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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.

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 .