Diff

diff is one of the oldest UNIX commands (was included in UNIX around 1976). It compares the contents of the two files source file and target file (modified version) and produces "delta" -- lines that are changed or absent in either of files.  It was written by Hunt and McIlroy and based on the algorithm for file comparison that they created (see  J. W. Hunt and M. D. McIlroy, An algorithm for differential file comparison, Bell Telephone Laboratories CSTR #41 (1976), PostScript (text edited from OCR, figures redrawn)).  While it is the first it is still one of the best.

Both files in a classic UNIX diff are assumed to be text files. As file difference is an illusive concept that simplest approach is consider line indivisible and compute so called "longest common subsequence of lines"(lcs). Then  anything not in this lcs is declared to belong to the difference set -- the minimal set of lines that needs to be changed for the transformation of source to the target.

But both symbol based diff and diff based on words can be easily obtained by using appropriate filters before applying diff. String diff is essentially the problem that is studied in all algorithms for text file comparison as any text file can be trivially converted to the string is some alphabet with each line represented by one letter of this alphabet.

Difference by word is also a trivial modification of the basic program. Essentially you first need to covert the text into "one word per line" format and then use line-based diff. Several such modifications exists (wdiff, dwdiff, see below) or can be easily written in any scripting language.

diff proved to be useful in so many cases that it is difficult to enumerate them.  First and foremost this tool is used to discover differences between versions of a text file. In this role it is useful in keeping track of the evolution of a document or of a program. For example, often, a programmer needs to debug a software program with the codebase that contains several thousand (or even hundred thousand) lines. if the problem is present only is this version and is not present in older versions then you can diff the sources and use code browsers for the difference set to try to pinpoint the source of the problem. This approach can help dramatically narrow the slice of the program that probably contains the problematic code.

It is also used as a file compression method, since many versions of a (long) file can be represented by storing one (long) version of it and many (short) scripts of transforming the older version into newer versions of the same file. Another application is so called approximate string matching used, for instance, for the detection of misspelled words.  

If we use diff for program understanding (which is comparing two versions of the program is usually about), then along with diff tools, powerful tools for source code browsing and, especially, slicers, are also necessary. Some are built into the IDE and some are standalone. The grand daddy of all slicers is Xedit  that has built-in slicing capabilities since 1970th (famous "all" command).  Among code browsers the cscope for C/C++ created by AT&T is probably one of the first useful implementations that address the problem.

diff also lead to creation of news class programs that use generation of the difference set as a part of their operation. The most popular among those is patch written by Larry Wall.  Older version control systems were little more then diff and several shell scripts.

The external diff command compares two text files for differences. It determines which lines must be changed to make the two files identical. The diff command scans the two files and indicates editing changes that must be made to the first file to make it identical to the second file. The changes can be saved for use as an ed command script to change the first file. It can also compare directories. System V provides a second command to compare directories; refer to the dircmp command.

     diff [ -lrs ] [ -S name ] [ -cefhn ] [ -biwt ] dir1 dir2

     diff [ -cefhn ] [ -biwt ] file1 file2
     diff [ -cefhn ] [ -biwt ] - file2
     diff [ -cefhn ] [ -biwt ] file1 -

     diff [ -D string ] [ -biw ] file1 file2
     diff [ -D string ] [ -biw ] - file2
     diff [ -D string ] [ -biw ] file1 -

     diff [ -C string ] [ -biw ] file1 file2
     diff [ -C string ] [ -biw ] - file2
     diff [ -C string ] [ -biw ] file1 -

Options

The following list describes the options and their arguments that may be used to control how diff functions.

Comparison control options:

                    file1:    A     sample line of text here
                    file2:    A     sample line of text here

                     THE BIG dog ran fast.
                    and
                     The big dog ran fast.
                    match as equal lines.

                      if ( x == y )
                    and
                      if(x==y)
                    compare as equals.

Directory comparison options:

Mutually exclusive options:

Two arguments may be passed to the diff command. They can be iether files or directories:

                    diff adir afile

NEWS CONTENTS

• 20170827 : Diff A Directory Recursively, Ignoring All Binary Files ( Aug 27, 2017 , stackoverflow.com )
• 20170204 : Quickly find differences between two directories ( Feb 04, 2017 , www.cyberciti.biz )
• 20080911 : ColorDiff ( ColorDiff, Sep 11, 2008 )
• 20080911 : LinuxDevCenter.com -- Checking Differences with diff ( LinuxDevCenter.com -- Checking Differences with diff, )
• 20080911 : LinuxDevCenter.com -- Context diffs ( LinuxDevCenter.com -- Context diffs, )
• 20080911 : LinuxDevCenter.com -- ex Scripts Built by diff ( LinuxDevCenter.com -- ex Scripts Built by diff, )

Old News

[Aug 27, 2017] Diff A Directory Recursively, Ignoring All Binary Files

It is now possible to use -r to recursively compare directories

Aug 27, 2017 | stackoverflow.com

diff -r dir1/ dir2/ | sed '/Binary\ files\ /d' >outputfile

This recursively compares dir1 to dir2, sed removes the lines for binary files (begins with " Binary files "), then it's redirected to the outputfile.

