neulab/tldr huggingface dataset:
Sometimes you might want to force diff to consider files to be text. For example, you might be comparing text files that contain null characters; diff would erroneously decide that those are non-text files. Or you might be comparing documents that are in a format used by a word processing system that uses null characters to indicate special formatting. You can force diff to consider all files to be text files, and compare them line by line, by using the [-a] or [--text] option. If the files you compare using this option do not in fact contain text, they will probably contain few newline characters, and the diff output will consist of hunks showing differences between long lines of whatever characters the files contain.
You can also force diff to report only whether files differ (but not how). Use the [-q] or [--brief] option for this.
Normally, differing binary files count as trouble because the resulting diff output does not capture all the differences. This trouble causes diff to exit with status 2. However, this trouble cannot occur with the [-a] or [--text] option, or with the [-q] or [--brief] option, as these options both cause diff to generate a form of output that represents differences as requested.
In operating systems that distinguish between text and binary files, diff normally reads and writes all data as text. Use the [--binary] option to force diff to read and write binary data instead. This option has no effect on a POSIX-compliant system like GNU or traditional Unix. However, many personal computer operating systems represent the end of a line with a carriage return followed by a newline.  On such systems, diff normally ignores these carriage returns on input and generates them at the end of each output line, but with the [--binary] option diff treats each carriage return as just another input character, and does not generate a carriage return at the end of each output line. This can be useful when dealing with non-text files that are meant to be interchanged with POSIX-compliant systems.
The [--strip-trailing-cr] causes diff to treat input lines that end in carriage return followed by newline as if they end in plain newline. This can be useful when comparing text that is imperfectly imported from many personal computer operating systems. This option affects how lines are read, which in turn affects how they are compared and output.
If you want to compare two files byte by byte, you can use the cmp program with the [-l] or [--verbose] option to show the values of each differing byte in the two files. With GNU cmp, you can also use the [-b] or [--print-bytes] option to show the ASCII representation of those bytes.See Section “Invoking cmp”, for more information.
If diff3 thinks that any of the files it is comparing is binary (a non-text file), it normally reports an error, because such comparisons are usually not useful.  diff3 uses the same test as diff to decide whether a file is binary. As with diff, if the input files contain a few non-text bytes but otherwise are like text files, you can force diff3 to consider all files to be text files and compare them line by line by using the [-a] or [--text] option.
diff(Output) Formats diff has several mutually exclusive options for output format. The following sections describe each format, illustrating how diff reports the differences between two sample input files.
Two Sample Input Files Here are two sample files that we will use in numerous examples to illustrate the output of diff and how various options can change it.
This is the file lao:
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, so we may see their outcome. The two are the same, But after they are produced, they have different names.
This is the file tzu:
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, so we may see their outcome. 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!
In this example, the first hunk contains just the first two lines of lao, the second hunk contains the fourth line of lao opposing the second and third lines of tzu, and the last hunk contains just the last three lines of tzu.
Showing Differences in Their Context Usually, when you are looking at the differences between files, you will also want to see the parts of the files near the lines that differ, to help you understand exactly what has changed. These nearby parts of the files are called the context.
GNU diff provides two output formats that show context around the differing lines: context format and unified format.  It can optionally show in which function or section of the file the differing lines are found.
If you are distributing new versions of files to other people in the form of diff output, you should use one of the output formats that show context so that they can apply the diffs even if they have made small changes of their own to the files.  patch can apply the diffs in this case by searching in the files for the lines of context around the differing lines; if those lines are actually a few lines away from where the diff says they are, patch can adjust the line numbers accordingly and still apply the diff correctly.See Section “Imperfect”, for more information on using patch to apply imperfect diffs.
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 [-C lines], [--context[= 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.
Example of Context Format
Here is the output of diff -c lao tzu (see Section “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.
*** 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!
Example of Context Format with Less Context
Here is the output of diff -C 1 lao tzu (see Section “Sample diff Input”, for the complete contents of the two files). Notice that at most one context line is reported here.
