2.2.1 Context Format

The context output format shows several lines of context around the lines that differ. It is the standard format for distributing updates to source code.

To select this output format, use the --context[=lines] (-C lines) or -c option. The argument lines that some of these options take is the number of lines of context to show. If you do not specify lines, it defaults to three. For proper operation, patch typically needs at least two lines of context.

• An Example of Context Format
• An Example of Context Format with Less Context
• Detailed Description of Context Format

*** lao	2002-02-21 23:30:39.942229878 -0800
--- tzu	2002-02-21 23:30:50.442260588 -0800
***************
*** 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!

*** lao	2002-02-21 23:30:39.942229878 -0800
--- tzu	2002-02-21 23:30:50.442260588 -0800
***************
*** 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!

*** from-file from-file-modification-time
--- to-file to-file-modification time

***************
*** from-file-line-numbers ****
  from-file-line
  from-file-line…
--- to-file-line-numbers ----
  to-file-line
  to-file-line…

2.2.2 Unified 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 --unified[=lines] (-U lines), or -u option. The argument lines is the number of lines of context to show. When it is not given, it defaults to three.

In the early 1990s, only GNU diff could produce this format and only GNU patch could automatically apply diffs in this format. For proper operation, patch typically needs at least three lines of context.

• An Example of Unified Format
• Detailed Description of Unified Format

--- lao	2002-02-21 23:30:39.942229878 -0800
+++ tzu	2002-02-21 23:30:50.442260588 -0800
@@ -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!

--- from-file from-file-modification-time
+++ to-file to-file-modification-time

@@ from-file-line-numbers to-file-line-numbers @@
 line-from-either-file
 line-from-either-file…

2.2.3 Showing Which Sections Differences Are in

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 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.

• Showing Lines That Match Regular Expressions
• Showing C Function Headings

2.2.4 Showing Alternate File Names

If you are comparing two files that have meaningless or uninformative names, you might want diff to show alternate names in the header of the context and unified output formats. To do this, use the --label=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, diff reports an error. The --label option does not affect the file names in the pr header when the -l or --paginate option is used (see Paginating diff Output).

Here are the first two lines of the output from ‘diff -C 2 --label=original --label=modified lao tzu’:

*** original
--- modified

2.3.1 Controlling Side by Side Format

The --side-by-side (-y) option selects side by side format. Because side by side output lines contain two input lines, the output is wider than usual: normally 130 print columns, which can fit onto a traditional printer line. You can set the width of the output with the --width=columns (-W columns) option. The output is split into two halves of equal width, separated by a small gutter to mark differences; the right half is aligned to a tab stop so that tabs line up. Input lines that are too long to fit in half of an output line are truncated for output.

The --left-column option prints only the left column of two common lines. The --suppress-common-lines option suppresses common lines entirely.

2.3.2 An Example of Side by Side Format

Here is the output of the command ‘diff -y -W 72 lao tzu’ (see Two Sample Input Files, for the complete contents of the two files).

The Way that can be told of is n   <
The name that can be named is no   <
The Nameless is the origin of He        The Nameless is the origin of He
The Named is the mother of all t   |    The named is the mother of all t
                                   >
Therefore let there always be no        Therefore let there always be no
  so we may see their subtlety,           so we may see their subtlety,
And let there always be being,          And let there always be being,
  so we may see their outcome.            so we may see their outcome.
The two are the same,                   The two are the same,
But after they are produced,            But after they are produced,
  they have different names.              they have different names.
                                   >    They both may be called deep and
                                   >    Deeper and more profound,
                                   >    The door of all subtleties!

2.4.1 An Example of Normal Format

Here is the output of the command ‘diff lao tzu’ (see Two Sample Input Files, for the complete contents of the two files). Notice that it shows only the lines that are different between the two files.

1,2d0
< The Way that can be told of is not the eternal Way;
< The name that can be named is not the eternal name.
4c2,3
< The Named is the mother of all things.
---
> The named is the mother of all things.
> 
11a11,13
> They both may be called deep and profound.
> Deeper and more profound,
> The door of all subtleties!

2.4.2 Detailed Description of Normal Format

The normal output format consists of one or more hunks of differences; each hunk shows one area where the files differ. Normal format hunks look like this:

change-command
< from-file-line
< from-file-line…
---
> to-file-line
> to-file-line…

There are three types of change commands. Each consists of a line number or comma-separated range of lines in the first file, a single character indicating the kind of change to make, and a line number or comma-separated range of lines in the second file. All line numbers are the original line numbers in each file. The types of change commands are:

‘lar’

Add the lines in range r of the second file after line l of the first file. For example, ‘8a12,15’ means append lines 12–15 of file 2 after line 8 of file 1; or, if changing file 2 into file 1, delete lines 12–15 of file 2.

