Making custom versions of packages in Ubuntu/Debian

Had to make some changes in the source code of a couple of packages on my system and decided to document the process for future reference. Surprisingly, it's way easier than I thought it would be.

As an example, suppose we want to make the default graphical viewer Eye of Gnome not jump back to the first picture after the last one in a directory (and, similarly, not jump in the opposite direction, either).

First, we need to get the source code for the program
$ mkdir eog; cd eog; apt-get source eog

Then let's install all the needed dependencies to compile the package:
$ sudo apt-get install build-dep eog

All right, time to make the changes we need. In this case, we need to slightly modify the corresponding part in eog_thumb_view_select_single() in src/eog-thumb-view.c to look like this:

        case EOG_THUMB_VIEW_SELECT_LEFT:
            gtk_tree_path_prev (path);
            break;
        case EOG_THUMB_VIEW_SELECT_RIGHT:
            if (gtk_tree_path_get_indices (path) [0] != n_items - 1)
                gtk_tree_path_next (path);
            break;

Now that we've made the changes, let's make our custom package:
$ dpkg-buildpackage -b

The program has been compiled and assembled in one neat package. All that is left is to install it, replacing the standard version:
$ sudo debi

Hooray, we have our custom version of Eye of Gnome installed and operational!

Now if I only knew a way to apply patches and rebuild the packages automatically every time they get an update..