patch — Apply a patch made by diff
patch [options]
Note that patch reads from standard input by default. This can be overridden with the -i command, but I never bother.
To apply a simple patch
patch -p0 < simple.patch
To see what changes would be made by a patch without actually making the changes
patch -p0 --dry-run < simple.patch
To remove 3 of the leading directory names in the patch
patch -p3 < not-so-simple.patch
To reverse apply a bad patch
patch -p0 -R < bad.patch