systemctl

The systemctl command is a powerful utility used to interact with systemd, the init system and service manager on many Linux distributions. It allows you to manage services, system states, and various system resources.

Overview of `systemctl`

systemctl provides control over systemd services, which includes starting, stopping, restarting, enabling, disabling, and checking the status of services. Additionally, it can be used to manage system states (e.g., reboot, shutdown) and inspect logs and configurations.

General Syntax

systemctl [OPTIONS] COMMAND [NAME...]

Where:

COMMAND is the action you want to perform (e.g., start, stop, status).
NAME is the name of the service, target, or unit you want to manage (e.g., nginx.service, docker, or multi-user.target).

Common `systemctl` Commands

Starting a Service:
- Starts a systemd service.
```
systemctl start <service-name>
```
- Example: To start the apache2 service:
  systemctl start apache2
Stopping a Service:
- Stops a running systemd service.
```
systemctl stop <service-name>
```
- Example: To stop the apache2 service:
  systemctl stop apache2
Restarting a Service:
- Restarts a service by stopping and then starting it again.
```
systemctl restart <service-name>
```
- Example: To restart the apache2 service:
  systemctl restart apache2
Reloading a Service:
- Reloads a service’s configuration without stopping it. Some services support reloading (like web servers).
```
systemctl reload <service-name>
```
- Example: To reload the apache2 service:
  systemctl reload apache2
Checking the Status of a Service:
- Displays the current status of a service, including whether it's running, its PID, and any recent log entries.
```
systemctl status <service-name>
```
- Example: To check the status of apache2:
  systemctl status apache2
Enabling a Service to Start at Boot:
- Enables a service to start automatically when the system boots.
```
systemctl enable <service-name>
```
- Example: To enable apache2 to start at boot:
  systemctl enable apache2
Disabling a Service from Starting at Boot:
- Disables a service from automatically starting when the system boots.
```
systemctl disable <service-name>
```
- Example: To disable apache2 from starting at boot:
  systemctl disable apache2
Masking a Service:
- Prevents a service from being started, either manually or automatically, by creating a symbolic link to /dev/null.
```
systemctl mask <service-name>
```
- Example: To mask the apache2 service:
  systemctl mask apache2
Unmasking a Service:
- Reverses the effect of mask by removing the symbolic link.
```
systemctl unmask <service-name>
```
- Example: To unmask apache2:
  systemctl unmask apache2
Viewing Logs for a Service:
- Displays the logs for a specific service managed by systemd.
```
journalctl -u <service-name>
```
- Example: To view the logs for apache2:
  journalctl -u apache2
Viewing the System Logs:
- Displays the entire system journal, including messages from all services and system events.
```
journalctl
```
Rebooting the System:
- Reboots the entire system.
```
systemctl reboot
```
Shutting Down the System:
- Shuts down the system gracefully.
```
systemctl poweroff
```
Suspending the System:
- Suspends the system (puts it to sleep).
```
systemctl suspend
```
Viewing All Active Units:
- Lists all active systemd units (services, sockets, etc.).
```
systemctl list-units
```
Viewing All Loaded Units:
- Lists all loaded systemd units, including inactive ones.
```
systemctl list-units --all
```
Checking Systemd Targets:
- Lists the systemd targets. Targets represent different states or "runlevels" in systemd. For example, multi-user.target is similar to runlevel 3 in traditional SysVinit systems.
```
systemctl list-units --type=target
```
Switching Runlevels (Changing Targets):
- You can switch between different systemd targets (which are similar to runlevels).
```
systemctl isolate <target-name>
```
- Example: To switch to the multi-user target (which is equivalent to runlevel 3):
  systemctl isolate multi-user.target
Checking System Boot Logs:
- You can view the logs related to the system’s boot process.
```
journalctl -b
```

Example Usage

1. Start the Apache service:

systemctl start apache2

2. Check the status of a service:

systemctl status apache2

3. Enable a service to start at boot:

systemctl enable apache2

4. Disable a service from starting at boot:

systemctl disable apache2

5. Restart a service after making configuration changes:

systemctl restart apache2

6. Check all active services:

systemctl list-units --type=service

7. Reboot the system:

systemctl reboot

8. View logs for the `nginx` service:

journalctl -u nginx

9. Power off the system:

systemctl poweroff

Conclusion

The systemctl command is an essential tool for managing a system running systemd. It provides control over services, targets (runlevels), and logs, enabling administrators to configure, troubleshoot, and maintain their Linux systems effectively. From simple service management to complex system configurations, systemctl is the go-to utility for modern Linux administration.

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Overview of systemctl