The unified output format is a variation on the context format that is more compact because it omits redundant context lines. To select this output format, use the --unified[=lines] (-U lines), or -u option. The argument lines is the number of lines of context to show. When it is not given, it defaults to three.
In the early 1990s, only GNU
diff could produce this format and
patch could automatically apply diffs in this
format. For proper operation,
patch typically needs at
least three lines of context.