Linux共有七种执行等级:
run level 0:hait(关机)
run level 1:single-user mode(mode for administrative tasks)
run level 2:multi-user mode
run level 3:multi-user mode(纯命令行模式)
run level 4:not used/user-definable
run level 5:含有图形界面模式
run level 6:重启