Contributing to the Linux Kernel

To make changes in the kernel, you need to know all about the diff and patch commands.
Using diff

The actual usage of diff and patch is not complicated. While these commands, like many GNU tools, support many options allowing users to refine the way these tools work, these options are not actually required for everyday use. For more information on all the command-line options of these utilities, check out their info pages.

At its simplest, a diff command line for comparing two files would be:

diff old.txt new.txt > oldnew.patch

This will create a patch in the POSIX format that could later be applied to files similar to old.txt. Please note that the output of diff generally appears on standard output (STDOUT) and we have used redirection to get that information into the patch file. For GNU projects, we generally want the results in unified diff format, so we add the -u (or --unified) option to the command line:

diff -u old.txt new.txt > oldnew.patch
Generally, however, a comparison of two source trees is often desired. These trees would be multiple revisions of a single project or something similar. The command I generally prefer for this would be:
diff -ruN old new > oldnew.txt
In this example, I have added two new switches. The first, -r (or --recursive), indicates we want to take a recursive look at directories instead of files. The last switch, -N (or --new-file), indicates we do not want to ignore whether files have been added or removed from either set. In that case, if the new directory included a file called foo.txt but the old directory didn't, the patch would behave as if there was a zero-byte file called foo.txt in the old directory and add it into the patch.

To actually get good use out of the diff command as a form of revision control, a bit of legwork must be done first. I'll discuss this later on.

Using patch

Generally, once a diff is generated or downloaded, the process of patching the file is even simpler. Based on our first example above, we could do something like this:

patch < oldnew.patch

This command would read the patch file from the standard input and apply it to whatever files were in the current directory. Most patch formats include information on the name of the file being patched. In our first example, it would have specified that old.txt was the original file, and the patch command will look for a file by that name here. If that file could not be found, it would then prompt you for the name of the file to which the patch should be applied.

A number of things can go wrong during the patch process. Occasionally, a diff may be made backwards, or you may want to reverse a patch. By using the --reverse option to patch, you can make new.txt old.txt again. Additionally, the patch utility can detect whether the file being patched already contains the patch you are applying. In this case, patch will ask you whether you want it to reverse or attempt to apply the patch anyway. Finally, the patch could fail. If this happens, a file named old.txt.rej (or something similar) will be created, and patch will exit. At that point, it is up to you to look at the contents of the .rej file (which will be in a patch format) and manually apply the contents to the source. (I have occasionally gotten a reject file to apply by using a larger --fuzz value of patch, but this can lead to patch application errors and subtle bugs that you'll be scratching your head about later.) Once the problems are worked through, however, you will have effectively merged two sets of changes into one.

Obviously, these examples are a bit contrived. In real-world practice, patch files are generally not applied by the same people who made them. Instead, you will probably be either a provider of a specific patch (a source maintainer) or one who applies a specific patch (an end user).

Using patch-kernel

Although somewhat beyond the scope of this document, the Linux kernel actually includes a script which will aid you in keeping it up to date with the latest revisions. The patch-kernel script is located in the /usr/src/linux/scripts directory and will apply, in order, all the patches necessary to bring your kernel up to the latest revision, provided you have already downloaded them and put them in that directory. If you don't actually intend to participate in the Linux development effort, but just want to keep up with the latest and greatest source, this handy script will allow you to bypass the real workings of patch until you start developing. Once you start adding your own changes to your source tree, I recommend you use the manual method.