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PROLOG | NAME | SYNOPSIS | DESCRIPTION | OPTIONS | OPERANDS | STDIN | INPUT FILES | ENVIRONMENT VARIABLES | ASYNCHRONOUS EVENTS | STDOUT | STDERR | OUTPUT FILES | EXTENDED DESCRIPTION | EXIT STATUS | CONSEQUENCES OF ERRORS | APPLICATION USAGE | EXAMPLES | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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CMP(1P) POSIX Programmer's Manual CMP(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.
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.1‐2017, 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 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:
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.1‐2017, 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
arguments).
LC_MESSAGES
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.
Default.
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.
If the -s option is used and an error occurs, it is unspecified
whether a diagnostic message is written to standard error.
None.
None.
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.
Default.
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.1‐2017. 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 <newline>, without any implication that the file
is organized into lines.
Since the behavior of -s differs between implementations as to
whether error messages are written, the only way to ensure
consistent behavior of cmp when -s is used is to redirect standard
error to /dev/null.
If error messages are wanted, instead of using -s standard output
should be redirected to /dev/null, and anything written to
standard error should be discarded if the exit status is 1. For
example:
silent_cmp() {
# compare files with no output except error messages
message=$(cmp "$@" 2>&1 >/dev/null)
status=$?
case $status in
(0|1) ;;
(*) printf '%s\n' "$message" ;;
esac
return $status
}
None.
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 allowed.
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.
Future versions of this standard may require that diagnostic
messages are written to standard error when the -s option is
specified.
comm(1p), diff(1p)
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
http://www.opengroup.org/unix/online.html .
Any typographical or formatting errors that appear in this page
are most likely to have been introduced during the conversion of
the source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2017 CMP(1P)
Pages that refer to this page: comm(1p), diff(1p)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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