Recently, a friend of mine returned to a manuscript after several months. The manuscript had half a dozen versions, and she could no longer remember how each one differed. Listening to her problem, I had a blinding flash of the obvious: diff [1], and related commands like diff3 [2] and merge [3], can be as much help to her as they have been to coders over the decades.

diff is a utility that compares two files line by line. For coders, diff is a command that defines Unix-like operating systems like Linux. Although file comparison utilities are as old as Unix, diff itself was first released in 1974 for text files, with support for binary files added later. diff presents users with a summary of the comparison in two different formats, which can also be merged into a single file. diff3 [2], a similar utility, operates in a like manner on three files, although it does not support binary formats. More sophisticated tools like patch have been developed, but diff is still installed by default in many distributions, and its output files, or diffs, remain a standard name for any patch, just as the grep command has given its name to any file search.

Basic Comparisons

Typing info diff (the man page is incomplete) quickly shows how diff can be as useful to a writer as a programmer. The command follows the standard format of a command followed by options and two files. The first file is the original, or any file if, as in my friend's case, the original is unknown or irrelevant:

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