The unified output format starts with a two-line header, which looks like this:
--- from-file from-file-modification-time +++ to-file to-file-modification-time
The time stamp looks like ‘2002-02-21 23:30:39.942229878 -0800’ to indicate the date, time with fractional seconds, and time zone. The fractional seconds are omitted on hosts that do not support fractional time stamps.
You can change the header's content with the --label=label option. See Alternate Names.
Next come one or more hunks of differences; each hunk shows one area where the files differ. Unified format hunks look like this:
@@ from-file-line-numbers to-file-line-numbers @@ line-from-either-file line-from-either-file...
If a hunk contains just one line, only its start line number appears. Otherwise its line numbers look like ‘start,count’. An empty hunk is considered to start at the line that follows the hunk.
If a hunk and its context contain two or more lines, its line numbers look like ‘start,count’. Otherwise only its end line number appears. An empty hunk is considered to end at the line that precedes the hunk.
The lines common to both files begin with a space character. The lines that actually differ between the two files have one of the following indicator characters in the left print column: