Comparing and Merging Files
2.3.2 Unified Format
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 `-U lines',
`--unified[=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.
At present, only GNU
diff can produce this format and
patch can automatically apply diffs in this
format. For proper operation,
patch typically needs at
least three lines of context.