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

The default old group format is the same as the changed group format if
it is specified; otherwise it is a format that outputs the line group as-is.

@item --new-group-format=@var{format}
These line groups are hunks containing only lines from the second
file.  The default new group format is same as the the changed group
format if it is specified; otherwise it is a format that outputs the
line group as-is.

@item --changed-group-format=@var{format}
These line groups are hunks containing lines from both files.  The
default changed group format is the concatenation of the old and new
group formats.

@item --unchanged-group-format=@var{format}
These line groups contain lines common to both files.  The default
unchanged group format is a format that outputs the line group as-is.
@end table

In a line group format, ordinary characters represent themselves;
conversion specifications start with @samp{%} and have one of the
following forms.

@table @samp
@item %<
stands for the lines from the first file, including the trailing newline.
Each line is formatted according to the old line format (@pxref{Line Formats}).

@item %>
stands for the lines from the second file, including the trailing newline.
Each line is formatted according to the new line format.

@item %=
stands for the lines common to both files, including the trailing newline.
Each line is formatted according to the unchanged line format.

@item %%
stands for @samp{%}.

@item %c'@var{C}'
where @var{C} is a single character, stands for @var{C}.
@var{C} may not be a backslash or an apostrophe.
For example, @samp{%c':'} stands for a colon, even inside
the then-part of an if-then-else format, which a colon would
normally terminate.

@item %c'\@var{O}'
where @var{O} is a string of 1, 2, or 3 octal digits,
stands for the character with octal code @var{O}.
For example, @samp{%c'\0'} stands for a null character.

@item @var{F}@var{n}
where @var{F} is a @code{printf} conversion specification and @var{n} is one
of the following letters, stands for @var{n}'s value formatted with @var{F}.

@table @samp
@item e
The line number of the line just before the group in the old file.

@item f
The line number of the first line in the group in the old file;
equals @var{e} + 1.

@item l
The line number of the last line in the group in the old file.

@item m
The line number of the line just after the group in the old file;
equals @var{l} + 1.

@item n
The number of lines in the group in the old file; equals @var{l} - @var{f} + 1.

@item E, F, L, M, N
Likewise, for lines in the new file.

@end table

The @code{printf} conversion specification can be @samp{%d},
@samp{%o}, @samp{%x}, or @samp{%X}, specifying decimal, octal,
lower case hexadecimal, or upper case hexadecimal output
respectively.  After the @samp{%} the following options can appear in
sequence: a @samp{-} specifying left-justification; an integer
specifying the minimum field width; and a period followed by an
optional integer specifying the minimum number of digits.
For example, @samp{%5dN} prints the number of new lines in the group
in a field of width 5 characters, using the @code{printf} format @code{"%5d"}.

@item (@var{A}=@var{B}?@var{T}:@var{E})
If @var{A} equals @var{B} then @var{T} else @var{E}.
@var{A} and @var{B} are each either a decimal constant
or a single letter interpreted as above.
This format spec is equivalent to @var{T} if
@var{A}'s value equals @var{B}'s; otherwise it is equivalent to @var{E}.

For example, @samp{%(N=0?no:%dN) line%(N=1?:s)} is equivalent to
@samp{no lines} if @var{N} (the number of lines in the group in the the
new file) is 0, to @samp{1 line} if @var{N} is 1, and to @samp{%dN lines}
otherwise.
@end table

@node Line Formats, Detailed If-then-else, Line Group Formats, If-then-else
@subsection Line Formats
@cindex line formats

Line formats control how each line taken from an input file is
output as part of a line group in if-then-else format.

For example, the following command outputs text with a one-column
change indicator to the left of the text.  The first column of output
is @samp{-} for deleted lines, @samp{|} for added lines, and a space
for unchanged lines.  The formats contain newline characters where
newlines are desired on output.

@example
diff \
   --old-line-format='-%l
' \
   --new-line-format='|%l
' \
   --unchanged-line-format=' %l
' \
   old new
@end example

To specify a line format, use one of the following options.  You should
quote @var{format}, since it often contains shell metacharacters.

@table @samp
@item --old-line-format=@var{format}
formats lines just from the first file.

@item --new-line-format=@var{format}
formats lines just from the second file.

@item --unchanged-line-format=@var{format}
formats lines common to both files.

@item --line-format=@var{format}
formats all lines; in effect, it sets all three above options simultaneously.
@end table

In a line format, ordinary characters represent themselves;
conversion specifications start with @samp{%} and have one of the
following forms.

@table @samp
@item %l
stands for the the contents of the line, not counting its trailing
newline (if any).  This format ignores whether the line is incomplete;
@xref{Incomplete Lines}.

@item %L
stands for the the contents of the line, including its trailing newline
(if any).  If a line is incomplete, this format preserves its
incompleteness.

@item %%
stands for @samp{%}.

@item %c'@var{C}'
where @var{C} is a single character, stands for @var{C}.
@var{C} may not be a backslash or an apostrophe.
For example, @samp{%c':'} stands for a colon.

@item %c'\@var{O}'
where @var{O} is a string of 1, 2, or 3 octal digits,
stands for the character with octal code @var{O}.
For example, @samp{%c'\0'} stands for a null character.

@item @var{F}n
where @var{F} is a @code{printf} conversion specification,
stands for the line number formatted with @var{F}.
For example, @samp{%.5dn} prints the line number using the
@code{printf} format @code{"%.5d"}.  @xref{Line Group Formats}, for
more about printf conversion specifications.

@end table

The default line format is @samp{%l} followed by a newline character.