-- Shannon VanWagner

[Feb 04, 2017] Quickly find differences between two directories

You will be surprised, but GNU diff use in Linux understands the situation when two arguments are directories and behaves accordingly

Feb 04, 2017 | www.cyberciti.biz

The diff command compare files line by line. It can also compare two directories:

# Compare two folders using diff ##
diff /etc /tmp/etc_old  

Rafal Matczak September 29, 2015, 7:36 am

§ Quickly find differences between two directories
And quicker:

 diff -y <(ls -l ${DIR1}) <(ls -l ${DIR2})  

[Sep 11, 2008] ColorDiff

Perl-based wrapper

ColorDiff is a wrapper for diff. It produces the same output as diff, but with coloured syntax highlighting at the commandline to improve readability. The output is similar to how a diff-generated patch might appear in Vim or Emacs with the appropriate syntax highlighting options enabled. The colour schemes can be read from a central configuration file or from a local user ~/.colordiffrc file.

LinuxDevCenter.com -- Checking Differences with diff

The diff command displays different versions of lines that are found when comparing two files. (There's also a GNU version on the CD-ROM.) It prints a message that uses ed-like notation (a for append, c for change, and d for delete) to describe how a set of lines has changed. This is followed by the lines themselves. The < character precedes lines from the first file and > precedes lines from the second file.

LinuxDevCenter.com -- Context diffs

The output of diff -e shows compact formats with just the differences between the files. But, in many cases, context diff listings are more useful. Context diffs show the changed lines and the lines around them. (This can be a headache if you're trying to read the listing on a terminal and there are many changed lines fairly close to one another: the context will make a huge "before" section, with the "after" section several screenfuls ahead. In that case, the more compact diff formats can be useful.)

On many versions of diff (including the GNU version used on Linux), the -c option shows context around each change. By itself, -c shows three lines above and below each change. Here's an example of a C++ file before and after some edits; the -c2 option shows two lines of context:

LinuxDevCenter.com -- ex Scripts Built by diff

The -e option of diff produces an editing script usable with either ex or ed, instead of the usual output. This script consists of a sequence of a (add), c (change), and d (delete) commands necessary to re-create file2 from file1 (the first and second files specified on the diff command line).

Obviously there is no need to completely re-create the first file from the second, because you could do that easily with cp. However, by editing the script produced by diff, you can come up with some desired combination of the two versions.

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Diff - Wikipedia, the free encyclopedia

diff - Linux Command - Unix Command

GNU Diff

diff [options] from-file to-file

If from-file is a directory and to-file is not, diff compares the file in from-file whose file name is that of to-file, and vice versa. The non-directory file must not be -.

If both from-file and to-file are directories, diff compares corresponding files in both directories, in alphabetical order; this comparison is not recursive unless the -r or --recursive option is given. diff never compares the actual contents of a directory as if it were a file. The file that is fully specified may not be standard input, because standard input is nameless and the notion of ``file with the same name'' does not apply.

diff options begin with -, so normally from-file and to-file may not begin with -. However, -- as an argument by itself treats the remaining arguments as file names even if they begin with -.

Options

-lines

Show lines (an integer) lines of context. This option does not specify an output format by itself; it has no effect unless it is combined with -c or -u. This option is obsolete. For proper operation, patch typically needs at least two lines of context.

-a

Treat all files as text and compare them line-by-line, even if they do not seem to be text.

-b

Ignore changes in amount of white space.

-B

Ignore changes that just insert or delete blank lines.

--brief

Report only whether the files differ, not the details of the differences.

-c

Use the context output format.

-C lines

--context[=lines]

Use the context output format, showing lines (an integer) lines of context, or three if lines is not given. For proper operation, patch typically needs at least two lines of context.

--changed-group-format=format

Use format to output a line group containing differing lines from both files in if-then-else format.

-d

Change the algorithm to perhaps find a smaller set of changes. This makes diff slower (sometimes much slower).

-D name

Make merged if-then-else format output, conditional on the preprocessor macro name.

-e

--ed

Make output that is a valid ed script.

--exclude=pattern

When comparing directories, ignore files and subdirectories whose basenames match pattern.

--exclude-from=file

When comparing directories, ignore files and subdirectories whose basenames match any pattern contained in file.

