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1 What Comparison Means

There are several ways to think about the differences between two files. One way to think of the differences is as a series of lines that were deleted from, inserted in, or changed in one file to produce the other file. diff compares two files line by line, finds groups of lines that differ, and reports each group of differing lines. It can report the differing lines in several formats, which have different purposes.

GNU diff can show whether files are different without detailing the differences. It also provides ways to suppress certain kinds of differences that are not important to you. Most commonly, such differences are changes in the amount of white space between words or lines. diff also provides ways to suppress differences in alphabetic case or in lines that match a regular expression that you provide. These options can accumulate; for example, you can ignore changes in both white space and alphabetic case.

Another way to think of the differences between two files is as a sequence of pairs of bytes that can be either identical or different. cmp reports the differences between two files byte by byte, instead of line by line. As a result, it is often more useful than diff for comparing binary files. For text files, cmp is useful mainly when you want to know only whether two files are identical, or whether one file is a prefix of the other.

To illustrate the effect that considering changes byte by byte can have compared with considering them line by line, think of what happens if a single newline character is added to the beginning of a file. If that file is then compared with an otherwise identical file that lacks the newline at the beginning, diff will report that a blank line has been added to the file, while cmp will report that almost every byte of the two files differs.

diff3 normally compares three input files line by line, finds groups of lines that differ, and reports each group of differing lines. Its output is designed to make it easy to inspect two different sets of changes to the same file.

These commands compare input files without necessarily reading them. For example, if diff is asked simply to report whether two files differ, and it discovers that the files have different sizes, it need not read them to do its job.

1.1 Hunks

When comparing two files, diff finds sequences of lines common to both files, interspersed with groups of differing lines called hunks. Comparing two identical files yields one sequence of common lines and no hunks, because no lines differ. Comparing two entirely different files yields no common lines and one large hunk that contains all lines of both files. In general, there are many ways to match up lines between two given files. diff tries to minimize the total hunk size by finding large sequences of common lines interspersed with small hunks of differing lines.

For example, suppose the file F contains the three lines ‘a’, ‘b’, ‘c’, and the file G contains the same three lines in reverse order ‘c’, ‘b’, ‘a’. If diff finds the line ‘c’ as common, then the command ‘diff F G’ produces this output:

< a
< b
> b
> a

But if diff notices the common line ‘b’ instead, it produces this output:

< a
> c
< c
> a

It is also possible to find ‘a’ as the common line. diff does not always find an optimal matching between the files; it takes shortcuts to run faster. But its output is usually close to the shortest possible. You can adjust this tradeoff with the --minimal (-d) option (see diff Performance Tradeoffs).

1.2 Suppressing Differences in Blank and Tab Spacing

The --ignore-tab-expansion (-E) option ignores the distinction between tabs and spaces on input. A tab is considered to be equivalent to the number of spaces to the next tab stop (see Preserving Tab Stop Alignment).

The --ignore-trailing-space (-Z) option ignores white space at line end.

The --ignore-space-change (-b) option is stronger than -E and -Z combined. It ignores white space at line end, and considers all other sequences of one or more white space characters within a line to be equivalent. With this option, diff considers the following two lines to be equivalent, where ‘$’ denotes the line end:

Here lyeth  muche rychnesse  in lytell space.   -- John Heywood$
Here lyeth muche rychnesse in lytell space. -- John Heywood   $

The --ignore-all-space (-w) option is stronger still. It ignores differences even if one line has white space where the other line has none. White space characters include tab, vertical tab, form feed, carriage return, and space; some locales may define additional characters to be white space. With this option, diff considers the following two lines to be equivalent, where ‘$’ denotes the line end and ‘^M’ denotes a carriage return:

Here lyeth  muche  rychnesse in lytell space.--  John Heywood$
  He relyeth much erychnes  seinly tells pace.  --John Heywood   ^M$

For many other programs newline is also a white space character, but diff is a line-oriented program and a newline character always ends a line. Hence the -w or --ignore-all-space option does not ignore newline-related changes; it ignores only other white space changes.

1.3 Suppressing Differences Whose Lines Are All Blank

The --ignore-blank-lines (-B) option ignores changes that consist entirely of blank lines. With this option, for example, a file containing

1.  A point is that which has no part.

2.  A line is breadthless length.
-- Euclid, The Elements, I

is considered identical to a file containing

1.  A point is that which has no part.
2.  A line is breadthless length.

-- Euclid, The Elements, I

Normally this option affects only lines that are completely empty, but if you also specify an option that ignores trailing spaces, lines are also affected if they look empty but contain white space. In other words, -B is equivalent to ‘-I '^$'’ by default, but it is equivalent to -I '^[[:space:]]*$' if -b, -w or -Z is also specified.

1.4 Suppressing Differences Whose Lines All Match a Regular Expression

To ignore insertions and deletions of lines that match a grep-style regular expression, use the --ignore-matching-lines=regexp (-I regexp) option. You should escape regular expressions that contain shell metacharacters to prevent the shell from expanding them. For example, ‘diff -I '^[[:digit:]]'’ ignores all changes to lines beginning with a digit.

However, -I only ignores the insertion or deletion of lines that contain the regular expression if every changed line in the hunk—every insertion and every deletion—matches the regular expression. In other words, for each nonignorable change, diff prints the complete set of changes in its vicinity, including the ignorable ones.

You can specify more than one regular expression for lines to ignore by using more than one -I option. diff tries to match each line against each regular expression.

1.5 Suppressing Case Differences

GNU diff can treat lower case letters as equivalent to their upper case counterparts, so that, for example, it considers ‘Funky Stuff’, ‘funky STUFF’, and ‘fUNKy stuFf’ to all be the same. To request this, use the -i or --ignore-case option.

1.6 Summarizing Which Files Differ

When you only want to find out whether files are different, and you don’t care what the differences are, you can use the summary output format. In this format, instead of showing the differences between the files, diff simply reports whether files differ. The --brief (-q) option selects this output format.

This format is especially useful when comparing the contents of two directories. It is also much faster than doing the normal line by line comparisons, because diff can stop analyzing the files as soon as it knows that there are any differences.

You can also get a brief indication of whether two files differ by using cmp. For files that are identical, cmp produces no output. When the files differ, by default, cmp outputs the byte and line number where the first difference occurs, or reports that one file is a prefix of the other. You can use the -s, --quiet, or --silent option to suppress that information, so that cmp produces no output and reports whether the files differ using only its exit status (see Invoking cmp).

Unlike diff, cmp cannot compare directories; it can only compare two files.

1.7 Binary Files and Forcing Text Comparisons

If diff thinks that either of the two files it is comparing is binary (a non-text file), it normally treats that pair of files much as if the summary output format had been selected (see Summarizing Which Files Differ), and reports only that the binary files are different. This is because line by line comparisons are usually not meaningful for binary files. This does not count as trouble, even though the resulting output does not capture all the differences.

diff determines whether a file is text or binary by checking the first few bytes in the file; the exact number of bytes is system dependent, but it is typically several thousand. If every byte in that part of the file is non-null, diff considers the file to be text; otherwise it considers the file to be binary.

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 --text (-a) 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 --brief (-q) option for this.

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 --verbose (-l) 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 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.

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