Run multiple parameterized instances of one systemd template unit (name@.service, %i vs %I)

Documented steps

1. Create a template whose filename ends in '@': /etc/systemd/system/worker@.service: [Unit] Description=worker for queue %I [Service] ExecStart=/usr/local/bin/worker --queue=%i Restart=on-failure [Install] WantedBy=multi-user.target
2. %i is the escaped instance name (the text between '@' and '.service'); %I is the unescaped form. Other useful specifiers: %n full unit name, %N unit name without suffix, %d credentials dir, %S state dir root.
3. Start instances by naming them: `systemctl enable --now worker@payments.service worker@emails.service`. Each instance is an independent unit with its own cgroup, logs and restart state.
4. Query one instance's logs: `journalctl -u worker@payments.service`; all instances: `journalctl -u 'worker@*'`.
5. Instance names containing '/' or other special characters must be escaped with `systemd-escape`: `systemctl start worker@$(systemd-escape 'queues/high').service` — inside the unit %I recovers the original string.
6. Per-instance configuration: use EnvironmentFile=/etc/worker/%i.env in the template, or a per-instance drop-in directory worker@payments.service.d/.
7. To start a default set at boot, create a target or add [Install] WantedBy plus DefaultInstance=main so `systemctl enable worker@.service` enables worker@main.