systemd targets: runlevels the new way

What a target is

A target is neither a process nor a service. It is a .target unit that does nothing on its own, but through Wants= / Requires= it pulls in a set of other units. Reaching a target means "every required service is up."

An analogy from SystemV: a target is like a runlevel, but it works through dependencies instead of numbered scripts in /etc/rc3.d/.

Mapping to SystemV runlevels

The aliases runlevel3.target → multi-user.target are kept for backward compatibility with old scripts and the init 3 command.

default.target: where the system boots into

/etc/systemd/system/default.target is a symlink to the canonical target:

ls -l /etc/systemd/system/default.target

This is what systemd picks up at startup when the kernel cmdline does not pass systemd.unit=.... You do not need to change it by hand with ln -s. There is a command for it:

systemctl get-default                            # current one

sudo systemctl set-default multi-user.target     # for servers without a GUI

System targets: the full boot chain

They form a dependency tree, going from low-level to high-level:

sysinit.target          ← mounts, swap, sysctl, journald

     ↓

basic.target            ← timers.target, sockets.target, slices.target

     ↓

multi-user.target       ← all the usual daemons (sshd, nginx, postgres)

     ↓

graphical.target        ← + display-manager.service (gdm/lightdm)

     ↓

default.target          ← symlink to one of the above

Alongside these there are "synchronization" targets:

• network.target is reached when all network units are configured (this does NOT mean the host is online)
• network-online.target is reached when the network is actually ready and an IP is assigned (for services that are useless without connectivity)
• local-fs.target is reached when all local filesystems from mount-and-fstab are mounted
• remote-fs.target is reached when all NFS/SMB mounts are in place
• time-sync.target is reached when the clock is synchronized (see chrony-and-ntp)

How a target collects its units

Two mechanisms:

1) Through the [Install] section of a service. When you run systemctl enable nginx, a symlink is created:

/etc/systemd/system/multi-user.target.wants/nginx.service

 → /usr/lib/systemd/system/nginx.service

That .wants/ directory (and .requires/) is what the target picks up at startup.

2) Through the WantedBy= directive in the target file itself, for system targets.

cat /usr/lib/systemd/system/graphical.target

# [Unit]

# Description=Graphical Interface

# Requires=multi-user.target

# Wants=display-manager.service

# After=multi-user.target display-manager.service

# AllowIsolate=yes

AllowIsolate=yes is critical. Without it a target cannot be made the default, and you cannot switch to it through isolate.

Switching between targets on the fly

sudo systemctl isolate multi-user.target    # drop to CLI without a reboot

sudo systemctl isolate graphical.target     # bring the GUI back

sudo systemctl isolate rescue.target        # repair without a reboot

sudo systemctl rescue                       # same as isolate rescue.target

sudo systemctl emergency                    # emergency shell

isolate stops everything that the new target does NOT need and starts what it does. The name is odd, but that is the one.

Passing a target through GRUB at boot

If the system does not boot into default.target, you can override it once from GRUB. At the menu screen, press e to edit, and on the linux ... line append:

systemd.unit=rescue.target

Then Ctrl+X to boot. This applies to this boot only; the default is not changed.

Debugging: what a target pulls in

systemctl list-units --type=target              # active targets

systemctl list-dependencies multi-user.target   # the tree of all units

systemctl list-dependencies --reverse nginx     # which targets nginx belongs to

systemctl cat multi-user.target                 # the resulting file

Creating your own target rarely makes sense, for example for a group of custom services that you switch on and off together. Most of the time WantedBy=multi-user.target in ordinary units is enough.