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cmp — compare two files


cmp [−l|−s] file1 file2


The cmp utility shall compare two files. The cmp utility shall write no output if the
files are the same. Under default options, if they differ, it shall write to standard
output the byte and line number at which the first difference occurred. Bytes and lines
shall be numbered beginning with 1.


The cmp utility shall conform to the Base Definitions volume of POSIX.1‐2008, Section
12.2, Utility Syntax Guidelines.

The following options shall be supported:

−l (Lowercase ell.) Write the byte number (decimal) and the differing bytes (octal)
for each difference.

−s Write nothing for differing files; return exit status only.


The following operands shall be supported:

file1 A pathname of the first file to be compared. If file1 is '−', the standard input
shall be used.

file2 A pathname of the second file to be compared. If file2 is '−', the standard
input shall be used.

If both file1 and file2 refer to standard input or refer to the same FIFO special, block
special, or character special file, the results are undefined.


The standard input shall be used only if the file1 or file2 operand refers to standard
input. See the INPUT FILES section.


The input files can be any file type.


The following environment variables shall affect the execution of cmp:

LANG Provide a default value for the internationalization variables that are unset or
null. (See the Base Definitions volume of POSIX.1‐2008, Section 8.2,
Internationalization Variables for the precedence of internationalization
variables used to determine the values of locale categories.)

LC_ALL If set to a non-empty string value, override the values of all the other
internationalization variables.

LC_CTYPE Determine the locale for the interpretation of sequences of bytes of text data
as characters (for example, single-byte as opposed to multi-byte characters in

Determine the locale that should be used to affect the format and contents of
diagnostic messages written to standard error and informative messages written
to standard output.

NLSPATH Determine the location of message catalogs for the processing of LC_MESSAGES.




In the POSIX locale, results of the comparison shall be written to standard output. When
no options are used, the format shall be:

"%s %s differ: char %d, line %d\n", file1, file2,
<byte number>, <line number>

When the −l option is used, the format shall be:

"%d %o %o\n", <byte number>, <differing byte>,
<differing byte>

for each byte that differs. The first <differing byte> number is from file1 while the
second is from file2. In both cases, <byte number> shall be relative to the beginning of
the file, beginning with 1.

No output shall be written to standard output when the −s option is used.


The standard error shall be used only for diagnostic messages. If the −l option is used
and file1 and file2 differ in length, or if the −s option is not used and file1 and file2
are identical for the entire length of the shorter file, in the POSIX locale the following
diagnostic message shall be written:

"cmp: EOF on %s%s\n", <name of shorter file>, <additional info>

The <additional info> field shall either be null or a string that starts with a <blank>
and contains no <newline> characters. Some implementations report on the number of lines
in this case.






The following exit values shall be returned:

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.

>1 An error occurred.



The following sections are informative.


Although input files to cmp can be any type, the results might not be what would be
expected on character special device files or on file types not described by the System
Interfaces volume of POSIX.1‐2008. Since this volume of POSIX.1‐2008 does not specify the
block size used when doing input, comparisons of character special files need not compare
all of the data in those files.

For files which are not text files, line numbers simply reflect the presence of a
<newline>, without any implication that the file is organized into lines.




The global language in Section 1.4, Utility Description Defaults indicates that using two
mutually-exclusive options together produces unspecified results. Some System V
implementations consider the option usage:

cmp −l −s ...

to be an error. They also treat:

cmp −s −l ...

as if no options were specified. Both of these behaviors are considered bugs, but are

The word char in the standard output format comes from historical usage, even though it is
actually a byte number. When cmp is supported in other locales, implementations are
encouraged to use the word byte or its equivalent in another language. Users should not
interpret this difference to indicate that the functionality of the utility changed
between locales.

Some implementations report on the number of lines in the identical-but-shorter file case.
This is allowed by the inclusion of the <additional info> fields in the output format. The
restriction on having a leading <blank> and no <newline> characters is to make parsing for
the filename easier. It is recognized that some filenames containing white-space
characters make parsing difficult anyway, but the restriction does aid programs used on
systems where the names are predominantly well behaved.



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