*** 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!
Detailed Description of Context Format
The context output format starts with a two-line header, which looks like this:
*** from-file from-file-modification-time --- to-file to-file-modification time
The time stamp normally looks like 2002-02-21 23:30:39.942229878 -0800 to indicate the date, time with fractional seconds, and time zone in ftp://ftp.isi.edu/in-notes/rfc2822.txt (The fractional seconds are omitted on hosts that do not support fractional time stamps.) However, a traditional time stamp like Thu Feb 21 23:30:39 2002 is used if the LC_TIME locale category is either C or POSIX.
You can change the header's content with the [--label= label] option; see 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:
*************** *** from-file-line-numbers **** from-file-line from-file-line... --- to-file-line-numbers ---- to-file-line to-file-line...
If a hunk contains two or more lines, its line numbers look like start, end.  Otherwise only its end line number appears. An empty hunk is considered to end at the line that precedes the hunk.
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:
!       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 !  in the part of this hunk for the other file.
+       An “inserted” line in the second file that corresponds to nothing in the first file.
-       A “deleted” line in the first file that corresponds to nothing in the second file.
If all of the changes in a hunk are insertions, the lines of from-file are omitted. If all of the changes are deletions, the lines of to-file are omitted.
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 [-U lines], [--unified[= 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.
At present, only GNU diff can produce this format and only GNU patch can automatically apply diffs in this format. For proper operation, patch typically needs at least three lines of context.
Example of Unified Format
Here is the output of the command diff -u lao tzu (see Section “Sample diff Input”, for the complete contents of the two files):
--- 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!
Detailed Description of Unified Format
The unified output format starts with a two-line header, which looks like this:
--- from-file from-file-modification-time +++ to-file to-file-modification-time
The time stamp looks like 2002-02-21 23:30:39.942229878 -0800 to indicate the date, time with fractional seconds, and time zone. The fractional seconds are omitted on hosts that do not support fractional time stamps.
You can change the header's content with the [--label= label] option; seeSee Section “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:
@@ from-file-line-numbers to-file-line-numbers @@ line-from-either-file line-from-either-file...
If a hunk contains just one line, only its start line number appears. Otherwise its line numbers look like start, count.  An empty hunk is considered to start at the line that follows the hunk.
If a hunk and its context contain two or more lines, its line numbers look like start, count.  Otherwise only its end line number appears. An empty hunk is considered to end at the line that precedes the hunk.
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 print column:
+       A line was added here to the first file.
-       A line was removed here from the first file.
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
To show in which sections differences occur for files that are not source code for C or similar languages, use the [-F regexp] or [--show-function-line= regexp] option.  diff considers lines that match the grep -style regular expression regexp to be the beginning of a section of the file. Here are suggested regular expressions for some common languages:
^[[:alpha:]$_] C, C++, Prolog
^(      Lisp
^@node  Texinfo
This option does not automatically select an output format; in order to use it, you must select the context format (see Section “Context Format”) or unified format (see Section “Unified Format”).  In other output formats it has no effect.
The [-F] or [--show-function-line] option finds the nearest unchanged line that precedes each hunk of differences and matches the given regular expression. Then it adds that line to the end of the line of asterisks in the context format, or to the @@ line in unified format. If no matching line exists, this option leaves the output for that hunk unchanged. If that line is more than 40 characters long, it outputs only the first 40 characters. You can specify more than one regular expression for such lines; diff tries to match each line against each regular expression, starting with the last one given. This means that you can use [-p] and [-F] together, if you wish.
Showing C Function Headings
To show in which functions differences occur for C and similar languages, you can use the [-p] or [--show-c-function] option. This option automatically defaults to the context output format (see Section “Context Format”), with the default number of lines of context. You can override that number with [-C lines] elsewhere in the command line. You can override both the format and the number with [-U lines] elsewhere in the command line.
The [-p] or [--show-c-function] option is equivalent to [-F '^[[:alpha:]$_]'] if the unified format is specified, otherwise [-c -F '^[[:alpha:]$_]'] (see Section “Specified Headings”).  GNU diff provides this option for the sake of convenience.
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 Section “Pagination”).
