Systems & Programming
Shell 條件判斷與迴圈：test、if、case、for 與 while
Switch and loop in Shell, with `test` and `if else`.
Systems & Programming
Shell 變數教程：字串、陣列與 echo
Variables in Shell, string and array, echo command, variable reference.
Shell & CLI
Shell 脚本编写：变量、条件、循环、函数与重定向。