How it works...

We are again querying systemd using systemctl, only this time instead of printing running services, we're getting everything that the init system is aware of.

Service files, and indeed all other types of unit file, generally exist in /usr/lib/systemd/system. From this location, files are symlinked into the runlevel directories at /etc/systemd/system/.

As seen in the following, we're going to ls this directory:

$ ls -la /etc/systemd/system
total 8
drwxr-xr-x. 13 root root 4096 May 12 2018 .
drwxr-xr-x. 4 root root 151 May 12 2018 ..
drwxr-xr-x. 2 root root 32 May 12 2018 basic.target.wants
lrwxrwxrwx. 1 root root 46 May 12 2018 dbus-org.freedesktop.NetworkManager.service -> /usr/lib/systemd/system/NetworkManager.service
lrwxrwxrwx. 1 root root 57 May 12 2018 dbus-org.freedesktop.nm-dispatcher.service -> /usr/lib/systemd/system/NetworkManager-dispatcher.service
lrwxrwxrwx. 1 root root 37 May 12 2018 default.target -> /lib/systemd/system/multi-user.target
drwxr-xr-x. 2 root root 87 May 12 2018 default.target.wants
drwxr-xr-x. 2 root root 38 May 12 2018 dev-virtio\x2dports-org.qemu.guest_agent.0.device.wants
drwxr-xr-x. 2 root root 32 May 12 2018 getty.target.wants
drwxr-xr-x. 2 root root 35 May 12 2018 local-fs.target.wants
drwxr-xr-x. 2 root root 4096 May 12 2018 multi-user.target.wants
drwxr-xr-x. 2 root root 48 May 12 2018 network-online.target.wants
drwxr-xr-x. 2 root root 31 May 12 2018 remote-fs.target.wants
drwxr-xr-x. 2 root root 51 May 12 2018 sockets.target.wants
drwxr-xr-x. 2 root root 217 May 12 2018 sysinit.target.wants
drwxr-xr-x. 2 root root 44 May 12 2018 system-update.target.wants

Note we have a few targets, which are basically the different runlevels of a system; most of the day-to-day services you'll interact with live in multi-user.target.wants, which are basically the services required for a multi-user session (the typical mode of operation).

Running ls again in this sub-directory reveals the symlinks mentioned earlier and their location on disk:

$ ls -la /etc/systemd/system/multi-user.target.wants/
total 8
drwxr-xr-x. 2 root root 4096 May 12 2018 .
drwxr-xr-x. 13 root root 4096 May 12 2018 ..
lrwxrwxrwx. 1 root root 38 May 12 2018 auditd.service -> /usr/lib/systemd/system/auditd.service
lrwxrwxrwx. 1 root root 39 May 12 2018 chronyd.service -> /usr/lib/systemd/system/chronyd.service
lrwxrwxrwx. 1 root root 37 May 12 2018 crond.service -> /usr/lib/systemd/system/crond.service
lrwxrwxrwx. 1 root root 42 May 12 2018 irqbalance.service -> /usr/lib/systemd/system/irqbalance.service
lrwxrwxrwx. 1 root root 46 May 12 2018 NetworkManager.service -> /usr/lib/systemd/system/NetworkManager.service
lrwxrwxrwx. 1 root root 41 May 12 2018 nfs-client.target -> /usr/lib/systemd/system/nfs-client.target
lrwxrwxrwx. 1 root root 39 May 12 2018 postfix.service -> /usr/lib/systemd/system/postfix.service
lrwxrwxrwx. 1 root root 40 May 12 2018 remote-fs.target -> /usr/lib/systemd/system/remote-fs.target
lrwxrwxrwx. 1 root root 46 May 12 2018 rhel-configure.service ->
...

These services are all enabled to start in a multi-user session.