Diff, Patch, and Friends

When someone changes a file that other people have copies of (source code, documentation, or just about any other text file), they often send patches instead of (or in addition to) making the entire new file available. If you have the old file and the patches, you might wish that you could have a program apply the patches. You might think that normal diff format, which was made to look like input to the ed program, would be the best way to accomplish this.

As it turns out, this is not true.

A program called patch has been written which is specifically designed to apply patches to files (change the files as specified in the patch). It correctly recognizes all the formats of patches and applies them. With unified and context diffs, patch can usually apply patches, even if lines have been added or removed from the file, by looking for unchanged context lines. Only if the context lines have themselves been changed is patch likely to fail.

To apply patches with patch, you normally have a file containing the patch (we'll call it patchfile), and then run patch:

patch < patchfile

Patch is very verbose. If it gets confused by anything, it stops and asks you in English (it was written by a linguist, not a computer scientist) what you want to do. If you want to learn more about patch, the man page is unusually readable.

If you read the RCS article in the May issue (Take Command: Keeping Track of Change, LJ #25, May 1996), you may have noticed that the article talked a bit about a program called rcsdiff. rcsdiff is really just a front end to diff. That is, it looks for arguments that it understands (such as revision numbers and the filename) and prepares two files representing the two versions of the file you are examining. It then calls diff with the remaining options. The RCS article used -u to get the unified format without explaining what it meant, but you can use -c to get context diffs, or use -U lines to choose the amount of context you get in a unified diff, or use any other diff options you like.

You may notice that rcsdiff produces more verbose output than normal diff. From the RCS article:

rcsdiff -u -r1.3 -r1.6 foo
==============================================
RCS file: foo,v
retrieving revision 1.3
retrieving revision 1.6
diff -u -r1.3 -r1.6
--- foo 1996/02/01 00:34:15     1.3
+++ foo 1996/02/01 01:05:28     1.6
 -1,2 +1,6
 This is a test of the emergency
-RCS system.  This is only a test.
+RCS version control system.
+This is only a test.
+
+I'm now adding a few lines for
+the next version.

It looks just like a normal unified diff except for the first 5 lines.

This doesn't prevent you from sending patches to people. The patch program is extremely good about ignoring extraneous information. It can even ignore news or mail headers, extra comments written in a file outside a patch, and people's signatures following patches. Patch tells you when it is determining whether text is part of a patch or not by saying “Hmm...”

If you don't care how two files differ, but just want to know whether they differ, the cmp program will tell you. It works not only for text files, but also for binary files. In this example, the files 5 and 6 are different; 2 and 4 are the same:

cmp 5 6
5 6 differ: char 159, line 4
cmp 2 4

Notice that when two files are the same, cmp doesn't say anything at all. It only tells you explicitly if the files have been changed. For use in writing shell scripts, cmp also returns true if the files are the same and false if they don't, as shown by this shell session:

if cmp 5 6 ; then
  echo "same"
else
  echo "different"
fi
5 6 differ: char 159, line 4
different
if cmp 2 4 ; then
  echo "same"
else
  echo "different"
fi
same

There are several other programs with related functionality. In particular, diff3 can be used to merge together two different files that have both been edited from a common ancestor file. That common ancestor must exist in order for diff3 to work correctly.

The info pages which are shipped with diff are probably installed on your system. If you want to learn more about diff, try the command info diff or use info mode from within emacs or jed.

diff, wdiff, patch, and emacs are available via ftp from the canonical GNU ftp archive, prep.ai.mit.edu, in the directory /pub/gnu/