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

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
       diff — compare two files

SYNOPSIS
       diff [−c|−e|−f|−u|−C n|−U n] [−br] file1 file2

DESCRIPTION
       The diff utility shall compare the contents  of	file1  and  file2  and
       write  to  standard output a list of changes necessary to convert file1
       into file2.  This list should be minimal. No output shall  be  produced
       if the files are identical.

OPTIONS
       The  diff  utility  shall  conform  to  the  Base Definitions volume of
       POSIX.1‐2008, Section 12.2, Utility Syntax Guidelines.

       The following options shall be supported:

       −b	 Cause any amount of white space at the end of a  line	to  be
		 treated as a single <newline> (that is, the white-space char‐
		 acters preceding the <newline> are ignored) and other strings
		 of  white-space  characters,  not including <newline> charac‐
		 ters, to compare equal.

       −c	 Produce output in a form that provides three lines of	copied
		 context.

       −C n	 Produce output in a form that provides n lines of copied con‐
		 text (where n shall be	 interpreted  as  a  positive  decimal
		 integer).

       −e	 Produce  output  in a form suitable as input for the ed util‐
		 ity, which can then be used to convert file1 into file2.

       −f	 Produce output in an alternative form, similar in  format  to
		 −e, but not intended to be suitable as input for the ed util‐
		 ity, and in the opposite order.

       −r	 Apply diff recursively to files and directories of  the  same
		 name when file1 and file2 are both directories.

		 The diff utility shall detect infinite loops; that is, enter‐
		 ing a previously visited directory that is an ancestor of the
		 last  file  encountered.   When  it detects an infinite loop,
		 diff shall write a diagnostic message to standard  error  and
		 shall	either recover its position in the hierarchy or termi‐
		 nate.

       −u	 Produce output in a form that provides three lines of unified
		 context.

       −U n	 Produce  output  in  a	 form that provides n lines of unified
		 context (where n shall be interpreted as a non-negative deci‐
		 mal integer).

OPERANDS
       The following operands shall be supported:

       file1, file2
		 A  pathname  of a file to be compared. If either the file1 or
		 file2 operand is '−', the standard input shall be used in its
		 place.

       If  both	 file1 and file2 are directories, diff shall not compare block
       special files, character special files, or FIFO special	files  to  any
       files  and  shall  not  compare	regular files to directories.  Further
       details are as specified in  Diff  Directory  Comparison	 Format.   The
       behavior	 of  diff  on  other file types is implementation-defined when
       found in directories.

       If only one of file1 and file2 is a directory, diff shall be applied to
       the  non-directory  file	 and  the file contained in the directory file
       with a filename that is the same as the	last  component	 of  the  non-
       directory file.

STDIN
       The  standard input shall be used only if one of the file1 or file2 op‐
       erands references standard input. See the INPUT FILES section.

INPUT FILES
       The input files may be of any type.

ENVIRONMENT VARIABLES
       The following environment variables shall affect the execution of diff:

       LANG	 Provide a default value for  the  internationalization	 vari‐
		 ables	that are unset or null. (See the Base Definitions vol‐
		 ume of POSIX.1‐2008, Section 8.2, Internationalization	 Vari‐
		 ables	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  and input
		 files).

       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.

       LC_TIME	 Determine the locale for affecting the format of  file	 time‐
		 stamps written with the −C and −c options.

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

       TZ	 Determine the timezone used for calculating  file  timestamps
		 written  with	a  context  format. If TZ is unset or null, an
		 unspecified default timezone shall be used.

ASYNCHRONOUS EVENTS
       Default.

STDOUT
   Diff Directory Comparison Format
       If both file1 and file2 are directories, the following  output  formats
       shall be used.

