Builtin Commands

  [I/O]           read   echo   printf
                  readarray   mapfile
  [Run Code]      source .   eval   trap
  [Set Options]   set   shopt
  [Working Dir]   cd   pwd   pushd   popd   dirs
  [Completion]    complete   compgen   compopt   compadjust   compexport
  [Shell Process] exec   X logout 
                  umask   X ulimit   times
  [Child Process] jobs   wait   ampersand &
                  fg   X bg   X kill   X disown 
  [External]      test [   getopts
  [Introspection] help   hash   type   X caller
  [Word Lookup]   command   builtin
  [Interactive]   alias   unalias   history   X fc   X bind
X [Unsupported]   enable