‘fct’

Replace the lines in range f of the first file with lines in range t of the second file. This is like a combined add and delete, but more compact. For example, ‘5,7c8,10’ means change lines 5–7 of file 1 to read as lines 8–10 of file 2; or, if changing file 2 into file 1, change lines 8–10 of file 2 to read as lines 5–7 of file 1.

‘rdl’

Delete the lines in range r from the first file; line l is where they would have appeared in the second file had they not been deleted. For example, ‘5,7d3’ means delete lines 5–7 of file 1; or, if changing file 2 into file 1, append lines 5–7 of file 1 after line 3 of file 2.

2.5.1 ed Scripts

diff can produce commands that direct the ed text editor to change the first file into the second file. Long ago, this was the only output mode that was suitable for editing one file into another automatically; today, with patch, it is almost obsolete. Use the --ed (-e) option to select this output format.

Like the normal format (see Showing Differences Without Context), this output format does not show any context; unlike the normal format, it does not include the information necessary to apply the diff in reverse (to produce the first file if all you have is the second file and the diff).

If the file d contains the output of ‘diff -e old new’, then the command ‘(cat d && echo w) | ed - old’ edits old to make it a copy of new. More generally, if d1, d2, …, dN contain the outputs of ‘diff -e old new1’, ‘diff -e new1 new2’, …, ‘diff -e newN-1 newN’, respectively, then the command ‘(cat d1 d2 … dN && echo w) | ed - old’ edits old to make it a copy of newN.

• Example ed Script
• Detailed Description of ed Format

11a
They both may be called deep and profound.
Deeper and more profound,
The door of all subtleties!
.
4c
The named is the mother of all things.

.
1,2d

change-command
to-file-line
to-file-line…
.

2.5.2 Forward ed Scripts

diff can produce output that is like an ed script, but with hunks in forward (front to back) order. The format of the commands is also changed slightly: command characters precede the lines they modify, spaces separate line numbers in ranges, and no attempt is made to disambiguate hunk lines consisting of a single period. Like ed format, forward ed format cannot represent incomplete lines.

Forward ed format is not very useful, because neither ed nor patch can apply diffs in this format. It exists mainly for compatibility with older versions of diff. Use the -f or --forward-ed option to select it.

2.5.3 RCS Scripts

The RCS output format is designed specifically for use by the Revision Control System, which is a set of free programs used for organizing different versions and systems of files. Use the --rcs (-n) option to select this output format. It is like the forward ed format (see Forward ed Scripts), but it can represent arbitrary changes to the contents of a file because it avoids the forward ed format’s problems with lines consisting of a single period and with incomplete lines. Instead of ending text sections with a line consisting of a single period, each command specifies the number of lines it affects; a combination of the ‘a’ and ‘d’ commands are used instead of ‘c’. Also, if the second file ends in a changed incomplete line, then the output also ends in an incomplete line.

Here is the output of ‘diff -n lao tzu’ (see Two Sample Input Files, for the complete contents of the two files):

d1 2
d4 1
a4 2
The named is the mother of all things.

a11 3
They both may be called deep and profound.
Deeper and more profound,
The door of all subtleties!

2.6.1 Line Group Formats

Line group formats let you specify formats suitable for many applications that allow if-then-else input, including programming languages and text formatting languages. A line group format specifies the output format for a contiguous group of similar lines.

For example, the following command compares the TeX files old and new, and outputs a merged file in which old regions are surrounded by ‘\begin{em}’-‘\end{em}’ lines, and new regions are surrounded by ‘\begin{bf}’-‘\end{bf}’ lines.

diff \
   --old-group-format='\begin{em}
%<\end{em}
' \
   --new-group-format='\begin{bf}
%>\end{bf}
' \
   old new

The following command is equivalent to the above example, but it is a little more verbose, because it spells out the default line group formats.

diff \
   --old-group-format='\begin{em}
%<\end{em}
' \
   --new-group-format='\begin{bf}
%>\end{bf}
' \
   --unchanged-group-format='%=' \
   --changed-group-format='\begin{em}
%<\end{em}
\begin{bf}
%>\end{bf}
' \
   old new

Here is a more advanced example, which outputs a diff listing with headers containing line numbers in a “plain English” style.

diff \
   --unchanged-group-format='' \
   --old-group-format='-------- %dn line%(n=1?:s) deleted at %df:
%<' \
   --new-group-format='-------- %dN line%(N=1?:s) added after %de:
%>' \
   --changed-group-format='-------- %dn line%(n=1?:s) changed at %df:
%<-------- to:
%>' \
   old new

To specify a line group format, use diff with one of the options listed below. You can specify up to four line group formats, one for each kind of line group. You should quote format, because it typically contains shell metacharacters.

