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DIFF(1) General Commands Manual DIFF(1)

diff —
differential file and directory comparator

diff [-abdipTtw] [-c | -e | -f | -n | -q | -u] [-I pattern] [-L label] file1 file2

diff [-abdilpTtw] [-I pattern] [-L label] -C number file1 file2

diff [-abdiltw] [-I pattern] -D string file1 file2

diff [-abdilpTtw] [-I pattern] [-L label] -U number file1 file2

diff [-abdilNPprsTtw] [-c | -e | -f | -n | -q | -u] [-I pattern] [-L label] [-S name] [-X file] [-x pattern] dir1 dir2

The diff utility compares the contents of file1 and file2 and writes to the standard output the list of changes necessary to convert one file into the other. No output is produced if the files are identical.

Output options (mutually exclusive):

number
Like -c but produces a diff with number lines of context.
Produces a diff with 3 lines of context. With -c the output format is modified slightly: the output begins with identification of the files involved and their creation dates and then each change is separated by a line with fifteen *'s. The lines removed from file1 are marked with ‘- ’; those added to file2 are marked ‘+ ’. Lines which are changed from one file to the other are marked in both files with ‘! ’. Changes which lie within 3 lines of each other are grouped together on output.
string
Creates a merged version of file1 and file2 on the standard output, with C preprocessor controls included so that a compilation of the result without defining string is equivalent to compiling file1, while defining string will yield file2.
Produces output in a form suitable as input for the editor utility, ed(1), which can then be used to convert file1 into file2.

Extra commands are added to the output when comparing directories with -e, so that the result is a sh(1) script for converting text files which are common to the two directories from their state in dir1 to their state in dir2.

Identical output to that of the -e flag, but in reverse order. It cannot be digested by ed(1).
Produces a script similar to that of -e, but in the opposite order and with a count of changed lines on each insert or delete command. This is the form used by rcsdiff(1).
Just print a line when the files differ. Does not output a list of changes.
number
Like -u but produces a diff with number lines of context.
Produces a unified diff with 3 lines of context. A unified diff is similar to the context diff produced by the -c option. However, unlike with -c, all lines to be changed (added and/or removed) are present in a single section.

Comparison options:

Treat all files as ASCII text. Normally diff will simply print “Binary files ... differ” if files contain binary characters. Use of this option forces diff to produce a diff.
Causes trailing blanks (spaces and tabs) to be ignored, and other strings of blanks to compare equal.
Try very hard to produce a diff as small as possible. This may consume a lot of processing power and memory when processing large files with many changes.
pattern
Ignores changes, insertions, and deletions whose lines match the extended regular expression pattern. Multiple -I patterns may be specified. All lines in the change must match some pattern for the change to be ignored. See re_format(7) for more information on regular expression patterns.
Ignores the case of letters. E.g., “A” will compare equal to “a”.
label
Print label instead of the first (and second, if this option is specified twice) file name and time in the context or unified diff header.
With unified and context diffs, show with each change the first 40 characters of the last line before the context beginning with a letter, an underscore or a dollar sign. For C source code following standard layout conventions, this will show the prototype of the function the change applies to.
Print a tab rather than a space before the rest of the line for the normal, context or unified output formats. This makes the alignment of tabs in the line consistent.
Will expand tabs in output lines. Normal or -c output adds character(s) to the front of each line which may screw up the indentation of the original source lines and make the output listing difficult to interpret. This option will preserve the original source's indentation.
Is similar to -b but causes whitespace (blanks and tabs) to be totally ignored. E.g., “if ( a == b )” will compare equal to “if(a==b)”.

Directory comparison options:

If a file is found in only one directory, act as if it was found in the other directory too but was of zero size.
If a file is found only in dir2, act as if it was found in dir1 too but was of zero size.
Causes application of diff recursively to common subdirectories encountered.
name
Re-starts a directory diff in the middle, beginning with file name.
Causes diff to report files which are the same, which are otherwise not mentioned.
file
Exclude files and subdirectories from comparison whose basenames match lines in file. Multiple -X options may be specified.
pattern
Exclude files and subdirectories from comparison whose basenames match pattern. Patterns are matched using shell-style globbing via fnmatch(3). Multiple -x options may be specified.

If both arguments are directories, diff sorts the contents of the directories by name, and then runs the regular file diff algorithm, producing a change list, on text files which are different. Binary files which differ, common subdirectories, and files which appear in only one directory are described as such. In directory mode only regular files and directories are compared. If a non-regular file such as a device special file or FIFO is encountered, a diagnostic message is printed.

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.

If either file1 or file2 is ‘-’, the standard input is used in its place.

The default (without -e, -c, or -n options) output contains lines of these forms, where XX, YY, ZZ, QQ are line numbers respective of file order.

aYY
At (the end of) line XX of file1, append the contents of line YY of file2 to make them equal.
aYY,ZZ
Same as above, but append the range of lines, YY through ZZ of file2 to line XX of file1.
dYY
At line XX delete the line. The value YY tells to which line the change would bring file1 in line with file2.
dZZ
Delete the range of lines XX through YY in file1.
cYY
Change the line XX in file1 to the line YY in file2.
cZZ
Replace the range of specified lines with the line ZZ.
cZZ,QQ
Replace the range XX,YY from file1 with the range ZZ,QQ from file2.

These lines resemble ed(1) subcommands to convert file1 into file2. The line numbers before the action letters pertain to file1; those after pertain to file2. Thus, by exchanging a for d and reading the line in reverse order, one can also determine how to convert file2 into file1. As in ed(1), identical pairs (where num1 = num2) are abbreviated as a single number.

/tmp/diff.XXXXXXXX
Temporary file used when comparing a device or the standard input. Note that the temporary file is unlinked as soon as it is created so it will not show up in a directory listing.

The diff utility exits with one of the following values:

0
No differences were found.
1
Differences were found.
>1
An error occurred.

cmp(1), comm(1), diff3(1), ed(1), patch(1), sdiff(1)

James W. Hunt and M. Douglas McIlroy, An Algorithm for Differential File Comparison, Computing Science Technical Report, Bell Laboratories 41, June 1976.

The diff utility is compliant with the IEEE Std 1003.1-2008 (“POSIX.1”) specification.

The flags [-aDdIiLlNnPpqSsTtwXx] are extensions to that specification.

A diff command appeared in Version 6 AT&T UNIX.
November 24, 2015 OpenBSD-current