diff.texi

这是一个同样来自贝尔实验室的和UNIX有着渊源的操作系统, 其简洁的设计和实现易于我们学习和理解

@noindent
You can change the header's content with the @samp{-L @var{label}} or
@samp{--label=@var{label}} option; see @ref{Alternate Names}.

Next come one or more hunks of differences; each hunk shows one area
where the files differ.  Context format hunks look like this:

@example
***************
*** @var{from-file-line-range} ****
  @var{from-file-line}
  @var{from-file-line}@dots{}
--- @var{to-file-line-range} ----
  @var{to-file-line}
  @var{to-file-line}@dots{}
@end example

The lines of context around the lines that differ start with two space
characters.  The lines that differ between the two files start with one
of the following indicator characters, followed by a space character:

@table @samp
@item !
A line that is part of a group of one or more lines that changed between
the two files.  There is a corresponding group of lines marked with
@samp{!} in the part of this hunk for the other file.

@item +
An ``inserted'' line in the second file that corresponds to nothing in
the first file.

@item -
A ``deleted'' line in the first file that corresponds to nothing in the
second file.
@end table

If all of the changes in a hunk are insertions, the lines of
@var{from-file} are omitted.  If all of the changes are deletions, the
lines of @var{to-file} are omitted.

@node Example Context, Less Context, Detailed Context, Context Format
@subsubsection An Example of Context Format

Here is the output of @samp{diff -c lao tzu} (@pxref{Sample diff Input},
for the complete contents of the two files).  Notice that up to three
lines that are not different are shown around each line that is
different; they are the context lines.  Also notice that the first two
hunks have run together, because their contents overlap.

@example
*** lao	Sat Jan 26 23:30:39 1991
--- tzu	Sat Jan 26 23:30:50 1991
***************
*** 1,7 ****
- The Way that can be told of is not the eternal Way;
- The name that can be named is not the eternal name.
  The Nameless is the origin of Heaven and Earth;
! The Named is the mother of all things.
  Therefore let there always be non-being,
    so we may see their subtlety,
  And let there always be being,
--- 1,6 ----
  The Nameless is the origin of Heaven and Earth;
! The named is the mother of all things.
! 
  Therefore let there always be non-being,
    so we may see their subtlety,
  And let there always be being,
***************
*** 9,11 ****
--- 8,13 ----
  The two are the same,
  But after they are produced,
    they have different names.
+ They both may be called deep and profound.
+ Deeper and more profound,
+ The door of all subtleties!
@end example

@node Less Context, , Example Context, Context Format
@subsubsection An Example of Context Format with Less Context

Here is the output of @samp{diff --context=1 lao tzu} (@pxref{Sample
diff Input}, for the complete contents of the two files).  Notice that
at most one context line is reported here.

@example
*** lao	Sat Jan 26 23:30:39 1991
--- tzu	Sat Jan 26 23:30:50 1991
***************
*** 1,5 ****
- The Way that can be told of is not the eternal Way;
- The name that can be named is not the eternal name.
  The Nameless is the origin of Heaven and Earth;
! The Named is the mother of all things.
  Therefore let there always be non-being,
--- 1,4 ----
  The Nameless is the origin of Heaven and Earth;
! The named is the mother of all things.
! 
  Therefore let there always be non-being,
***************
*** 11 ****
--- 10,13 ----
    they have different names.
+ They both may be called deep and profound.
+ Deeper and more profound,
+ The door of all subtleties!
@end example

@node Unified Format, Sections, Context Format, Context
@subsection Unified Format
@cindex unified output format
@cindex @samp{+-} output format

The unified output format is a variation on the context format that is
more compact because it omits redundant context lines.  To select this
output format, use the @samp{-U @var{lines}},
@samp{--unified@r{[}=@var{lines}@r{]}}, or @samp{-u}
option.  The argument @var{lines} is the number of lines of context to
show.  When it is not given, it defaults to three.

At present, only GNU @code{diff} can produce this format and only GNU
@code{patch} can automatically apply diffs in this format.  For proper
operation, @code{patch} typically needs at least two lines of context.

@menu
* Detailed Unified::	A detailed description of unified format.
* Example Unified::	Sample output in unified format.
@end menu

@node Detailed Unified, Example Unified, , Unified Format
@subsubsection Detailed Description of Unified Format

The unified output format starts with a two-line header, which looks
like this:

@example
--- @var{from-file} @var{from-file-modification-time}
+++ @var{to-file} @var{to-file-modification-time}
@end example

@noindent
You can change the header's content with the @samp{-L @var{label}} or
@samp{--label=@var{label}} option; see @xref{Alternate Names}.

Next come one or more hunks of differences; each hunk shows one area
where the files differ.  Unified format hunks look like this:

@example
@@@@ @var{from-file-range} @var{to-file-range} @@@@
 @var{line-from-either-file}
 @var{line-from-either-file}@dots{}
@end example

The lines common to both files begin with a space character.  The lines
that actually differ between the two files have one of the following
indicator characters in the left column:

@table @samp
@item +
A line was added here to the first file.