       In  the	POSIX  locale, each file that is present in only one directory
       shall be reported using the following format:

	   "Only in %s: %s\n", <directory pathname>, <filename>

       In the POSIX locale, subdirectories that are common to the two directo‐
       ries may be reported with the following format:

	   "Common subdirectories: %s and %s\n", <directory1 pathname>,
	       <directory2 pathname>

       For  each  file common to the two directories, if the two files are not
       to be compared: if the two files have  the  same	 device	 ID  and  file
       serial  number,	or are both block special files that refer to the same
       device, or are both character special files  that  refer	 to  the  same
       device,	in  the POSIX locale the output format is unspecified.	Other‐
       wise, in the POSIX locale an unspecified format shall be used that con‐
       tains the pathnames of the two files.

       For  each file common to the two directories, if the files are compared
       and are identical, no output shall be written. If the two files differ,
       the following format is written:

	   "diff %s %s %s\n", <diff_options>, <filename1>, <filename2>

       where <diff_options> are the options as specified on the command line.

       All directory pathnames listed in this section shall be relative to the
       original command line arguments. All other names	 of  files  listed  in
       this section shall be filenames (pathname components).

   Diff Binary Output Format
       In the POSIX locale, if one or both of the files being compared are not
       text files, it is implementation-defined whether diff uses  the	binary
       file  output format or the other formats as specified below. The binary
       file output format shall contain the pathnames of two files being  com‐
       pared and the string "differ".

       If  both	 files being compared are text files, depending on the options
       specified, one of the following formats shall be used to write the dif‐
       ferences.

   Diff Default Output Format
       The  default  (without  −e, −f, −c, −C, −u, or −U options) diff utility
       output shall contain lines of these forms:

	   "%da%d\n", <num1>, <num2>

	   "%da%d,%d\n", <num1>, <num2>, <num3>

	   "%dd%d\n", <num1>, <num2>

	   "%d,%dd%d\n", <num1>, <num2>, <num3>

	   "%dc%d\n", <num1>, <num2>

	   "%d,%dc%d\n", <num1>, <num2>, <num3>

	   "%dc%d,%d\n", <num1>, <num2>, <num3>

	   "%d,%dc%d,%d\n", <num1>, <num2>, <num3>, <num4>

       These lines resemble ed subcommands to convert file1 into  file2.   The
       line  numbers  before  the action letters shall pertain to file1; those
       after shall pertain to file2.  Thus, by exchanging a for d and  reading
       the  line in reverse order, one can also determine how to convert file2
       into file1.  As in ed, identical pairs (where num1= num2) are  abbrevi‐
       ated as a single number.

       Following  each of these lines, diff shall write to standard output all
       lines affected in the first file using the format:

	   "< %s", <line>

       and all lines affected in the second file using the format:

	   "> %s", <line>

       If there are lines affected in both file1 and file2 (as with the c sub‐
       command),  the  changes	are  separated with a line consisting of three
       <hyphen> characters:

	   "−−−\n"

   Diff −e Output Format
       With the −e option, a script shall be produced that  shall,  when  pro‐
       vided as input to ed, along with an appended w (write) command, convert
       file1 into file2.  Only the a  (append),	 c  (change),  d  (delete),  i
       (insert),  and  s  (substitute)	commands  of  ed shall be used in this
       script. Text lines, except those consisting  of	the  single  character
       <period> ('.'), shall be output as they appear in the file.

   Diff −f Output Format
       With  the −f option, an alternative format of script shall be produced.
       It is similar to that produced by −e, with the following differences:

	1. It is expressed in  reverse	sequence;  the	output	of  −e	orders
	   changes  from  the  end  of	the file to the beginning; the −f from
	   beginning to end.

	2. The command form <lines> <command-letter> used by −e	 is  reversed.
	   For example, 10c with −e would be c10 with −f.

	3. The	form  used  for	 ranges	 of line numbers is <space>-separated,
	   rather than <comma>-separated.

   Diff −c or −C Output Format
       With the −c or −C option, the output format shall consist  of  affected
       lines along with surrounding lines of context. The affected lines shall
       show which ones need to be deleted or changed in file1, and those added
       from  file2.  With the −c option, three lines of context, if available,
       shall be written before and after  the  affected	 lines.	 With  the  −C
       option,	the  user  can	specify how many lines of context are written.
       The exact format follows.

