Quick-n-Dirty Guide to Creating and Applying diff-style Patches

1) DO YOU HAVE DIFF AND PATCH?
------------------------------

In a shell, type:

which diff
which patch

This should return the paths to diff and patch, probably /usr/bin/diff
and /usr/bin/patch, respectively. If it finds them, continue to Step
2. If it does not, you have a serious problem, as diff and patch are
two of the most fundamental utilities in a Unix system. Hopefully,
your $PATH environment variable just does not contain the proper paths
to the binaries. Try typing in a shell:

for i in /bin /usr/bin /usr/local; do
    find $i -name diff
    find $i name patch
done

If this returns a full path to diff and patch, note that path, then
add it to your $PATH by typing:

export PATH=$PATH:diff_path:patch_path

where diff_path is the path to the diff binary (take the /diff off
the end of what the find command reported for diff) and patch_path
is the path to the patch binary (take the /patch off the end of what
the find command reported for patch). Now, repeat this step, and the
'which' commands should find both diff and patch. If they do, continue
to Step 2. If not, try contacting your local LUG for help. *** Link to
the Help Me! QND ***


2) CREATING PATCHES WITH DIFF
-----------------------------

In a shell, change to the directory where the file you wish to make a
patch for is located. Copy the original file (before your edits) to
the same directory (you *did* make a backup copy of it before you
edited it, right?). Now, type in the shell:

diff -uN original_file new_file >patch.original_file

where original_file is the filename of the original file and new_file
is the filename of the new file (the one containing your edits).

This technique also works for creating a patch for a whole directory
tree (e.g. creating a Linux kernel patch). Just add the -r flag to
your diff command:

diff -uNr original_dir new_dir >patch.original_dir

Here is a concrete example of creating a patch against the vanilla
Linux kernel's source tree:

cd /usr/src
diff -uNr linux-2.4.19 linux-2.4.19-my_version \
          >patch.linux-2.4.19-my_version

That is all there is to creating patches with diff. Move on to Step 3
to see how to apply them with patch.


3) APPLYING PATCHES WITH PATCH
------------------------------

Copy the patch that you generated in Step 2 to the directory
containing the file to which you want to apply the patch. Now, type in
your shell:

patch <patch_file

where patch_file is the filename of the patch file. Note that the
filename of the file to be patched must be the same as it was in Step
2.

If you want to apply a patch to a whole directory tree, add a -p1 to
your patch command:

patch -p1 <patch_file

Here is a concrete example of applying a patch to your Linux kernel
source tree:

cd /usr/src
patch -p1 <patch.linux-2.4.19-my_version

Congratulations, you are finished.



REFERENCES
----------

For more information on diff and patch, see:

http://www.gnu.org/software/diffutils/diffutils.html
http://www.gnu.org/manual/diffutils/html_mono/diff.html