Perto da parte inferior da página man bash é a seção SHELL BUILTIN COMMANDS onde diz
compgen [option] [word]
Generate possible completion matches for word according to the
options, which may be any option accepted by the complete builtin
with the exception of -p and -r, and write the matches to the stan-
dard output. When using the -F or -C options, the various shell
variables set by the programmable completion facilities, while
available, will not have useful values.
The matches will be generated in the same way as if the pro-
grammable completion code had generated them directly from a com-
pletion specification with the same flags. If word is specified,
only those completions matching word will be displayed.
The return value is true unless an invalid option is supplied, or
no matches were generated.
Logo abaixo está o comando complete
builtin que explica os switches.