One simple solution would be to have the run.sh symlink a fixed name (say hull-last_static.log) to the output from its last run (perhaps when it finishes as its final action). Then your code would just use readlink to obtain the last run's logfile name, or simply open the symlink directly.

Alternately you could use opendir/readdir and sort the results by the modification time (obtained with stat to find the newest file.