--expand-tabs

Expand tabs to spaces in the output, to preserve the alignment of tabs in the input files.

-f

Make output that looks vaguely like an ed script but has changes in the order they appear in the file.

-F regexp

In context and unified format, for each hunk of differences, show some of the last preceding line that matches regexp.

--forward-ed

Make output that looks vaguely like an ed script but has changes in the order they appear in the file.

-h

This option currently has no effect; it is present for Unix compatibility.

-H

Use heuristics to speed handling of large files that have numerous scattered small changes.

--horizon-lines=lines

Do not discard the last lines lines of the common prefix and the first lines lines of the common suffix.

-i

Ignore changes in case; consider upper- and lower-case letters equivalent.

-I regexp

Ignore changes that just insert or delete lines that match regexp.

--ifdef=name

Make merged if-then-else format output, conditional on the preprocessor macro name.

--ignore-all-space

Ignore white space when comparing lines.

--ignore-blank-lines

Ignore changes that just insert or delete blank lines.

--ignore-case

Ignore changes in case; consider upper- and lower-case to be the same.

--ignore-matching-lines=regexp

Ignore changes that just insert or delete lines that match regexp.

--ignore-space-change

Ignore changes in amount of white space.

--initial-tab

Output a tab rather than a space before the text of a line in normal or context format. This causes the alignment of tabs in the line to look normal.

-l

Pass the output through pr to paginate it.

-L label

--label=label

Use label instead of the file name in the context format and unified format headers.

--left-column

Print only the left column of two common lines in side by side format.

--line-format=format

Use format to output all input lines in in-then-else format.

--minimal

Change the algorithm to perhaps find a smaller set of changes. This makes diff slower (sometimes much slower).

-n

Output RCS-format diffs; like -f except that each command specifies the number of lines affected.

-N

--new-file

In directory comparison, if a file is found in only one directory, treat it as present but empty in the other directory.

--new-group-format=format

Use format to output a group of lines taken from just the second file in if-then-else format.

--new-line-format=format

Use format to output a line taken from just the second file in if-then-else format.

--old-group-format=format

Use format to output a group of lines taken from just the first file in if-then-else format.

--old-line-format=format

Use format to output a line taken from just the first file in if-then-else format.

-p

Show which C function each change is in.

-P

When comparing directories, if a file appears only in the second directory of the two, treat it as present but empty in the other.

--paginate

Pass the output through pr to paginate it.

-q

Report only whether the files differ, not the details of the differences.

-r

When comparing directories, recursively compare any subdirectories found.

--rcs

Output RCS-format diffs; like -f except that each command specifies the number of lines affected.

--recursive

When comparing directories, recursively compare any subdirectories found.

--report-identical-files

-s

Report when two files are the same.

-S file

When comparing directories, start with the file file. This is used for resuming an aborted comparison.

--from-file=file

Compare file to all operands. file can be a directory.

--to-file=file

Compare all operands to file. file can be a directory.

--sdiff-merge-assist

Print extra information to help sdiff. sdiff uses this option when it runs diff. This option is not intended for users to use directly.

--show-c-function

Show which C function each change is in.

--show-function-line=regexp

In context and unified format, for each hunk of differences, show some of the last preceding line that matches regexp.

--side-by-side

Use the side by side output format.

--speed-large-files

Use heuristics to speed handling of large files that have numerous scattered small changes.

--starting-file=file

When comparing directories, start with the file file. This is used for resuming an aborted comparison.

--suppress-common-lines

Do not print common lines in side by side format.

-t

Expand tabs to spaces in the output, to preserve the alignment of tabs in the input files.

-T

Output a tab rather than a space before the text of a line in normal or context format. This causes the alignment of tabs in the line to look normal.

--text

Treat all files as text and compare them line-by-line, even if they do not appear to be text.

-u

Use the unified output format.

--unchanged-group-format=format

Use format to output a group of common lines taken from both files in if-then-else format.

--unchanged-line-format=format

Use format to output a line common to both files in if-then-else format.

--unidirectional-new-file

When comparing directories, if a file appears only in the second directory of the two, treat it as present but empty in the other.

-U lines

--unified[=lines]

Use the unified output format, showing lines (an integer) lines of context, or three if lines is not given. For proper operation, patch typically needs at least two lines of context.

-v

--version

Output the version number of diff.

-w

Ignore white space when comparing lines.

-W columns

--width=columns

Use an output width of columns in side by side format.

-x pattern

When comparing directories, ignore files and subdirectories whose basenames match pattern.

-X file

When comparing directories, ignore files and subdirectories whose basenames match any pattern contained in file.

-y

Use the side by side output format.

Etc

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