Linux systemctl Command – Control Systemd Services
The systemctl command is an essential tool for managing systemd services and other system components in modern Linux distributions. It is used to start, stop, restart, enable, disable, and monitor system services, making it a fundamental tool for system administrators.
Syntax of systemctl
OPTIONS→ Flags that modify the behavior ofsystemctl.COMMAND→ The action to be performed (e.g., start, stop, status).SERVICE→ The name of the service you want to control (e.g.,nginx,apache2).
Common systemctl Commands
1. Start a Service
To start a service immediately:
Example:
This will start the nginx web server.
2. Stop a Service
To stop a running service:
Example:
This will stop the nginx service.
3. Restart a Service
To restart a service (stops and then starts the service again):
Example:
This will restart the nginx service.
4. Reload a Service
If a service supports reloading its configuration without stopping, use:
Example:
This reloads the nginx service's configuration.
5. Check the Status of a Service
To check the current status of a service (running, stopped, etc.):
Example:
This shows the status of the nginx service, including whether it is active, inactive, or failed.
6. Enable a Service
To enable a service to start automatically at boot time:
Example:
This will ensure that nginx starts automatically when the system boots.
7. Disable a Service
To disable a service from starting at boot time:
Example:
This prevents nginx from starting automatically at boot time.
8. Mask a Service
To prevent a service from being started manually or automatically:
Example:
This will create a symbolic link to /dev/null and make it impossible to start the nginx service, even if it's enabled.
9. Unmask a Service
To remove the mask from a service and allow it to be started again:
Example:
This will allow nginx to be started again after being masked.
10. View All Services
To list all active and inactive services:
This will display all services, including their current state (active, inactive, etc.).
11. View All Failed Services
To list all services that have failed:
This will display any services that have failed to start or encountered an issue.
12. View Logs for a Service
To view logs for a specific service using journalctl:
Example:
This will show the logs for the nginx service.
Useful Options for systemctl
1. Show the Status of Systemd
To view general information about the systemd system and its status:
2. Start a Service in the Background (--quiet)
To suppress output when starting a service, use the --quiet option:
3. View Service Dependency Tree (list-dependencies)
To see the dependency tree for a specific service:
Example:
This shows the other services or units that the nginx service depends on.
4. Set Default Target (Runlevel) (set-default)
To set the default target (similar to runlevels in older Linux systems):
Example:
This sets the default target to the graphical environment (GUI).
Examples
1. Start a Service
This starts the apache2 web server.
2. Enable a Service to Start on Boot
This enables apache2 to start automatically at boot.
3. Check the Status of a Service
This shows the current status of the apache2 service.
4. Restart a Service
This restarts the apache2 service.
5. View Logs of a Service
This shows the logs for the apache2 service.
Conclusion
The systemctl command is a powerful tool for managing systemd services in Linux. It allows you to control services, check their status, and enable or disable them from starting automatically at boot. Whether you're restarting a service, viewing logs, or managing the startup behavior, systemctl is essential for system administrators.
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