This manual describes how to compare and merge files using GNU diff and patch. Comparing and merging files is a common activity for software developers. These programs make it easy to find and apply changes. GNU diff is part of the GNU diffutils package, a complete set of programs for handling differences between groups of files. GNU patch allows these differences to be distributed in an efficient format. This is a printed copy of the offical GNU diffutils manual. It documents all the diffutils programs (diff, cmp, sdiff, diff3), plus GNU patch. GNU diff and patch are free software. For each copy of this manual sold, $1 will be donated to the Free Software Foundation.