From help compgen:$ help compgencompgen: compgen [-abcdefgjksuv] [-o option]  [-A action] [-G globpat] [-W wordlist]  [-F function] [-C command] [-X filterpat] [-P prefix] [-S suffix] [word]    Display possible completions depending on the options.    Intended to be used from within a shell function generating possible    completions.  If the optional WORD argument is supplied, matches against    WORD are generated.    Exit Status:    Returns success unless an invalid option is supplied or an error occurs.So, What do options [-abcdefgjksuv] stands for?In other words I want to know how to use all options. 
Understand `compgen` builtin command
bash;documentation;shell builtin;compgen
Options for compgen command are the same as complete, except -p and -r. From compgen man page:compgen 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 standard outputFor options [abcdefgjksuv]:-a means Names of alias-b means Names of shell builtins-c means Names of all commands-d means Names of directory-e means Names of exported shell variables-f means Names of file and functions-g means Names of groups-j means Names of job-k means Names of Shell reserved words-s means Names of service-u means Names of userAlias names-v means Names of shell variablesYou can see complete man page here.