Targets and the boot sequence

A target is a unit that groups other units, the way a runlevel groups services. Reaching a target means everything pulled into it is up. atom boots by picking one target as the goal and starting everything it depends on, in dependency order, in parallel where it can.

The default target

The goal of a normal boot is set by default.target, which on Sinty OS points at graphical.target:

/etc/systemd/system/default.target -> /usr/lib/systemd/system/graphical.target

So a normal boot brings the system all the way up to the graphical login.

The target stack

The common targets, from earliest to latest:

Target	What it means is ready
local-fs.target	Local filesystems are mounted.
sysinit.target	Early boot setup has run (devices, low-level services).
sockets.target / timers.target	Socket- and timer-activated units are armed.
basic.target	The basic system is up, the point most services order after.
network.target	Networking is considered up for ordering purposes.
multi-user.target	The full non-graphical system: system services and daemons.
graphical.target	The graphical layer on top of multi-user, the greeter and session.
shutdown.target	Reached on the way down; conflicting with it stops a unit at shutdown.

Each layer orders after the one before it, so sysinit runs after local-fs, multi-user after basic, and graphical after multi-user. Within a layer, independent units start together.

Booting to the desktop

Putting it together, a boot looks like this:

1. The kernel starts sinit as PID 1.
2. sinit reads default.target (graphical.target) and works out everything it depends on.
3. It brings up the stack in order: filesystems, early setup, the basic system, then the multi-user services.
4. graphical.target pulls in the greeter, and you reach the login screen.
5. After you log in, your desktop session starts on top.

How a unit joins a target

A unit is pulled into a target by being enabled. Enabling a unit whose [Install] section says WantedBy=multi-user.target creates a symlink under that target’s .wants directory:

/etc/systemd/system/multi-user.target.wants/example.service
    -> /usr/lib/systemd/system/example.service

That symlink is what makes the target pull the unit in at boot. You normally create it with atomctl enable rather than by hand; see atomctl.