
- [Rough Notes](#rough-notes)
  - [Commands](#commands)
  - [Init Systems](#init-systems)
  - [Service Managers](#service-managers)
- [Important Reading](#important-reading)
- [References](#references)

----

## Rough Notes

- [systemd](https://en.wikipedia.org/wiki/Systemd) is for both system init and service management
- [supervisord](http://supervisord.org/) is only for service managment

### Commands
- The [`service`](https://manpages.ubuntu.com/manpages/jammy/en/man8/service.8.html) command is a "wrapper"/"high-level" command that will call the respective "low-level" manager such as [`systemctl`](https://manpages.ubuntu.com/manpages/jammy/man1/systemctl.1.html)
- The [`service`](https://manpages.ubuntu.com/manpages/jammy/en/man8/service.8.html) command allows for basic interatctions such as `status`, `start`, `reload`, etc.
- The [`systemctl`](https://manpages.ubuntu.com/manpages/jammy/man1/systemctl.1.html) command allows for advanced commands such as checking if a service is enabled (activated when booting): `systemctl is-enabled sshd.service`

**Quick Reference**
- Enable service to run at startup
    ```bash
    sudo systemctl enable sshd.service
    ```
- Disable service from running at startup
    ```bash
    sudo systemctl disable sshd.service
    ```

### Init Systems
- Init systems are those that run in process #1 - responsible for bootstrapping the system
- May also be responsible for starting and managing services (long running, background process)
- Ubuntu used to use [upstart](https://upstart.ubuntu.com/) but switched to [systemd](https://en.wikipedia.org/wiki/Systemd) in version 15.04 (2015)
- Alpine linux uses [OpenRC](https://wiki.alpinelinux.org/wiki/OpenRC)
- `systemd` is linux specific - (BSD distros such as FreeBSD have their own ["BSD-style init"](https://docs.freebsd.org/en/articles/linux-users/#startup))
- Although highly adopted, there is a lot of contreversy around `systemd` (see: [[Reddit] ELI5: The SystemD vs. init/upstart controversy](https://www.reddit.com/r/linux/comments/132gle/comment/c709592/?utm_source=share&utm_medium=web2x&context=3))

### Service Managers
Outside of init systems, there are also dedicated service managers such as [Supervisor (supervisord)](http://supervisord.org/)
> It shares some of the same goals of programs like launchd, daemontools, and runit. Unlike some of these programs, it is not meant to be run as a substitute for init as “process id 1”. Instead it is meant to be used to control processes related to a project or a customer, and is meant to start like any other program at boot time.

*[[supervisord.org] Supervisor: A Process Control System](http://supervisord.org/#supervisor-a-process-control-system)*

- Advantages of using something like `supervisord` are that it is OS agnostic and since it is outside of process #1, processes can be managed by users other than root/sudo

## Important Reading
- [[StackExchange] What is the difference between "init" and "service manager"?](https://unix.stackexchange.com/a/489949)
- [[gentoo wiki] Comparison of init systems](https://wiki.gentoo.org/wiki/Comparison_of_init_systems)

## References
- [[Stack Overflow] Difference between Systemctl and service command](https://stackoverflow.com/a/43539946)
- [[Ask Ubuntu] Difference between systemctl and service commands](https://askubuntu.com/questions/903354/difference-between-systemctl-and-service-commands)

Posts tagged "os"

Linux Service Managers and Init Systems