       The name and last modification time of each file shall be output in the
       following format:

	   "*** %s %s\n", file1, <file1 timestamp>
	   "−−− %s %s\n", file2, <file2 timestamp>

       Each <file> field shall be the pathname of the corresponding file being
       compared. The pathname written for standard input is unspecified.

       In the POSIX locale, each <timestamp> field shall be equivalent to  the
       output from the following command:

	   date "+%a %b %e %T %Y"

       without	the trailing <newline>, executed at the time of last modifica‐
       tion of the corresponding file (or the current time,  if	 the  file  is
       standard input).

       Then,  the  following  output formats shall be applied for every set of
       changes.

       First, a line shall be written in the following format:

	   "***************\n"

       Next, the range of lines in file1 shall be  written  in	the  following
       format if the range contains two or more lines:

	   "*** %d,%d ****\n", <beginning line number>, <ending line number>

       and the following format otherwise:

	   "*** %d ****\n", <ending line number>

       The  ending  line  number  of an empty range shall be the number of the
       preceding line, or 0 if the range is at the start of the file.

       Next, the affected lines along with lines of context (unaffected lines)
       shall  be  written.  Unaffected lines shall be written in the following
       format:

	   "  %s", <unaffected_line>

       Deleted lines shall be written as:

	   "− %s", <deleted_line>

       Changed lines shall be written as:

	   "! %s", <changed_line>

       Next, the range of lines in file2 shall be  written  in	the  following
       format if the range contains two or more lines:

	   "−−− %d,%d −−−−\n", <beginning line number>, <ending line number>

       and the following format otherwise:

	   "−−− %d −−−−\n", <ending line number>

       Then,  lines of context and changed lines shall be written as described
       in the previous formats. Lines added from file2 shall be written in the
       following format:

	   "+ %s", <added_line>

   Diff −u or −U Output Format
       The  −u or −U options behave like the −c or −C options, except that the
       context lines are not repeated;	instead,  the  context,	 deleted,  and
       added lines are shown together, interleaved.  The exact format follows.

       The name and last modification time of each file shall be output in the
       following format:

	   "--- %s%s%s %s0, file1, <file1 timestamp>, <file1 frac>, <file1 zone>
	   "+++ %s%s%s %s0, file2, <file2 timestamp>, <file2 frac>, <file2 zone>

       Each <file> field shall be the pathname of the corresponding file being
       compared,  or  the single character '−' if standard input is being com‐
       pared. However, if the pathname contains a <tab> or a <newline>, or  if
       it  does	 not  consist  entirely	 of characters taken from the portable
       character set, the behavior is implementation-defined.

       Each <timestamp> field shall be equivalent to the output from the  fol‐
       lowing command:

	   date '+%Y-%m-%d %H:%M:%S'

       without	the trailing <newline>, executed at the time of last modifica‐
       tion of the corresponding file (or the current time,  if	 the  file  is
       standard input).

       Each <frac> field shall be either empty, or a decimal point followed by
       at least one decimal digit, indicating the fractional-seconds part  (if
       any) of the file timestamp. The number of fractional digits shall be at
       least the number needed to represent the file's timestamp without  loss
       of information.

       Each <zone> field shall be of the form "shhmm", where "shh" is a signed
       two-digit decimal number in the range −24 through +25, and "mm"	is  an
       unsigned	 two-digit decimal number in the range 00 through 59.  It rep‐
       resents the timezone of the timestamp as the number of hours  (hh)  and
       minutes	(mm)  east  (+)	 or west (−) of UTC for the timestamp.	If the
       hours and minutes are both zero, the sign shall be  '+'.	  However,  if
       the  timezone  is  not an integral number of minutes away from UTC, the
       <zone> field is implementation-defined.

       Then, the following output formats shall be applied for	every  set  of
       changes.