@item -
A line was removed here from the first file.
@end table

@node Example Unified, , Detailed Unified, Unified Format
@subsubsection An Example of Unified Format

Here is the output of the command @samp{diff -u lao tzu}
(@pxref{Sample diff Input}, for the complete contents of the two files):

@example
--- lao	Sat Jan 26 23:30:39 1991
+++ tzu	Sat Jan 26 23:30:50 1991
@@@@ -1,7 +1,6 @@@@
-The Way that can be told of is not the eternal Way;
-The name that can be named is not the eternal name.
 The Nameless is the origin of Heaven and Earth;
-The Named is the mother of all things.
+The named is the mother of all things.
+
 Therefore let there always be non-being,
   so we may see their subtlety,
 And let there always be being,
@@@@ -9,3 +8,6 @@@@
 The two are the same,
 But after they are produced,
   they have different names.
+They both may be called deep and profound.
+Deeper and more profound,
+The door of all subtleties!
@end example

@node Sections, Alternate Names, Unified Format, Context
@subsection Showing Which Sections Differences Are in
@cindex headings
@cindex section headings

Sometimes you might want to know which part of the files each change
falls in.  If the files are source code, this could mean which function
was changed.  If the files are documents, it could mean which chapter or
appendix was changed.  GNU @code{diff} can show this by displaying the
nearest section heading line that precedes the differing lines.  Which
lines are ``section headings'' is determined by a regular expression.

@menu
* Specified Headings::	Showing headings that match regular expressions.
* C Function Headings::	Showing headings of C functions.
@end menu

@node Specified Headings, C Function Headings, , Sections
@subsubsection Showing Lines That Match Regular Expressions
@cindex specified headings
@cindex regular expression matching headings

To show in which sections differences occur for files that are not
source code for C or similar languages, use the @samp{-F @var{regexp}}
or @samp{--show-function-line=@var{regexp}} option.  @code{diff}
considers lines that match the argument @var{regexp} to be the beginning
of a section of the file.  Here are suggested regular expressions for
some common languages:

@c Please add to this list, e.g. Fortran, Pascal.
@table @samp
@item ^[A-Za-z_]
C, C++, Prolog
@item ^(
Lisp
@item ^@@\(chapter\|appendix\|unnumbered\|chapheading\)
Texinfo
@end table

This option does not automatically select an output format; in order to
use it, you must select the context format (@pxref{Context Format}) or
unified format (@pxref{Unified Format}).  In other output formats it
has no effect.

The @samp{-F} and @samp{--show-function-line} options find the nearest
unchanged line that precedes each hunk of differences and matches the
given regular expression.  Then they add that line to the end of the
line of asterisks in the context format, or to the @samp{@@@@} line in
unified format.  If no matching line exists, they leave the output for
that hunk unchanged.  If that line is more than 40 characters long, they
output only the first 40 characters.  You can specify more than one
regular expression for such lines; @code{diff} tries to match each line
against each regular expression, starting with the last one given.  This
means that you can use @samp{-p} and @samp{-F} together, if you wish.

@node C Function Headings, , Specified Headings, Sections
@subsubsection Showing C Function Headings
@cindex C function headings
@cindex function headings, C

To show in which functions differences occur for C and similar
languages, you can use the @samp{-p} or @samp{--show-c-function} option.
This option automatically defaults to the context output format
(@pxref{Context Format}), with the default number of lines of context.
You can override that number with @samp{-C @var{lines}} elsewhere in the
command line.  You can override both the format and the number with
@samp{-U @var{lines}} elsewhere in the command line.

The @samp{-p} and @samp{--show-c-function} options are equivalent to
@samp{-F'^[_a-zA-Z$]'} if the unified format is specified, otherwise
@samp{-c -F'^[_a-zA-Z$]'} (@pxref{Specified Headings}).  GNU @code{diff}
provides them for the sake of convenience.

@node Alternate Names, , Sections, Context
@subsection Showing Alternate File Names
@cindex alternate file names
@cindex file name alternates

If you are comparing two files that have meaningless or uninformative
names, you might want @code{diff} to show alternate names in the header
of the context and unified output formats.  To do this, use the @samp{-L
@var{label}} or @samp{--label=@var{label}} option.  The first time
you give this option, its argument replaces the name and date of the
first file in the header; the second time, its argument replaces the
name and date of the second file.  If you give this option more than
twice, @code{diff} reports an error.  The @samp{-L} option does not
affect the file names in the @code{pr} header when the @samp{-l} or
@samp{--paginate} option is used (@pxref{Pagination}).

Here are the first two lines of the output from @samp{diff -C2
-Loriginal -Lmodified lao tzu}:

@example
*** original
--- modified
@end example

@node Side by Side, Scripts, Context, Output Formats
@section Showing Differences Side by Side
@cindex side by side
@cindex two-column output
@cindex columnar output

@code{diff} can produce a side by side difference listing of two files.
The files are listed in two columns with a gutter between them.  The
gutter contains one of the following markers:

@table @asis
@item white space
The corresponding lines are in common.  That is, either the lines are
identical, or the difference is ignored because of one of the
@samp{--ignore} options (@pxref{White Space}).

@item @samp{|}
The corresponding lines differ, and they are either both complete
or both incomplete.

@item @samp{<}
The files differ and only the first file contains the line.

@item @samp{>}
The files differ and only the second file contains the line.

@item @samp{(}
Only the first file contains the line, but the difference is ignored.

@item @samp{)}
Only the second file contains the line, but the difference is ignored.

@item @samp{\}
The corresponding lines differ, and only the first line is incomplete.

@item @samp{/}
The corresponding lines differ, and only the second line is incomplete.
@end table

Normally, an output line is incomplete if and only if the lines that it
contains are incomplete; @xref{Incomplete Lines}.  However, when an