Tip 10: diff and patch

Suppose you have two scenarios:

1) Someone patched a bug and you want to apply their changes to your code (which may already be modified). 2) You fix a bug in someones code but you don't have access to commit to their repository.

The solution for both of these is to use diff and patch.

diff

The unix "diff" command shows the incremental differences in a text file or in a subdirectory of files. This is handy because you can only transfer what changed (and a little more for context) rather than redistribute everything. To get the differences between your code and a directory of the original code, you can simply do:

diff -rupN original/ new/ > original.patch

The -r specifies recursive, the -u specifies to output context lines, and the -p tells it to give specific C function names for context too. -N says to treat absent files as empty.

patch

Once you have this difference file, you can apply these differences using the "patch" program. To update the original in our example, simply do a:

cd original
patch < ../original.patch

Sometimes there will be issues with path names in the patch. For example, if your original directory was in /user/bob/research and the new one was not, you can chop off these paths using the -p option to patch. For example,

patch -p4 < ../original.patch

would use the directories under research. More in the man page.

SVN and CVS

Subversion comes with a handy way to generate a diff with another SVN revision. At any time, if you type:

svn diff

it will show the diff between your copy and the current one in the repository. You can simply send this to someone else as a patch file using:

svn diff -upN > original.patch

You can do a similar patch in CVS with:

cvs diff -upN > original.patch

and then apply the patch with:

patch -p0 < original.patch

Note, that you could use this to "undo" local changes and apply them back again later!