fax4CUPS consists of some PPD (PostScript Printer Definition) files and a shell scripts that acts as a back end for CUPS (the Common Unix Printing System). Essentially, you print with lpr and the fax is sent. fax4CUPS supports efax, mgetty-fax, capisuite and HylaFAX. To use the HylaFAX backend, you also need grep and sudo.