Final answer:
To set an environment variable in a systemd service, you must add an Environment directive in the .service file under the [Service] section, then reload the systemctl daemon, and restart the service.
Step-by-step explanation:
To set an environment variable in a systemd service, you will need to modify the service's unit file. This is located typically under /etc/systemd/system/ for custom services or /lib/systemd/system/ for default service files provided by installed packages. Here is the step-by-step process:
Open the .service file for the service you want to modify with a text editor, such as vim or nano.
Add an Environment directive under the [Service] section of the file. For example: Environment="VAR_NAME=var_value".
If you need to set multiple variables, you can add multiple Environment lines or use a single line with space-separated variables, like: Environment="VAR1=value1" "VAR2=value2".
After you have made your changes, save the file.
Reload the systemctl daemon to apply the changes with the command systemctl daemon-reload.
Restart the service to pick up the new environment variables with systemctl restart service_name.
This approach is generally more secure and maintainable than hard-coding variables directly into service execution commands. Also, it allows for better separation of configuration from code.