Many modern shells (including bash, tcsh, and zsh) support parameter and/or option tab-completion. This support usually comes from completion files, which contain the definitions for how tab completion will work for a certain command. Ports sometimes ship with their own completion files, or porters may have created them themselves.

When available, completion files should always be installed. It is not necessary to make an option for it. If an option is used, though, always enable it in OPTIONS_DEFAULT.

Do not register any dependencies on the shells themselves.