View source for 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 ==

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 > original.patch