--old-group-format=format

These line groups are hunks containing only lines from the first file. 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.

--new-group-format=format

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

--changed-group-format=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.

--unchanged-group-format=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.

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

‘%<’

stands for the lines from the first file, including the trailing newline. Each line is formatted according to the old line format (see Line Formats).

‘%>’

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

‘%=’

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

‘%%’

stands for ‘%’.

‘%c'C'’

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

‘%c'\O'’

where O is a string of 1, 2, or 3 octal digits, stands for the character with octal code O. For example, ‘%c'\0'’ stands for a null character.

‘Fn’

where F is a printf conversion specification and n is one of the following letters, stands for n’s value formatted with F.

‘e’

The line number of the line just before the group in the old file.

‘f’

The line number of the first line in the group in the old file; equals e + 1.

‘l’

The line number of the last line in the group in the old file.

‘m’

The line number of the line just after the group in the old file; equals l + 1.

‘n’

The number of lines in the group in the old file; equals l - f + 1.

‘E, F, L, M, N’

Likewise, for lines in the new file.

The printf conversion specification can be ‘%d’, ‘%o’, ‘%x’, or ‘%X’, specifying decimal, octal, lower case hexadecimal, or upper case hexadecimal output respectively. After the ‘%’ the following options can appear in sequence: a series of zero or more flags; an integer specifying the minimum field width; and a period followed by an optional integer specifying the minimum number of digits. The flags are ‘-’ for left-justification, ‘'’ for separating the digit into groups as specified by the LC_NUMERIC locale category, and ‘0’ for padding with zeros instead of spaces. For example, ‘%5dN’ prints the number of new lines in the group in a field of width 5 characters, using the printf format "%5d".

‘(A=B?T:E)’

If A equals B then T else E. A and B are each either a decimal constant or a single letter interpreted as above. This format spec is equivalent to T if A’s value equals B’s; otherwise it is equivalent to E.

For example, ‘%(N=0?no:%dN) line%(N=1?:s)’ is equivalent to ‘no lines’ if N (the number of lines in the group in the new file) is 0, to ‘1 line’ if N is 1, and to ‘%dN lines’ otherwise.

2.6.2 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-character change indicator to the left of the text. The first character of output is ‘-’ for deleted lines, ‘|’ for added lines, and a space for unchanged lines. The formats contain newline characters where newlines are desired on output.

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

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

--old-line-format=format

formats lines just from the first file.

--new-line-format=format

formats lines just from the second file.

--unchanged-line-format=format

formats lines common to both files.

--line-format=format

formats all lines; in effect, it sets all three above options simultaneously.

In a line format, ordinary characters represent themselves; conversion specifications start with ‘%’ and have one of the following forms.

‘%l’

stands for the contents of the line, not counting its trailing newline (if any). This format ignores whether the line is incomplete; See Incomplete Lines.

‘%L’

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

‘%%’

stands for ‘%’.

‘%c'C'’

where C is a single character, stands for C. C may not be a backslash or an apostrophe. For example, ‘%c':'’ stands for a colon.

‘%c'\O'’

where O is a string of 1, 2, or 3 octal digits, stands for the character with octal code O. For example, ‘%c'\0'’ stands for a null character.

‘Fn’

where F is a printf conversion specification, stands for the line number formatted with F. For example, ‘%.5dn’ prints the line number using the printf format "%.5d". See Line Group Formats, for more about printf conversion specifications.

The default line format is ‘%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 ‘%l’ or ‘%L’ in a line format is just after a tab stop (e.g. by preceding ‘%l’ or ‘%L’ with a tab character), or you should use the -t or --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 normal diff format. You can tailor this command to get fine control over diff output.

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

2.6.3 An Example of If-then-else Format

Here is the output of ‘diff -DTWO lao tzu’ (see Two Sample Input Files, for the complete contents of the two files):

#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 /* ! 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 */

2.6.4 Detailed Description of If-then-else Format

For lines common to both files, 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, diff uses the old line group format; if the hunk contains only lines from the second file, diff uses the new group format; otherwise, 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 --ifdef=name is equivalent to the following sequence of options using shell syntax:

--old-group-format='#ifndef name
%<#endif /* ! name */
' \
--new-group-format='#ifdef name
%>#endif /* name */
' \
--unchanged-group-format='%=' \
--changed-group-format='#ifndef name
%<#else /* name */
%>#endif /* name */
'

You should carefully check the diff output for proper nesting. For example, when using the -D name or --ifdef=name option, you should check that if the differing lines contain any of the C preprocessor directives ‘#ifdef’, ‘#ifndef’, ‘#else’, ‘#elif’, or ‘#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 patch -D name option behaves like the diff -D name option, except it operates on a file and a diff to produce a merged file. See Options to patch.