CMP(1P) POSIX Programmer's Manual CMP(1P)PROLOG
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.
NAMEcmp — compare two files
SYNOPSIScmp [−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 dif‐
fer, 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 dif‐
fering 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
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 vari‐
ables that are unset or null. (See the Base Definitions vol‐
ume of POSIX.1‐2008, Section 8.2, Internationalization Vari‐
ables 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).
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
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>,
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
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 diagnos‐
tic 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 imple‐
mentations 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.
CONSEQUENCES OF ERRORS
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 com‐
pare 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 orga‐
nized into lines.
The global language in Section 1.4, Utility Description Defaults indi‐
cates 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.
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
Some implementations report on the number of lines in the identical-
but-shorter file case. This is allowed by the inclusion of the <addi‐
tional 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
The Base Definitions volume of POSIX.1‐2008, 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, 2013 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
cal and Electronics Engineers, Inc and The Open Group. (This is
POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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.unix.org/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
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IEEE/The Open Group 2013 CMP(1P)