man cd -->查詢不到，所以會提示bash的內置命令
. alias bg bind break builtin caller cd command compgen complete compopt continue declare dirs disown echo enable eval exec exit export false fc fg getopts hash help history jobs kill let local logout mapfile popd printf pushd pwd read readonly return set shift shopt source suspend test times trap true type typeset ulimit umask unalias unset wait - bash built-in commands see bash(1)