Here are the first two lines of the output from diff -C 2 --label=original --label=modified lao tzu:
*** original --- modified
Showing Differences Side by Side 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:
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 [--ignore] options (see Section “White Space”).
|       The corresponding lines differ, and they are either both complete or both incomplete.
<       The files differ and only the first file contains the line.
>       The files differ and only the second file contains the line.
(       Only the first file contains the line, but the difference is ignored.
)       Only the second file contains the line, but the difference is ignored.
\       The corresponding lines differ, and only the first line is incomplete.
/       The corresponding lines differ, and only the second line is incomplete.
Normally, an output line is incomplete if and only if the lines that it contains are incomplete;See Section “Incomplete Lines”.  However, when an output line represents two differing lines, one might be incomplete while the other is not. In this case, the output line is complete, but its the gutter is marked \ if the first line is incomplete, / if the second line is.
Side by side format is sometimes easiest to read, but it has limitations. It generates much wider output than usual, and truncates lines that are too long to fit. Also, it relies on lining up output more heavily than usual, so its output looks particularly bad if you use varying width fonts, nonstandard tab stops, or nonprinting characters.
You can use the sdiff command to interactively merge side by side differences.See Section “Interactive Merging”, for more information on merging files.
Controlling Side by Side Format
The [-y] or [--side-by-side] 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 [-W columns] or [--width= 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.
Example of Side by Side Format
Here is the output of the command diff -y -W 72 lao tzu (see Section “Sample diff Input”, 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!
Showing Differences Without Context The “normal” diff output format shows each hunk of differences without any surrounding context.  Sometimes such output is the clearest way to see how lines have changed, without the clutter of nearby unchanged lines (although you can get similar results with the context or unified formats by using 0 lines of context). However, this format is no longer widely used for sending out patches; for that purpose, the context format (see Section “Context Format”) and the unified format (see Section “Unified Format”) are superior. Normal format is the default for compatibility with older versions of diff and the POSIX standard. Use the [--normal] option to select this output format explicitly.
Example of Normal Format
Here is the output of the command diff lao tzu (see Section “Sample diff Input”, 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!
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:
la r    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.
fc t    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.
rd l    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.
Making Edit Scripts Several output modes produce command scripts for editing from-file to produce to-file.
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 [-e] or [--ed] option to select this output format.
Like the normal format (see Section “Normal”), 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)
Here is the output of diff -e lao tzu (see Section “Sample diff Input”, for the complete contents of the two files):
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
Detailed Description of ed(Format)
The ed output format consists of one or more hunks of differences. The changes closest to the ends of the files come first so that commands that change the number of lines do not affect how ed interprets line numbers in succeeding commands.  ed format hunks look like this:
change-command to-file-line to-file-line... .
Because ed uses a single period on a line to indicate the end of input, GNU diff protects lines of changes that contain a single period on a line by writing two periods instead, then writing a subsequent ed command to change the two periods into one. The ed format cannot represent an incomplete line, so if the second file ends in a changed incomplete line, diff reports an error and then pretends that a newline was appended.
There are three types of change commands. Each consists of a line number or comma-separated range of lines in the first file and a single character indicating the kind of change to make. All line numbers are the original line numbers in the file. The types of change commands are:
la      Add text from the second file after line l in the first file. For example, 8a means to add the following lines after line 8 of file 1.
rc      Replace the lines in range r in the first file with the following lines. Like a combined add and delete, but more compact. For example, 5,7c means change lines 5--7 of file 1 to read as the text file 2.
rd      Delete the lines in range r from the first file. For example, 5,7d means delete lines 5--7 of file 1.
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.
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 [-n] or [--rcs] option to select this output format. It is like the forward ed format (see Section “Forward ed”), 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 Section “Sample diff Input”, 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!
Merging Files with If-then-else You can use diff to merge two files of C source code. The output of diff in this format contains all the lines of both files. Lines common to both files are output just once; the differing parts are separated by the C preprocessor directives #ifdef name or #ifndef name, #else, and #endif. When compiling the output, you select which version to use by either defining or leaving undefined the macro name.
