Diff, Patch, and Friends

Diff is designed to show you the differences between files, line by line. It is fundamentally simple to use, but takes a little practice. Don't let the length of this article scare you; you can get some use out of diff by reading only the first page or two. The rest of the article is for those who aren't satisfied with very basic uses.

While diff is often used by developers to show differences between different versions of a file of source code, it is useful for far more than source code. For example, diff comes in handy when editing a document which is passed back and forth between multiple people, perhaps via e-mail. At Linux Journal, we have experience with this. Often both the editor and an author are working on an article at the same time, and we need to make sure that each (correct) change made by each person makes its way into the final version of the article being edited. The changes can be found by looking at the differences between two files.

However, it is hard to show off how helpful diff can be in finding these kinds of differences. To demonstrate with files large enough to really show off diff's capabilities would require that we devote the entire magazine to this one article. Instead, because few of our readers are likely to be fluent in Latin, at least compared to those fluent in English, we will give a Latin example from Winnie Ille Pu, a translation by Alexander Leonard of A. A. Milne's Winnie The Pooh (ISBN 0-525-48335-7). This will make it harder for the average reader to see differences at a glance and show how useful these tools can be in finding changes in much larger documents.

Quickly now, find the differences between these two passages:

Ecce Eduardus Ursus scalis nunc tump-tump-tump
occipite gradus pulsante post Christophorum
Robinum descendens. Est quod sciat unus et solus
modus gradibus desendendi, non nunquam autem
sentit, etiam alterum modum exstare, dummodo
pulsationibus desinere et de no modo meditari
possit. Deinde censet alios modos non esse. En,
nunc ipse in imo est, vobis ostentari paratus.
Winnie ille Pu.

Ecce Eduardus Ursus scalis nunc tump-tump-tump
occipite gradus pulsante post Christophorum
Robinum descendens. Est quod sciat unus et solus
modus gradibus descendendi, nonnunquam autem
sentit, etiam alterum modum exstare, dummodo
pulsationibus desinere et de eo modo meditari
possit. Deinde censet alios modos non esse. En,
nunc ipse in imo est, vobis ostentari paratus.
Winnie ille Pu.

You may be able to find one or two changes after some careful comparison, but are you sure you have found every change? Probably not: tedious, character-by-character comparison of two files should be the computer's job, not yours.

Use the diff program to avoid eyestrain and insanity:

diff -u 1 2
--- 1   Sat Apr 20 22:11:53 1996
+++ 2   Sat Apr 20 22:12:01 1996
 -1,9 +1,9
 Ecce Eduardus Ursus scalis nunc tump-tump-tump
 occipite gradus pulsante post Christophorum
 Robinum descendens. Est quod sciat unus et solus
-modus gradibus desendendi, non nunquam autem
+modus gradibus descendendi, nonnunquam autem
 sentit, etiam alterum modum exstare, dummodo
-pulsationibus desinere et de no modo meditari
+pulsationibus desinere et de eo modo meditari
 possit. Deinde censet alios modos non esse. En,
 nunc ipse in imo est, vobis ostentari paratus.
 Winnie ille Pu.

There are several things to notice here:

Perhaps the main thing to notice is that you didn't need this description of how to interpret diff's output in order to find the differences. It is rather easy to compare two adjacent lines and see the differences.