       First,  the range of lines in each file shall be written in the follow‐
       ing format:

	   "@@ -%s +%s @@", <file1 range>, <file2 range>

       Each <range> field shall be of the form:

	   "%1d", <beginning line number>

       if the range contains exactly one line, and:

	   "%1d,%1d", <beginning line number>, <number of lines>

       otherwise. If a range is empty, its beginning line number shall be  the
       number  of  the	line  just  before  the range, or 0 if the empty range
       starts the file.

       Next, the affected lines along with lines of context shall be  written.
       Each  non-empty	unaffected line shall be written in the following for‐
       mat:

	   " %s", <unaffected_line>

       where the contents of the unaffected line shall be  taken  from	file1.
       It  is implementation-defined whether an empty unaffected line is writ‐
       ten as an empty line or a line containing a single  <space>  character.
       This  line  also represents the same line of file2, even though file2's
       line may contain different contents due to the −b.  Deleted lines shall
       be written as:

	   "-%s", <deleted_line>

       Added lines shall be written as:

	   "+%s", <added_line>

       The order of lines written shall be the same as that of the correspond‐
       ing file. A deleted line shall never be written	immediately  after  an
       added line.

       If  −U n is specified, the output shall contain no more than n consecu‐
       tive unaffected lines; and if the output contains an affected line  and
       this  line  is  adjacent to up to n consecutive unaffected lines in the
       corresponding file, the output shall contain  these  unaffected	lines.
       −u shall act like −U3.

STDERR
       The standard error shall be used only for diagnostic messages.

OUTPUT FILES
       None.

EXTENDED DESCRIPTION
       None.

EXIT STATUS
       The following exit values shall be returned:

	0    No differences were found.

	1    Differences were found.

       >1    An error occurred.

CONSEQUENCES OF ERRORS
       Default.

       The following sections are informative.

APPLICATION USAGE
       If  lines  at  the end of a file are changed and other lines are added,
       diff output may show this as a delete and add, as a  change,  or	 as  a
       change  and  add; diff is not expected to know which happened and users
       should not care about the difference in output as long  as  it  clearly
       shows the differences between the files.

EXAMPLES
       If dir1 is a directory containing a directory named x, dir2 is a direc‐
       tory containing a directory named x, dir1/x  and	 dir2/x	 both  contain
       files named date.out, and dir2/x contains a file named y, the command:

	   diff −r dir1 dir2

       could produce output similar to:

	   Common subdirectories: dir1/x and dir2/x
	   Only in dir2/x: y
	   diff −r dir1/x/date.out dir2/x/date.out
	   1c1
	   < Mon Jul  2 13:12:16 PDT 1990
	   −−−
	   > Tue Jun 19 21:41:39 PDT 1990

RATIONALE
       The  −h	option was omitted because it was insufficiently specified and
       does not add to applications portability.

       Historical implementations employ algorithms that do not always produce
       a  minimum list of differences; the current language about making every
       effort is the best this volume of POSIX.1‐2008 can do, as there	is  no
       metric  that  could be employed to judge the quality of implementations
       against any and all file contents. The statement ``This list should  be
       minimal'' clearly implies that implementations are not expected to pro‐
       vide the following output when comparing two 100-line files that differ
       in only one character on a single line:

	   1,100c1,100
	   all 100 lines from file1 preceded with "< "
	   −−−
	   all 100 lines from file2 preceded with "> "

       The  ``Only  in'' messages required when the −r option is specified are
       not used by most historical implementations if the −e  option  is  also
       specified.  It  is required here because it provides useful information
       that must be provided to update a target directory hierarchy to match a
       source hierarchy. The ``Common subdirectories'' messages are written by
       System V and 4.3 BSD when the −r option is specified. They are  allowed
       here  but are not required because they are reporting on something that
       is the same, not reporting a difference, and are not needed to update a
       target hierarchy.