To merge two files, use diff with the [-D name] or [--ifdef= name] option. The argument name is the C preprocessor identifier to use in the #ifdef and #ifndef directives.
For example, if you change an instance of wait (&s) to waitpid (-1, &s, 0) and then merge the old and new files with the [--ifdef=HAVE_WAITPID] option, then the affected part of your code might look like this:
do { #ifndef HAVE_WAITPID if ((w = wait (&s)) < 0  &&  errno != EINTR) #else /* HAVE_WAITPID */ if ((w = waitpid (-1, &s, 0)) < 0  &&  errno != EINTR) #endif /* HAVE_WAITPID */ return w; } while (w != child);
You can specify formats for languages other than C by using line group formats and line formats, as described in the next sections.
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 Section “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.
F n     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.
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 Section “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.
%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.
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 Section.Dq 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
Example of If-then-else Format
Here is the output of diff -DTWO lao tzu (see Section “Sample diff Input”, 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 */
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 Section “patch Options”.
Incomplete Lines When an input file ends in a non-newline character, its last line is called an incomplete line because its last character is not a newline. All other lines are called full lines and end in a newline character. Incomplete lines do not match full lines unless differences in white space are ignored (see Section “White Space”).
An incomplete line is normally distinguished on output from a full line by a following line that starts with \.  However, the RCS format (see Section “RCS”) outputs the incomplete line as-is, without any trailing newline or following line. The side by side format normally represents incomplete lines as-is, but in some cases uses a \ or / gutter marker;See Section “Side by Side”.  The if-then-else line format preserves a line's incompleteness with %L, and discards the newline with %l ;See Section “Line Formats”.  Finally, with the ed and forward ed output formats (see Section “Output Formats”) diff cannot represent an incomplete line, so it pretends there was a newline and reports an error.
For example, suppose F and G are one-byte files that contain just f and g, respectively. Then diff F G outputs
1c1 < f \ No newline at end of file --- > g \ No newline at end of file
(The exact message may differ in non-English locales.)  diff -n F G outputs the following without a trailing newline:
d1 1 a1 1 g
diff -e F G reports two errors and outputs the following:
1c g .
Comparing Directories You can use diff to compare some or all of the files in two directory trees. When both file name arguments to diff are directories, it compares each file that is contained in both directories, examining file names in alphabetical order as specified by the LC_COLLATE locale category. Normally diff is silent about pairs of files that contain no differences, but if you use the [-s] or [--report-identical-files] option, it reports pairs of identical files. Normally diff reports subdirectories common to both directories without comparing subdirectories' files, but if you use the [-r] or [--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, 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 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 [-N] or [--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 [--unidirectional-new-file] option instead of [-N].  This option is like [-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.See Section “Making Patches”, for more discussion of making patches for distribution.
To ignore some files while comparing directories, use the [-x pattern] or [--exclude= pattern] option. This option ignores any files or subdirectories whose base names match the shell pattern 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 pattern in quotes so that the shell does not expand it. For example, the option [-x '*.[ao]'] ignores any file whose name ends with .a or .o.
This option accumulates if you specify it more than once. For example, using the options [-x 'RCS' -x '*,v'] ignores any file or subdirectory whose base name is RCS or ends with ,v.
If you need to give this option many times, you can instead put the patterns in a file, one pattern per line, and use the [-X file] or [--exclude-from= file] option. Trailing white space and empty lines are ignored in the pattern file.
If you have been comparing two directories and stopped partway through, later you might want to continue where you left off. You can do this by using the [-S file] or [--starting-file= file] option. This compares only the file file and all alphabetically later files in the topmost directory level.
If two directories differ only in that file names are lower case in one directory and upper case in the upper, diff normally reports many differences because it compares file names in a case sensitive way. With the [--ignore-file-name-case] option, diff ignores case differences in file names, so that for example the contents of the file Tao in one directory are compared to the contents of the file TAO in the other. The [--no-ignore-file-name-case] option cancels the effect of the [--ignore-file-name-case] option, reverting to the default behavior.
If an [-x pattern] or [--exclude= pattern] option, or an [-X file] or [--exclude-from= file] option, is specified while the [--ignore-file-name-case] option is in effect, case is ignored when excluding file names matching the specified patterns.
