diff - differential file comparator
diff [ -acefmnbwr ] file1 ... file2
Diff tells what lines must be changed in two files to bring them into agreement. If one file is a directory, then a file in that directory with basename the same as that of the other file is used. If both files are directories, similarly named files in the two directories are compared by the method of diff for text files and cmp(1) otherwise. If more than two file names are given, then each argument is compared to the last argument as above. The -r option causes diff to process similarly named subdirectories recursively. When processing more than one file, diff prefixes file differences with a single line listing the two differing files, in the form of a diff command line. The -m flag causes this behavior even when processing single files.
The normal output contains lines of these forms:
n1 a n3,n4
n1,n2 d n3
n1,n2 c n3,n4
These lines resemble ed commands to convert file1 into file2. The numbers after the letters pertain to file2. In fact, by exchanging ’a’ for ’d’ and reading backward one may ascertain equally how to convert file2 into file1. As in ed, identical pairs where n1 = n2 or n3 = n4 are abbreviated as a single number.
Following each of these lines come all the lines that are affected in the first file flagged by ’<’, then all the lines that are affected in the second file flagged by ’>’.
The -b option causes trailing blanks (spaces and tabs) to be ignored and other strings of blanks to compare equal. The -w option causes all white-space to be removed from input lines before applying the difference algorithm.
The -n option prefixes each range with file: and inserts a space around the a, c, and d verbs. The -e option produces a script of a, c and d commands for the editor ed, which will recreate file2 from file1. The -f option produces a similar script, not useful with ed, in the opposite order. It may, however, be useful as input to a stream-oriented post-processor.
The -c option includes three lines of context around each change, merging changes whose contexts overlap. The -a flag displays the entire file as context.
Except in rare circumstances, diff finds a smallest sufficient set of file differences.
Exit status is the empty string for no differences, for some, and for trouble.
Editing scripts produced under the -e or -f option are naive about creating lines consisting of a single ’.’.
When running diff on directories, the notion of what is a text file is open to debate.