shell的内建命令

  bash,  :, ., [, alias, bg, bind, break, builtin, cd, command, compgen,
       complete, continue, declare, dirs, disown, echo, enable,  eval, exec,
       exit,  export,  fc, fg, getopts, hash, help, history, jobs, kill, let,
       local, logout, popd, printf, pushd, pwd, read, readonly, return,  set,
       shift,  shopt,  source, suspend,  test,  times,  trap, type, typeset,
       ulimit, umask, unalias, unset, wait  -  bash  built-in  commands