The normal output contains lines of these forms:

5

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. In this mode, diff prints - and + instead of < and > because the former are easier to distinguish when mixed. The -a flag displays the entire file as context.

Except in rare circumstances, diff finds a smallest sufficient set of file differences.

When running diff on directories, the notion of what is a text file is open to debate.