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man pages section 1: User Commands

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Updated: Wednesday, February 9, 2022



diff - compare two files


diff [-bitw] [-c | -e | -f | -h | -n | -u] file1 file2
diff [-bitw] [-C number | -U number] file1 file2
diff [-bitw] [-D string] file1 file2
diff [-bitw] [-c | -e | -f | -h | -n | -u] [-l] [-r] [-s]
     [-S name] directory1


The diff utility compares the contents of file1 and file2 and writes to standard output a list of changes necessary to convert file1 into file2. This list should be minimal. Except in rare circumstances, diff finds a smallest sufficient set of file differences. No output is produced if the files are identical.

The normal output contains lines of these forms:

n1 a n3,n4
n1,n2 d n3
n1,n2 c n3,n4

where n1 and n2 represent lines file1 and n3 and n4 represent lines in file2 These lines resemble ed(1) commands to convert file1 to file2. By exchanging a for d and reading backward, file2 can be converted to file1. As in ed, identical pairs, where n1= n2 or n3=n4, are abbreviated as a single number.

Following each of these lines come all the lines that are affected in the first file flagged by ‘ < ’, then all the lines that are affected in the second file flagged by ‘ > ’.


The following options are supported:


Ignores trailing blanks (spaces and tabs) and treats other strings of blanks as equivalent.


Ignores the case of letters. For example, ‘A’ compares equal to ‘a’.


Expands TAB characters in output lines. Normal or –c output adds character(s) to the front of each line that can adversely affect the indentation of the original source lines and make the output lines difficult to interpret. This option preserves the original source's indentation.


Ignores all blanks (SPACE and TAB characters) and treats all other strings of blanks as equivalent. For example, ‘if ( a = = b )’ compares equal to ‘if(a= =b)’.

The following options are mutually exclusive:


Produces a listing of differences with three lines of context. With this option, output format is modified slightly. That is, output begins with identification of the files involved and their creation dates, then each change is separated by a line with a dozen *'s. The lines removed from file1 are marked with '  '. The lines added to file2 are marked ' + '. Lines that are changed from one file to the other are marked in both files with ' ! '.

–C number

Produces a listing of differences identical to that produced by –c with number lines of context.

–D string

Creates a merged version of file1 and file2 with C preprocessor controls included so that a compilation of the result without defining string is equivalent to compiling file1, while defining string yields file2.


Produces a script of only a, c, and d commands for the editor ed, which re-creates file2 from file1. In connection with the –e option, the following shell program can help maintain multiple versions of a file. Only an ancestral file ($1) and a chain of version-to-version ed scripts ($2,$3,...) made by diff need be on hand. A “latest version” appears on the standard output.

(shift; cat $*; echo ´1,$p') | ed − $1

Produces a similar script, not useful with ed, in the opposite order.


Does a fast, uninspired job.

This option only works when changed stretches are short and well-separated. It does work on files of unlimited length.

Only –b is available with –h.

diff does not descend into directories with this option.


Produces a script similar to –e, but in the opposite order and with a count of changed lines on each insert or delete command.


Produces a listing of differences with three lines of context. The output is similar to that of the –c option, except that the context is “unified”. Removed and changed lines in file1 are marked by a '-' while lines added or changed in file2 are marked by a '+'. Both versions of changed lines appear in the output, while added, removed, and context lines appear only once. The identification of file1 and file2 is different, with “−−−” and “+++” being printed where “***” and “−−−” would appear with the –c option. Each change is separated by a line of the form

@@ -n1,n2 +
n3,n4 @@
–U number

Produces a listing of differences identical to that produced by –u with number lines of context.

The following options are used for comparing directories:


Produces output in long format. Before the diff, each text file is piped through pr(1) to paginate it. Other differences are remembered and summarized after all text file differences are reported.


Applies diff recursively to common subdirectories encountered.


Reports files that are identical. These identical files would not otherwise be mentioned.

–S name

Starts a directory diff in the middle, beginning with the file name.


The following operands are supported:


A path name of a file or directory to be compared. If either file1 or file2 is , the standard input is used in its place.


A path name of a directory to be compared.

If only one of file1 and file2 is a directory, diff is applied to the non-directory file and the file contained in the directory file with a filename that is the same as the last component of the non-directory file.


Example 1 Using the diff Command

In the following command, dir1 is a directory containing a directory named x, dir2 is a directory containing a directory named x, dir1/x and dir2/x both contain files named date.out, and dir2/x contains a file named y:

example% diff -r dir1 dir2
Common subdirectories: dir1/x and dir2/x

Only in dir2/x: y

diff -r dir1/x/date.out dir2/x/date.out


< Mon Jul  2 13:12:16 PDT 1990


> Tue Jun 19 21:41:39 PDT 1990

Environment Variables

See environ(7) for descriptions of the following environment variables that affect the execution of diff: LANG, LC_ALL, LC_CTYPE , LC_MESSAGES, LC_TIME, and NLSPATH.


Determines the locale for affecting the time zone used for calculating file timestamps written with the –C and –c options.

Exit Status

The following exit values are returned:


No differences were found.


Differences were found.

> 1

An error occurred.



Temporary file used for comparison


Executable file for the –h option


See attributes(7) for descriptions of the following attributes:

Interface Stability

See Also

bdiff(1), cmp(1), comm(1), diff(1g), dircmp(1), ed(1), patch(1g), pr(1), sdiff(1), wdiff(1), attributes(7), environ(7), standards(7)


Editing scripts produced under the –e or –f options are naive about creating lines consisting of a single period (.).

Missing NEWLINE at end of file indicates that the last line of the file in question did not have a NEWLINE. If the lines are different, they are flagged and output, although the output seems to indicate they are the same.