cmp(1p) — Linux manual page


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

PROLOG         top

       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.

NAME         top

       cmp — compare two files

SYNOPSIS         top

       cmp [-l|-s] file1 file2

DESCRIPTION         top

       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         top

       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         top

       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         top

       The standard input shall be used only if the file1 or file2
       operand refers to standard input. See the INPUT FILES section.

INPUT FILES         top

       The input files can be any file type.


       The following environment variables shall affect the execution of

       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).

                 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.



STDOUT         top

       In the POSIX locale, results of the comparison shall be written
       to standard output. When no options are used, the format shall

           "%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         top

       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         top




EXIT STATUS         top

       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.



       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

           silent_cmp() {
               # compare files with no output except error messages
               message=$(cmp "$@" 2>&1 >/dev/null)
               case $status in
               (0|1) ;;
               (*) printf '%s\n' "$message" ;;
               return $status

EXAMPLES         top


RATIONALE         top

       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


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

SEE ALSO         top

       comm(1p), diff(1p)

       The Base Definitions volume of POSIX.1‐2017, Chapter 8,
       Environment Variables, Section 12.2, Utility Syntax Guidelines

COPYRIGHT         top

       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 .

       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 .

IEEE/The Open Group               2017                           CMP(1P)

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