CMP(1)                     POSIX Programmer's Manual                    CMP(1)



PROLOG
       This manual page is part of the POSIX Programmer's Manual.  The OS
       implementation of this interface may differ (consult the corresponding
       OS manual page for details of OS behavior), or the interface may not be
       implemented on OS.

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
       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.

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 section.

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 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.

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 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.

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 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
           }

EXAMPLES
       None.

RATIONALE
       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 DIRECTIONS
       Future versions of this standard may require that diagnostic messages
       are written to standard error when the -s option is specified.

SEE ALSO
       comm, diff

       The Base Definitions volume of POSIX.1-2017, Chapter _8, Environment
       Variables, Section 12.2, Utility Syntax Guidelines

COPYRIGHT
       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(1)
