Section: POSIX Programmer's Manual (1P)
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
--- compare two files
cmp [-l|-s] file1 file2
utility shall compare two files. The
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.
utility shall conform to the Base Definitions volume of POSIX.1-2017,
, Utility Syntax Guidelines.
The following options shall be supported:
(Lowercase ell.) Write the byte number (decimal) and the differing
bytes (octal) for each difference.
Write nothing to standard output or standard error when files
differ; indicate differing files through exit status only.
It is unspecified whether a diagnostic message is written to
standard error when an error is encountered; if a message is
not written, the error is indicated through exit status only.
The following operands shall be supported:
A pathname of the first file to be compared. If
the standard input shall be used.
A pathname of the second file to be compared. If
the standard input shall be used.
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
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
Provide a default value for the internationalization variables that are
unset or null. (See the Base Definitions volume of POSIX.1-2017,
Section 8.2, Internationalization Variables
for the precedence of internationalization variables used to determine
the values of locale categories.)
If set to a non-empty string value, override the values of all the
other internationalization variables.
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 arguments).
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.
Determine the location of message catalogs for the processing of
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>
option is used, the format shall be:
"%d %o %o\n", <byte number>, <differing byte>,
for each byte that differs. The first <differing byte> number is
while the second is from
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
option is used.
The standard error shall be used only for diagnostic messages. If the
option is used and
differ in length, or if the
option is not used and
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
and contains no
characters. Some implementations report on the number of lines in
option is used and an error occurs, it is unspecified whether a
diagnostic message is written to standard error.
The following exit values shall be returned:
The files are identical.
The files are different; this includes the case where one file is
identical to the first part of the other.
An error occurred.
CONSEQUENCES OF ERRORS
The following sections are informative.
Although input files to
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.1-2017. Since this volume of POSIX.1-2017 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
without any implication that the file is organized into lines.
Since the behavior of
differs between implementations as to whether error messages are
written, the only way to ensure consistent behavior of
is used is to redirect standard error to
If error messages are wanted, instead of using
standard output should be redirected to
and anything written to standard error should be discarded if the exit
status is 1. For example:
# compare files with no output except error messages
message=$(cmp "$@" 2>&1 >/dev/null)
case $status in
(*) printf '%s\n' "$message" ;;
The global language in
, Utility Description Defaults
indicates that using two mutually-exclusive options together produces
unspecified results. Some System V implementations consider the option
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 allowed.
in the standard output format comes from historical usage, even though
it is actually a byte number. When
is supported in other locales, implementations are encouraged to use
or its equivalent in another language. Users should not interpret this
difference to indicate that the functionality of the utility changed
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
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.
Future versions of this standard may require that diagnostic messages
are written to standard error when the
option is specified.
The Base Definitions volume of POSIX.1-2017,
Chapter 8, Environment Variables,
Section 12.2, Utility Syntax Guidelines
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1-2017, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
In the event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
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