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Linux 2.6 - man page for diff (linux section 1posix)

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

       diff - compare two files

       diff [-c| -e| -f| -C n][-br] file1 file2

       The  diff utility shall compare the contents of file1 and file2 and write to standard out-
       put 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.

       The  diff  utility  shall  conform to the Base Definitions volume of IEEE Std 1003.1-2001,
       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 characters preceding the <newline> are ignored)
	      and other strings of white-space characters, not including <newline>s,  to  compare

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

       -C n   Produce  output in a form that provides n lines of context (where n shall be inter-
	      preted as a positive decimal integer).

       -e     Produce output in a form suitable as input for the ed utility, 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 utility, 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 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, char-
       acter  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 direc-

       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.

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

       The input files may be of any type.

       The following environment variables shall affect the execution of diff:

       LANG   Provide  a  default  value for the internationalization variables that are unset or
	      null. (See the Base Definitions volume of IEEE Std 1003.1-2001, Section 8.2, Inter-
	      nationalization 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  interna-
	      tionalization variables.

	      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  argu-
	      ments and input files).

	      Determine the locale that should be used to affect the format and contents of diag-
	      nostic messages written to standard error and informative messages written to stan-
	      dard output.

	      Determine  the  locale for affecting the format of file timestamps written with the
	      -C and -c options.

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

       TZ     Determine the timezone used for calculating file timestamps written with the -C and
	      -c options. If TZ is unset or null, an unspecified default timezone shall be used.


   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 directories 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, the
       following format shall be used in the POSIX locale:

	      "File %s is a %s while file %s is a %s\n", <directory1 pathname>,
		  <file type of directory1 pathname>, <directory2 pathname>,
		  <file type of directory2 pathname>

       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  (path-
       name components).

   Diff Binary Output Format
       In  the	POSIX  locale,	if one or both of the files being compared are not text files, an
       unspecified format shall be used that contains the pathnames of two files  being  compared
       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 differences.

   Diff Default Output Format
       The default (without -e, -f, -c, or -C 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 abbreviated  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	subcommand),  the
       changes are separated with a line consisting of three hyphens:


   Diff -e Output Format
       With  the  -e option, a script shall be produced that shall, when provided 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-sepa-

   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 con-
       text, 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 fol-

       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 modification  of  the  corre-
       sponding 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:


       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>

       The standard error shall be used only for diagnostic messages.



       The following exit values shall be returned:

	0     No differences were found.

	1     Differences were found.

       >1     An error occurred.


       The following sections are informative.

       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.

       If  dir1  is  a directory containing a directory named x, dir2 is a directory containing a
       directory named x, dir1/x and dir2/x both contain files named date.out,	and  dir2/x  con-
       tains 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
	      < Mon Jul  2 13:12:16 PDT 1990
	      > Tue Jun 19 21:41:39 PDT 1990

       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
       IEEE Std 1003.1-2001 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  provide  the
       following  output when comparing two 100-line files that differ in only one character on a
       single line:

	      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  his-
       torical 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 difference 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 the 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 asterisks after the range  of  affected  lines  in  the
       first  filename.  This was to provide a flag for old programs (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 prohibit
       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 asterisks (that is, utilities that cur-
       rently use diff and understand the new merged format will also understand the old unmerged
       format, but not vice versa).

       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  sub-
       stitute 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  sys-
       tems, and is not easily reproducible 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 historical  implementations.  The  message
       specified here is already in use when a directory is being compared to a non-directory. It
       is extended here to preclude 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. In most common usage, diff -r should indicate differences in  the  file  hierar-
       chies, 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 IEEE Std 1003.1-2001 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 ordering is, in fact,  not
       optimal,  in that it prints out all of the differences 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 sen-


       cmp , comm , ed , find

       Portions  of  this  text  are  reprinted  and  reproduced in electronic form from IEEE Std
       1003.1, 2003 Edition, Standard for Information Technology  --  Portable	Operating  System
       Interface  (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 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 orig-
       inal Standard can be obtained online at http://www.opengroup.org/unix/online.html .

IEEE/The Open Group			       2003					  DIFF(P)

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