Making diff(Output) Prettier diff provides several ways to adjust the appearance of its output. These adjustments can be applied to any output format.
Preserving Tab Stop Alignment The lines of text in some of the diff output formats are preceded by one or two characters that indicate whether the text is inserted, deleted, or changed. The addition of those characters can cause tabs to move to the next tab stop, throwing off the alignment of columns in the line. GNU diff provides two ways to make tab-aligned columns line up correctly.
The first way is to have diff convert all tabs into the correct number of spaces before outputting them; select this method with the [-t] or [--expand-tabs] option. To use this form of output with patch, you must give patch the [-l] or [--ignore-white-space] option (see Section “Changed White Space”, for more information).  diff normally assumes that tab stops are set every 8 print columns, but this can be altered by the [--tabsize= columns] option.
The other method for making tabs line up correctly is to add a tab character instead of a space after the indicator character at the beginning of the line.  This ensures that all following tab characters are in the same position relative to tab stops that they were in the original files, so that the output is aligned correctly. Its disadvantage is that it can make long lines too long to fit on one line of the screen or the paper. It also does not work with the unified output format, which does not have a space character after the change type indicator character. Select this method with the [-T] or [--initial-tab] option.
Paginating diff(Output) It can be convenient to have long output page-numbered and time-stamped. The [-l] or [--paginate] option does this by sending the diff output through the pr program. Here is what the page header might look like for diff -lc lao tzu:
2002-02-22 14:20                 diff -lc lao tzu                 Page 1
diff(Performance) Tradeoffs GNU diff runs quite efficiently; however, in some circumstances you can cause it to run faster or produce a more compact set of changes.
One way to improve diff performance is to use hard or symbolic links to files instead of copies. This improves performance because diff normally does not need to read two hard or symbolic links to the same file, since their contents must be identical. For example, suppose you copy a large directory hierarchy, make a few changes to the copy, and then often use diff -r to compare the original to the copy. If the original files are read-only, you can greatly improve performance by creating the copy using hard or symbolic links (e.g., with GNU cp -lR or cp -sR).  Before editing a file in the copy for the first time, you should break the link and replace it with a regular copy.
You can also affect the performance of GNU diff by giving it options that change the way it compares files. Performance has more than one dimension. These options improve one aspect of performance at the cost of another, or they improve performance in some cases while hurting it in others.
The way that GNU diff determines which lines have changed always comes up with a near-minimal set of differences. Usually it is good enough for practical purposes. If the diff output is large, you might want diff to use a modified algorithm that sometimes produces a smaller set of differences. The [-d] or [--minimal] option does this; however, it can also cause diff to run more slowly than usual, so it is not the default behavior.
When the files you are comparing are large and have small groups of changes scattered throughout them, you can use the [--speed-large-files] option to make a different modification to the algorithm that diff uses. If the input files have a constant small density of changes, this option speeds up the comparisons without changing the output. If not, diff might produce a larger set of differences; however, the output will still be correct.
Normally diff discards the prefix and suffix that is common to both files before it attempts to find a minimal set of differences. This makes diff run faster, but occasionally it may produce non-minimal output. The [--horizon-lines= lines] option prevents diff from discarding the last lines lines of the prefix and the first lines lines of the suffix. This gives diff further opportunities to find a minimal output.
Suppose a run of changed lines includes a sequence of lines at one end and there is an identical sequence of lines just outside the other end. The diff command is free to choose which identical sequence is included in the hunk.  In this case, diff normally shifts the hunk's boundaries when this merges adjacent hunks, or shifts a hunk's lines towards the end of the file. Merging hunks can make the output look nicer in some cases.
Comparing Three Files Use the program diff3 to compare three files and show any differences among them. ( diff3 can also merge files; see diff3 Merging).
The “normal” diff3 output format shows each hunk of differences without surrounding context.  Hunks are labeled depending on whether they are two- way or three-way, and lines are annotated by their location in the input files.
See Section.Dq Invoking diff3 , for more information on how to run diff3.