gistfile1.sh

# bash function
defcomp () {
  local name="$1" && shift
  local functionName="$( echo "${name}_completion" | tr - _ )"
  source <(cat <<EOM
$functionName () {
  local cur
  _get_comp_words_by_ref -n =: cur
  COMPREPLY=( \$(compgen -W "\$( $@ )" -- "\$cur") )
}
EOM
)
  command complete -F "$functionName" "$name"
}

# command line

% defcomp gemnames gem list --no-versions
% gemnames <tab>
addressable          ffi                  highline             mini_portile         rb-appscript         rspec-mocks          text-hyphen
bigdecimal           formatador           httparty             minitest             rb-fsevent           ruby2ruby            thor
builder              gherkin              httpauth             multi_json           rb-inotify           ruby_parser          tins
bundler              github               io-console           multi_test           rb-kqueue            safe_yaml            tty
coderay              github_api           json                 multi_xml            rdoc                 sexp_processor       vcr
colorize             github_cli           json_pure            multipart-post       reek                 simplecov            webmock
coveralls            growl                jwt                  nokogiri             rest-client          simplecov-html       
crack                growl_notify         launchy              oauth2               ripper_ruby_parser   slop                 
cucumber             guard                listen               pry                  roodi                spruz                
descendants_tracker  guard-cucumber       lumberjack           psych                rspec                term-ansicolor       
diff-lcs             guard-rspec          method_source        rack                 rspec-core           test-unit            
faraday              hashie               mime-types           rake                 rspec-expectations   text-format        
% gemnames f<tab>
faraday     ffi         formatador