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In Process
Martin Zeltin
Asked: 2021-03-20 10:14:40 +0800 CST

What is the starting process like for Ubuntu?

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I'm trying to learn about Linux and how Ubuntu with Gnome is started from the moment I turn on my computer.

I understand that first the kernel starts and after it has finished booting it runs the init program (upstart?). Then upstart looks at some config files (somewhere?) and runs X and gnome-shell?

So what is the sequence and where are all the config files that say when and what gets started?

boot

2 Answers

  1. No, we use systemd and have for a while now.

    Once the kernel is loaded into initial RAM disk, it begins the systemd initialization.

    systemd handles services management process using "targets". The "target" files in systemd are used for grouping different units and start up synchronization processes.

    The very first target executed by systemd is default.target (a symlink to graphical.target; /usr/lib/systemd/system/graphical.target)

    more /usr/lib/systemd/system/graphical.target 
#  SPDX-License-Identifier: LGPL-2.1+
#
#  This file is part of systemd.
#
#  systemd is free software; you can redistribute it and/or modify it
#  under the terms of the GNU Lesser General Public License as published by
#  the Free Software Foundation; either version 2.1 of the License, or
#  (at your option) any later version.

[Unit]
Description=Graphical Interface
Documentation=man:systemd.special(7)
Requires=multi-user.target
Wants=display-manager.service
Conflicts=rescue.service rescue.target
After=multi-user.target rescue.service rescue.target display-manager.service
AllowIsolate=yes

    This one triggers multi-user.target -> basic.target -> sysinit.target -> local-fs.target -> /etc/fstab, /etc/inittab

    This is a simplification as it is far more complex than just this: graphical.target shows you a couple of things about the connection to other targets. Each of those targets has the same kind of setup eventually leading to a desktop.

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  2. Thanks @Rinzwind for pointing me in the right direction. I ended up chasing through the systemd starting process and finally got all the way to how it starts Gnome Shell.

    For anyone else looking for this, here is how the process goes (At least for now in 2021 and Ubuntu 20.04). This might change in later versions I suppose.

    - Kernel boots
-- Kernel launches /usr/sbin/init (symlink to SysVinit, upstart or systemd)
--- systemd executes /usr/lib/systemd/system/default.target
---- default.target executes /etc/systemd/system/display-manager.service
----- display-manager.service is a symlink to /lib/systemd/system/gdm3.service
------ gdm3.service us a symlink to /lib/systemd/system/gdm.service
------- gdm.service executes /usr/sbin/gdm3
-------- gdm3 starts the X server and after the user logs in executes /usr/share/xsessions/ubuntu.desktop
--------- ubuntu.desktop executes /usr/bin/gnome-shell
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