. (‘Zi’ is a GNU extension to IEC 60027-2.)


yottabyte: 10^24 = 1,000,000,000,000,000,000,000,000.


2^80 = 1,208,925,819,614,629,174,706,176. (‘Yi’ is a GNU extension to IEC 60027-2.)

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