Diff, Patch, and Friends

“Kernel patches” may sound like magic, but the two tools used to create and apply patches are simple and easy to use—if they weren't, some Linux developers would be too lazy to use them...

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:

  • The file names and last dates of modification are shown in a “header” at the top. The dates may not mean anything if you are comparing files that have been passed back and forth by e-mail, but they become very useful in other circumstances.

  • The file names (in this case, 1 and 2—are preceded by --- and +++.

  • After the header comes a line that includes numbers. We will discuss that line later.

  • The lines that did not change between files are shown preceded by spaces; those that are different in the different files are shown preceded by a character which shows which file they came from. Lines which exist only in a file whose name is preceded by --- in the header are preceded by a - character, and vice-versa for lines preceded by a + character. Another way to remember this is to see that the lines preceded by a - character were removed from the first (---) file, and those preceded by a + character were added to the second (+++) file.

  • Three spelling changes have been made: “desendendi” has been corrected to “descendendi”, “non nunquam” has been corrected to “nonnunquam”, and “no” has been corrected to “eo”.

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.

______________________

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The first comment asked for

Anonymous's picture

The first comment asked for reasons. Here are a few.

1. Years of fighting with graphical tools can lead one to learn command line tools.

2. A major difference between the two is the former can be used in scripts.

3. Another significant difference is robustness. It's generally much easier to crash a graphical tool than a well-designed command line one.

4. Lastly, command line tools most often contain less code and require less system resources to operate, often making them better suited to work faster and more efficiently than graphical tools.

Smaller, faster and more reliable. Built for automation. A higher learning curve may be a trade off for higher performance.

Why?

Jonathan Allen's picture

Why in the world would I want to fight with command line tools when trying to compare and merge versions? Graphical diff and merge tools have existed for decades.

Now if you can show me a diff and merge tool that understands the syntax of the file being compared and I'll be far more interested. I have seen merge tools eat a brace way too often.

I think the images for figure

Anonymous's picture

I think the images for figures 1 and 2 are broken. When I follow the links to view these figures, the images don't show up in my browser window.

using diff and patch

John's picture

If I use diff -Naur to generate a patch symbolic links are not respected. e.g. see below;

How can I get patch/diff to respect symbolic links?

>---------------------------------------------------<
$ ll foo*
foo:
total 4
lrwxrwxrwx 1 foo users 5 Jun 9 13:04 link -> stuff
-rw-r--r-- 1 foo users 12 Jun 9 13:04 stuff

foo2:
total 4
lrwxrwxrwx 1 foo users 5 Jun 9 13:04 link -> stuff
-rw-r--r-- 1 foo users 12 Jun 9 13:04 stuff

$ diff -Naur foo foo2 > patch
$ mv foo2 foo2.orig
$ patch -p0 < patch
patching file foo/link
patching file foo/stuff

$ ll foo
total 8
-rw-r--r-- 1 foo users 27 Jun 9 13:08 link
-rw-r--r-- 1 foo users 27 Jun 9 13:08 stuff

>---------------------------------------------------<

Working with directories

Anonymous's picture

This article is missing info on patching multiple files.

See here: http://lists.gnu.org/archive/html/help-gnu-utils/2004-06/msg00024.html for examlpe.

Re: Diff, Patch, and Friends

Anonymous's picture

Nice article! I now link to it from
my "Howto contribute to an open source project" tutorial,
www.kegel.com/academy/opensource.html

Re: Diff, Patch, and Friends

Anonymous's picture

You may want to link to the manpages for the free versions
of diff and patch too, instead of only the GNU versions:

http://mirbsd.bsdadvocacy.org/man1/diff.htm
http://mirbsd.bsdadvocacy.org/man1/patch.htm

To the editor: ed(1) is by no means obsolete; I'm actually
faster with ed than with vi (whose modus operandi is
cruelly to a wordstar-compatible editor user like me).

http://mirbsd.bsdadvocacy.org/man1/ed.htm

You didn't mention diff3 either, did you?

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