Version Control with Git

Chapter 8. Diffs

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 the following code. 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.

$ cat initial                $ cat rewrite
Now 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 connoted by --- and the new file by +++. The @@ 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. However, a diff offers more ...