*nix command of the day:

diff -Naur old new > new.patch

Where “old” and “new” are files or directories to patch.

Example (a Drupal patch for IDs duplication):

$ patch -p0<fapi-5.x-dupe-id_111719-151.patch
patching file includes/form.inc
$

Creating patch for Drupal:
diff -up original.php new.php > filename.patch

another example:

diff -up path/to/file/example.module path/to/file/exampleNew.module > mypatchname.patch

See also:

• How to create and use a patch in Linux
• Creating patches in Drupal