A diff is a compact summary of the
differences (hence the name “diff”) between two items. For
example, given two files, the Unix and Linux diff command compares the files line by line and
summarizes the deviations in a diff, as shown in Example 8-1. In the example, initial is one version of some prose and
rewrite is a subsequent revision. The
-u option produces a unified diff, a
standardized format used widely to share modifications.
Example 8-1. Simple Unix diff
cat rewriteNow is the time Today is the time For all good men For all good men To come to the aid And women Of their country. To come to the aid Of their country. $
diff -u initial rewrite--- initial 1867-01-02 11:22:33.000000000 -0500 +++ rewrite 2000-01-02 11:23:45.000000000 -0500 @@ -1,4 +1,5 @@ -Now is the time +Today is the time For all good men +And women To come to the aid Of their country.
Let’s look at the diff in detail. In the header, the original
file is denoted by
--- and the new file
@@ line provides line number context for both file
versions. A line prefixed with a minus sign (
–) must be removed from the original file to
produce the new file. Conversely, a line with a leading plus sign (
+) must be added to the original file to produce
the new file. A line that begins with a space is the same in both files and
is provided by the
-u option as context.
By itself, a diff offers no reason or rationale for a change, nor does it justify the initial or final state. ...