在 RHEL/CentOS 7 之前，系統(tǒng)啟動是經(jīng)由 init 所負(fù)責(zé)，從 RHEL/CentOS 7 開始已改用 systemd 處理系統(tǒng)運作階層（Run Level），而傳統(tǒng) runlevel 已改為 target 表示。
本文介紹如何使用 systemctl 切換不同的運作層級與設(shè)定預(yù)設(shè)層級。
列出可用的運作層級
$ sudo systemctl list-units --type target
[sudo] password for $USER:
UNIT LOAD ACTIVE SUB DESCRIPTION
basic.target loaded active active Basic System
cryptsetup.target loaded active active Local Encrypted Volumes
getty.target loaded active active Login Prompts
graphical.target loaded active active Graphical Interface
local-fs-pre.target loaded active active Local File Systems (Pre)
local-fs.target loaded active active Local File Systems
multi-user.target loaded active active Multi-User System
network.target loaded active active Network
paths.target loaded active active Paths
remote-fs.target loaded active active Remote File Systems
slices.target loaded active active Slices
sockets.target loaded active active Sockets
swap.target loaded active active Swap
sysinit.target loaded active active System Initialization
timers.target loaded active active Timers
LOAD = Reflects whether the unit definition was properly loaded.
ACTIVE = The high-level unit activation state, i.e. generalization of SUB.
SUB = The low-level unit activation state, values depend on unit type.
15 loaded units listed. Pass --all to see loaded but inactive units, too.
To show all installed unit files use 'systemctl list-unit-files'.
顯示目前系統(tǒng)預(yù)設(shè)運作層級
$ sudo systemctl get-default
graphical.target
切換運作層級至文字模式（runlevel 3）
$ sudo systemctl isolate multi-user.target
切換運作層級至 GUI 模式（runlevel 5）
$ sudo systemctl isolate graphical.target
設(shè)定開機啟動至文字模式（runlevel 3）
$ sudo systemctl set-default multi-user.target
設(shè)定開機啟動至 GUI 模式（runlevel 5）
$ sudo systemctl set-default graphical.target
