使用systemd / systemctl 管理后台服务，关联 runlevel, service, daemon, init.d

List all running services

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# 列出所有service类型的unit
systemctl list-units --type=service --all

其他查看命令如下：

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systemctl # 列出所有的系统服务
systemctl list-units # 列出所有启动unit
systemctl list-unit-files # 列出所有启动文件
systemctl list-units –type=service –all # 列出所有service类型的unit
systemctl list-units –type=service –all grep cpu # 列出 cpu电源管理机制的服务
systemctl list-units –type=target –all # 列出所有target
systemctl list-dependencies # 查看当前运行级别target(mult-user)启动了哪些服务

Start/stop or enable/disable services

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# Start/stop a service immediately:
systemctl start foo.service
systemctl stop foo.service
systemctl restart foo.service
systemctl status foo.service

# service 自启动
systemctl enable foo.service
systemctl disable foo.service

# 0 indicates that it is enabled. 1 indicates that it is disabled
systemctl is-enabled foo.service; echo $?

runlevel 设置

How do I change the default runlevel?

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rm /etc/systemd/system/default.target

# Switch to runlevel 3 by default
ln -sf /lib/systemd/system/multi-user.target /etc/systemd/system/default.target

# Switch to runlevel 5 by default
ln -sf /lib/systemd/system/graphical.target /etc/systemd/system/default.target

Ubuntu 20.04 上没找到文件 /etc/systemd/system/default.target

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# 查看默认启动target
root# systemctl get-default
graphical.target

# 查看默认启动target
root# systemctl set-default multi-user.target

切换当前运行的runlevel

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# You can switch to ‘runlevel 3′ by running
systemctl isolate multi-user.target (or) systemctl isolate runlevel3.target

# You can switch to ‘runlevel 5′ by running
systemctl isolate graphical.target (or) systemctl isolate runlevel5.target

SystemD cheatsheet

service foobar start systemctl start foobar.service Used to start a service (not reboot persistent)
service foobar stop systemctl stop foobar.service Used to stop a service (not reboot persistent)
service foobar restart systemctl restart foobar.service Used to stop and then start a service
service foobar reload systemctl reload foobar.service When supported, reloads the config file without interrupting pending operations.
service foobar condrestart systemctl condrestart foobar.service Restarts if the service is already running.
service foobar status systemctl status foobar.service Tells whether a service is currently running.
ls /etc/rc.d/init.d/ ls /lib/systemd/system/.service /etc/systemd/system/.service Used to list the services that can be started or stopped
chkconfig foobar on systemctl enable foobar.service Turn the service on, for start at next boot, or other trigger.
chkconfig foobar off systemctl disable foobar.service Turn the service off for the next reboot, or any other trigger.
chkconfig foobar systemctl is-enabled foobar.service Used to check whether a service is configured to start or not in the current environment.
chkconfig foobar –list ls /etc/systemd/system/*.wants/foobar.service Used to list what levels this service is configured on or off
chkconfig foobar –add   Not needed, no equivalent.

List the current run level

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# systemctl list-units --type=target