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.1‐2008, 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 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 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.1‐2008, 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.

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.1‐2008.
       Since this volume of POSIX.1‐2008 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 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

       None.

SEE ALSO

       comm, diff

       The  Base  Definitions  volume  of  POSIX.1‐2008, 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, 2013 Edition,
       Standard  for  Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base
       Specifications Issue 7, Copyright (C) 2013 by the Institute of Electrical and Electronics Engineers,  Inc
       and  The  Open Group.  (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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.unix.org/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 .