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
IEEE Std 1003.1-2001, Section 12.2, Utility Syntax Guidelines.
The following options shall be supported:
-l (Lowercase ell.) Write the byte number (decimal) and the differ‐
ing 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 variables
that are unset or null. (See the Base Definitions volume of
IEEE Std 1003.1-2001, 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.
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>
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 file1
and file2 are identical for the entire length of the shorter file, in
the POSIX locale the following diagnostic message shall be written,
unless the -s option is specified:
"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>s. 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.
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
IEEE Std 1003.1-2001. Since this volume of IEEE Std 1003.1-2001 does
not specify the block size used when doing input, comparisons of char‐
acter 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 orga‐
nized into lines.
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 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
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>s 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
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 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 .
IEEE/The Open Group 2003 CMP(1P)