21 管理服务-改变运行级别
21.1 Target和运行级别
systemd用目标（target）替代了运行级别的概念，提供了更大的灵活性，如您可以继承一个已有的目标，并添加其他服务，来创建自己的目标。表7列举了systemd下的目标和常见runlevel的对应关系。
表 7 运行级别和systemd目标
| 运行级别 | systemd目标（target） | 描述 |
|---|---|---|
| runlevel0.target，poweroff.target | 关闭系统。 | |
| 1, s, single | runlevel1.target，rescue.target | 单用户模式。 |
| 2, 4 | runlevel2.target，runlevel4.target，multi-user.target | 用户定义/域特定运行级别。默认等同于3。 |
| 3 | runlevel3.target，multi-user.target | 多用户，非图形化。用户可以通过多个控制台或网络登录。 |
| 5 | runlevel5.target，graphical.target | 多用户，图形化。通常为所有运行级别3的服务外加图形化登录。 |
| 6 | runlevel6.target，reboot.target | 重启系统。 |
| emergency | emergency.target | 紧急Shell |
21.2 查看系统默认启动目标
查看当前系统默认的启动目标，命令如下：
# systemctl get-default
例如示例命令如下：
[root@superman-21 ~]# systemctl get-default
multi-user.target
[root@superman-21 ~]#
21.3 查看当前系统所有的启动目标
查看当前系统所有的启动目标，命令如下：
# systemctl list-units --type=target
例如示例命令如下：
[root@superman-21 ~]# systemctl list-units --type=target
UNIT LOAD ACTIVE SUB DESCRIPTION
basic.target loaded active active Basic System
bluetooth.target loaded active active Bluetooth Support
getty.target loaded active active Login Prompts
local-fs-pre.target loaded active active Preparation for Local File Systems
local-fs.target loaded active active Local File Systems
multi-user.target loaded active active Multi-User System
network-online.target loaded active active Network is Online
network-pre.target loaded active active Preparation for Network
network.target loaded active active Network
nfs-client.target loaded active active NFS client services
paths.target loaded active active Path Units
remote-fs-pre.target loaded active active Preparation for Remote File Systems
remote-fs.target loaded active active Remote File Systems
rpc_pipefs.target loaded active active rpc_pipefs.target
rpcbind.target loaded active active RPC Port Mapper
slices.target loaded active active Slice Units
sockets.target loaded active active Socket Units
sound.target loaded active active Sound Card
sshd-keygen.target loaded active active sshd-keygen.target
swap.target loaded active active Swaps
sysinit.target loaded active active System Initialization
timers.target loaded active active Timer Units
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.
22 loaded units listed. Pass --all to see loaded but inactive units, too.
To show all installed unit files use 'systemctl list-unit-files'.
[root@superman-21 ~]#
21.4 改变默认目标
改变系统默认的目标，在root权限下执行如下命令：
# systemctl set-default name.target
例如示例命令如下：
[root@superman-21 ~]# systemctl set-default multi-user.target
[root@superman-21 ~]#
21.5 改变当前目标
改变当前系统的目标，在root权限下执行如下命令：
# systemctl isolate name.target
例如示例命令如下：
[root@superman-21 ~]# systemctl isolate rescue.target
[root@superman-21 ~]#
21.6 切换到救援模式
改变当前系统为救援模式，在root权限下执行如下命令：
# systemctl rescue
例如示例命令如下：
[root@superman-21 ~]# systemctl rescue
[root@superman-21 ~]#
这条命令和“systemctl isolate rescue.target”类似。命令执行后会在串口有如下打印信息：
You are in rescue mode. After logging in, type "journalctl -xb" to viewsystem logs, systemctl reboot" to reboot, "systemctl default" or "exit"to boot into default mode.
Give root password for maintenance
(or press Control-D to continue):
说明： 用户需要重启系统，从救援模式进入正常模式。
21.7 切换到紧急模式
改变当前系统为紧急模式，在root权限下执行如下命令：
# systemctl emergency
这条命令和“systemctl isolate emergency.target”类似。命令执行后会在串口有如下打印信息：
You are in emergency mode. After logging in, type "journalctl -xb" to viewsystem logs, systemctl reboot" to reboot, "systemctl default" or "exit"to boot into default mode.
Give root password for maintenance
(or press Control-D to continue):
例如示例命令如下：
[root@superman-21 ~]# systemctl emergency
[root@superman-21 ~]#
说明： 用户需要重启系统，从紧急模式进入正常模式。
标签：target,一步,systemctl,active,loaded,openEuler,root,21 From： https://blog.51cto.com/u_237826/6077089