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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 (con-
sult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on
Linux.

NAME
cmp - compare two files

SYNOPSIS
cmp [ -l | -s ] file1 file2

DESCRIPTION
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.

OPTIONS
The cmp utility shall conform to the Base Definitions volume of IEEE Std 1003.1-2001, Section 12.2, Utility Syntax Guide-
lines.

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.

OPERANDS
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.

STDIN
The standard input shall be used only if the file1 or file2 operand refers to standard input. See the INPUT FILES sec-
tion.

INPUT FILES
The input files can be any file type.

ENVIRONMENT VARIABLES
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 Variables for the precedence of international-
ization 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 stan-
dard error and informative messages written to standard output.

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

ASYNCHRONOUS EVENTS
Default.

STDOUT
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.

STDERR
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 speci-
fied:

"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.

OUTPUT FILES
None.

EXTENDED DESCRIPTION
None.

EXIT STATUS
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
Default.

The following sections are informative.

APPLICATION USAGE
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 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.

EXAMPLES
None.

RATIONALE
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 lan-
guage. 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 inclu-
sion of the <additional 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
behaved.

FUTURE DIRECTIONS
None.