Detailed introduction to systemctl command in Linux

Linux Systemctl is a collection of system management daemons, tools and libraries used to replace the System V, service and chkconfig commands. The initial process is mainly responsible for controlling the systemd system and service manager. Through Systemctl –help, you can see that the command is mainly divided into: querying or sending control commands to the systemd service, commands for management unit services, commands related to service files, commands related to tasks, environments, and snapshots, configuration reloading of systemd services, and system Commands related to powering on and off.

1. List all available units

# systemctl list-unit-files

2. List all running units

# systemctl list-units

3. List all failed units

# systemctl –failed

4. Check whether a certain unit (such as crond.service) is enabled

##

# systemctl is-enabled crond.service

5. List all Service

# systemctl list-unit-files –type=service

6. How to start, restart, stop, reload services and check the status of services (such as httpd.service) in Linux

# systemctl start httpd.service
# systemctl restart httpd.service
# systemctl stop httpd.service
# systemctl reload httpd.service
# systemctl status httpd.service

Note: When we use systemctl's start, restart, stop and reload commands, the terminal will not output anything, only the status command can Printout.

7. How to activate the service and enable or disable the service at boot (that is, the mysql.service service is automatically started when the system starts)

# systemctl is-active mysql.service
# systemctl enable mysql.service
# systemctl disable mysql.service

8. How to block (so that it cannot be started) or display services (such as ntpdate.service)

# systemctl mask ntpdate.service
ln -s ‘/dev/null”/etc/systemd/system/ntpdate.service'
# systemctl unmask ntpdate.service
rm ‘/etc/systemd/system/ntpdate.service'

9. Use the systemctl command to kill the service

# systemctl kill crond

10. List all system mount points

# systemctl list-unit-files –type=mount

11. Mount, unmount, remount, reload the system mount point and check the mount point status in the system

# systemctl start tmp.mount
# systemctl stop tmp.mount
# systemctl restart tmp.mount
# systemctl reload tmp.mount
# systemctl status tmp.mount

12. Activate, enable or disable mount points at startup (automatically mounted at system startup)

# systemctl is-active tmp.mount
# systemctl enable tmp.mount
# systemctl disable tmp.mount

13. Mask (so that it cannot be enabled) or visible mount points in Linux

# systemctl mask tmp.mount
ln -s ‘/dev/null”/etc/systemd/system/tmp.mount'
# systemctl unmask tmp.mount
rm ‘/etc/systemd/system/tmp.mount'

14. List all available system sockets

# systemctl list-unit-files –type=socket

15. Check all Configuration details

# systemctl show mysql

16. Get the dependency list of a service (httpd)

# systemctl list-dependencies httpd.service

17. Start rescue mode

# systemctl rescue

18. Enter emergency mode

# systemctl emergency

19. List the currently used runlevels

# systemctl get-default

20. Start running level 5, that is, graphics mode

# systemctl isolate runlevel5.target

or

# systemctl isolate graphical.target

21. Start running level 3, that is, multi-user mode (command line)

# systemctl isolate runlevel3.target

or

# systemctl isolate multiuser.target

22. Set multi-user mode or graphics mode as the default running level

# systemctl set-default runlevel3.target
# systemctl set-default runlevel5.target

23. Restart, stop, suspend, hibernate or put the system into hybrid sleep

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# systemctl reboot
# systemctl halt
# systemctl suspend
# systemctl hibernate
# systemctl hybrid-sleep

For those who don’t know what run level is, here’s the explanation.

Runlevel 0: Shut down the system

Runlevel 1: Rescue, maintenance mode
Runlevel 3: Multi-user, no graphical system
Runlevel 4: Multi-user, no graphical system
Runlevel 5: Multi-user , Graphical system
Runlevel 6: Shut down and restart the machine

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