If the input contains tab characters and it is important that they line
up on output, you should ensure that @samp{%l} or @samp{%L} in a line
format is just after a tab stop (e.g.@: by preceding @samp{%l} or
@samp{%L} with a tab character), or you should use the @samp{-t} or
@samp{--expand-tabs} option.

Taken together, the line and line group formats let you specify many
different formats.  For example, the following command uses a format
similar to @code{diff}'s normal format.  You can tailor this command
to get fine control over @code{diff}'s output.

@example
diff \
   --old-line-format='< %l
' \
   --new-line-format='> %l
' \
   --old-group-format='%df%(f=l?:,%dl)d%dE
%<' \
   --new-group-format='%dea%dF%(F=L?:,%dL)
%>' \
   --changed-group-format='%df%(f=l?:,%dl)c%dF%(F=L?:,%dL)
%<---
%>' \
   --unchanged-group-format='' \
   old new
@end example

@node Detailed If-then-else, Example If-then-else, Line Formats, If-then-else
@subsection Detailed Description of If-then-else Format

For lines common to both files, @code{diff} uses the unchanged line
group format.  For each hunk of differences in the merged output
format, if the hunk contains only lines from the first file,
@code{diff} uses the old line group format; if the hunk contains only
lines from the second file, @code{diff} uses the new group format;
otherwise, @code{diff} uses the changed group format.

The old, new, and unchanged line formats specify the output format of
lines from the first file, lines from the second file, and lines common
to both files, respectively.

The option @samp{--ifdef=@var{name}} is equivalent to
the following sequence of options using shell syntax:

@example
--old-group-format='#ifndef @var{name}
%<#endif /* not @var{name} */
' \
--new-group-format='#ifdef @var{name}
%>#endif /* @var{name} */
' \
--unchanged-group-format='%=' \
--changed-group-format='#ifndef @var{name}
%<#else /* @var{name} */
%>#endif /* @var{name} */
'
@end example

You should carefully check the @code{diff} output for proper nesting.
For example, when using the the @samp{-D @var{name}} or
@samp{--ifdef=@var{name}} option, you should check that if the
differing lines contain any of the C preprocessor directives
@samp{#ifdef}, @samp{#ifndef}, @samp{#else}, @samp{#elif}, or
@samp{#endif}, they are nested properly and match.  If they don't, you
must make corrections manually.  It is a good idea to carefully check
the resulting code anyway to make sure that it really does what you
want it to; depending on how the input files were produced, the output
might contain duplicate or otherwise incorrect code.

The @code{patch} @samp{-D @var{name}} option behaves just like
the @code{diff} @samp{-D @var{name}} option, except it operates on
a file and a diff to produce a merged file; @xref{patch Options}.

@node Example If-then-else, , Detailed If-then-else, If-then-else
@subsection An Example of If-then-else Format

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

@example
#ifndef TWO
The Way that can be told of is not the eternal Way;
The name that can be named is not the eternal name.
#endif /* not TWO */
The Nameless is the origin of Heaven and Earth;
#ifndef TWO
The Named is the mother of all things.
#else /* TWO */
The named is the mother of all things.

#endif /* TWO */
Therefore let there always be non-being,
  so we may see their subtlety,
And let there always be being,
  so we may see their outcome.
The two are the same,
But after they are produced,
  they have different names.
#ifdef TWO
They both may be called deep and profound.
Deeper and more profound,
The door of all subtleties!
#endif /* TWO */
@end example

@node Comparing Directories, Adjusting Output, Output Formats, Top
@chapter Comparing Directories

You can use @code{diff} to compare some or all of the files in two
directory trees.  When both file name arguments to @code{diff} are
directories, it compares each file that is contained in both
directories, examining file names in alphabetical order.  Normally
@code{diff} is silent about pairs of files that contain no differences,
but if you use the @samp{-s} or @samp{--report-identical-files} option,
it reports pairs of identical files.  Normally @code{diff} reports
subdirectories common to both directories without comparing
subdirectories' files, but if you use the @samp{-r} or
@samp{--recursive} option, it compares every corresponding pair of files
in the directory trees, as many levels deep as they go.

For file names that are in only one of the directories, @code{diff}
normally does not show the contents of the file that exists; it reports
only that the file exists in that directory and not in the other.  You
can make @code{diff} act as though the file existed but was empty in the
other directory, so that it outputs the entire contents of the file that
actually exists.  (It is output as either an insertion or a
deletion, depending on whether it is in the first or the second
directory given.)  To do this, use the @samp{-N} or @samp{--new-file}
option.

If the older directory contains one or more large files that are not in
the newer directory, you can make the patch smaller by using the
@samp{-P} or @samp{--unidirectional-new-file} option instead of @samp{-N}.
This option is like @samp{-N} except that it only inserts the contents
of files that appear in the second directory but not the first (that is,
files that were added).  At the top of the patch, write instructions for
the user applying the patch to remove the files that were deleted before
applying the patch.  @xref{Making Patches}, for more discussion of
making patches for distribution.

To ignore some files while comparing directories, use the @samp{-x
@var{pattern}} or @samp{--exclude=@var{pattern}} option.  This option
ignores any files or subdirectories whose base names match the shell
pattern @var{pattern}.  Unlike in the shell, a period at the start of
the base of a file name matches a wildcard at the start of a pattern.
You should enclose @var{pattern} in quotes so that the shell does not
expand it.  For example, the option @samp{-x '*.[ao]'} ignores any file
whose name ends with @samp{.a} or @samp{.o}.

This option accumulates if you specify