       The  −c option, which writes output in a format using lines of context,
       has been included. The format is useful for a variety of reasons, among
       them being much improved readability and the ability to understand dif‐
       ference changes when the target file has line numbers that differ  from
       another	similar,  but  slightly	 different, copy. The patch utility is
       most valuable when working with difference  listings  using  a  context
       format. The BSD version of −c takes an optional argument specifying the
       amount of context. Rather than overloading −c and breaking the  Utility
       Syntax  Guidelines  for	diff, the standard developers decided to add a
       separate option for specifying a context diff with a  specified	amount
       of  context  (−C).   Also,  the	format	for context diffs was extended
       slightly in 4.3 BSD to allow multiple changes that are  within  context
       lines from each other to be merged together. The output format contains
       an additional four <asterisk> characters after the  range  of  affected
       lines  in  the  first filename. This was to provide a flag for old pro‐
       grams (like old versions of patch) that only understand the old context
       format.	The  version  of  context described here does not require that
       multiple changes within context lines be merged, but it does  not  pro‐
       hibit  it  either.  The extension is upwards-compatible, so any vendors
       that wish to retain the old version of diff can do  so  by  adding  the
       extra four <asterisk> characters (that is, utilities that currently use
       diff and understand the new merged format will also understand the  old
       unmerged format, but not vice versa).

       The −u and −U options of GNU diff have been included. Their output for‐
       mat, designed by Wayne Davison, takes up less space than −c and −C for‐
       mat,  and  in  many cases is easier to read. The format's timestamps do
       not vary by locale, so LC_TIME does not affect it.  The	format's  line
       numbers are rendered with the %1d format, not %d, because the file for‐
       mat notation rules would	 allow	extra  <blank>	characters  to	appear
       around the numbers.

       The  substitute	command	 was  added as an additional format for the −e
       option. This was added to provide implementations with a way to fix the
       classic	``dot  alone on a line'' bug present in many versions of diff.
       Since many implementations have fixed this bug, the standard developers
       decided	not  to standardize broken behavior, but rather to provide the
       necessary tool for fixing the bug. One way to fix this bug is to output
       two periods whenever a lone period is needed, then terminate the append
       command with a period, and then use the substitute command  to  convert
       the two periods into one period.

       The  BSD-derived	 −r  option was added to provide a mechanism for using
       diff to compare two file system trees.  This  behavior  is  useful,  is
       standard	 practice on all BSD-derived systems, and is not easily repro‐
       ducible with the find utility.

       The requirement that diff not compare files in some circumstances, even
       though  they  have the same name, is based on the actual output of his‐
       torical implementations.	 The specified behavior precludes the problems
       arising	from  running into FIFOs and other files that would cause diff
       to hang waiting for input with no indication to the user that diff  was
       hung.  An  earlier version of this standard specified the output format
       more precisely, but in practice this requirement was widely ignored and
       the  benefit of standardization seemed small, so it is now unspecified.
       In most common usage, diff −r should indicate differences in  the  file
       hierarchies,  not  the  difference of contents of devices pointed to by
       the hierarchies.

       Many early implementations of diff require seekable  files.  Since  the
       System  Interfaces  volume  of  POSIX.1‐2008  supports named pipes, the
       standard developers decided that such a restriction  was	 unreasonable.
       Note also that the allowed filename − almost always refers to a pipe.

       No directory search order is specified for diff.	 The historical order‐
       ing is, in fact, not optimal, in that it prints out all of the  differ‐
       ences  at  the current level, including the statements about all common
       subdirectories before recursing into those subdirectories.

       The message:

	   "diff %s %s %s\n", <diff_options>, <filename1>, <filename2>

       does not vary by locale because it is the representation of a  command,
       not an English sentence.

FUTURE DIRECTIONS
       None.

SEE ALSO
       cmp, comm, ed, find

       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 Electri‐
       cal 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.ker‐
       nel.org/doc/man-pages/reporting_bugs.html .

IEEE/The Open Group		     2013			      DIFF(1P)
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