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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 another file. diff compares two files line by line, finds groups of lines that differ, and reports each group of differing lines. See Hunks for more documentation. 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. See Suppressing Differences in Blank and Tab Spacing, Suppressing Differences in Blank Lines, Suppressing Case Differences, Suppressing Lines Matching a Regular Expression and Summarizing Which Files Differ.

Another way to think of the differences between two files is as a sequence of pairs of characters that can be either identical or different. For more, see Binary Files and Forcing Text Comparisons. cmp reports the differences between two files character by character, instead of line by line. As a result, it is 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.

To illustrate the effect that considering changes character by character 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 character of the two files differs. For more, see diff Output Formats and Merging with patch.

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.