Otdfbt

I don't want to remove the service, I just want to avoid its start on boot. I still need the option to start it manually later (with the systemctl start <service> command).

I tried to use systemctl disable <service>. It doesn't work, because it removes the service.

There is another possibility. In its service file,

[Install]
#WantedBy=multi-user.target

could be commented out (and then, systemctl daemon-reload). It works in the case of my own services, because their service files was written by me.

However, the service files belonging to distribution, are in /lib/systemd/system. Files in this directory are managed by the OS, i.e. they would be overwritten by updates, other parts of the system might assume that these are unmodified, and so on. Simply editing system files out of the /etc is a bad practice, and I don't want to do that. I don't want to edit configuration files in my /lib.

What to do?

systemctl disable is the correct way to do this; it still allows starting a unit manually, even if it doesn’t appear in systemctl --all’s output — to list all startable units, you should run systemctl list-unit-files instead. To render a unit un-startable, you need to mask it.

$ sudo systemctl stop unbound
$ sudo systemctl status unbound
● unbound.service - Unbound DNS server
 Loaded: loaded (/lib/systemd/system/unbound.service; enabled; vendor preset: enabled)
 Active: inactive (dead) since Fri 2019-05-03 13:12:41 CEST; 5s ago
 Docs: man:unbound(8)
 Main PID: 5320 (code=exited, status=0/SUCCESS)

$ sudo systemctl disable unbound
$ sudo systemctl status unbound
● unbound.service - Unbound DNS server
 Loaded: loaded (/lib/systemd/system/unbound.service; disabled; vendor preset: enabled)
 Active: inactive (dead)
 Docs: man:unbound(8)

$ sudo systemctl start unbound
$ sudo systemctl status unbound
● unbound.service - Unbound DNS server
 Loaded: loaded (/lib/systemd/system/unbound.service; disabled; vendor preset: enabled)
 Active: active (running) since Fri 2019-05-03 13:13:14 CEST; 1s ago
 Docs: man:unbound(8)
 Process: 30513 ExecStartPre=/usr/lib/unbound/package-helper chroot_setup (code=exited, status=0/SUCCESS)
 Process: 30518 ExecStartPre=/usr/lib/unbound/package-helper root_trust_anchor_update (code=exited, status=0/SUCCESS)
 Main PID: 30525 (unbound)
 Tasks: 1 (limit: 4915)
 CGroup: /system.slice/unbound.service
 └─30525 /usr/sbin/unbound -d

If you really want to, you can override system-provided services defined in /lib by adding files in /etc, and change their desired target; systemctl edit yourunit will do the right thing: it opens an editor, allowing you to override only the settings you care about, and it will store the result in the right place, as an override “snippet”. Updates made to non-overridden settings in the system-provided services (e.g. by package upgrades) will be taken into account transparently.