1.2Customizing System Startup

"Why won't this service start?" "I changed the config but it did not take effect"—answering these requires understanding where systemd reads its config from and what overrides what. Exam 201 focuses heavily on this hierarchy and practical systemctl commands.

1.2.1The systemd config file hierarchy

• /usr/lib/systemd/ holds the default unit files shipped by packages. Package updates overwrite this, so never edit here directly.
• /etc/systemd/ is where administrators put customizations. Override a default by placing a same-named file or a drop-in (a .d/ directory) here.
• /run/systemd/ holds runtime (volatile) configuration—transient units and state that disappear on reboot.
• Precedence: /etc/ overrides /usr/lib/, and /run/ overrides both transiently. systemd-delta lists which files are overridden where across the three layers.

1.2.2Unit operations with systemctl and failure modes

• Inspection: systemctl status <unit> (detailed state) and systemctl list-units (currently loaded units, narrow with --type=service, etc.).
• Runtime control: start/stop (act right now) vs enable/disable (toggle auto-start on future boots by creating/removing symlinks).
• Lockout: mask makes a unit completely unstartable (symlinks it to /dev/null, so even start fails) and unmask reverses it. Stronger than disable—it also blocks incidental starts pulled in by dependencies.
• rescue mode (rescue.target) is minimal-service, single-user maintenance with root login. emergency mode (emergency.target) is the last resort: it mounts the root filesystem read-only and does not even start what rescue does.

Trace the practical config-change flow. Avoid editing vendor units directly (e.g., /usr/lib/systemd/system/sshd.service); instead hand-craft a drop-in such as /etc/systemd/system/sshd.service.d/override.conf on the /etc/ side. This way the /etc/ layer overrides the /usr/lib/ default and survives package upgrades. When unsure which file wins where, run systemd-delta to see the three-layer overlay. After a change, apply it with systemctl stop sshd followed by systemctl start sshd. To permanently disable a service, use systemctl disable <unit>—but if you are worried it could still start incidentally via another unit's dependency, go further with systemctl mask <unit>. If the system enters a boot loop, append systemd.unit=rescue.target to the GRUB2 kernel line for minimal-config maintenance; if it is too broken even for that, use systemd.unit=emergency.target to get a shell with the root FS still mounted read-only and inspect/fix configuration files.

1.2.3Section summary

• Config layers = /usr/lib (default) < /etc (admin) < /run (runtime); check overlaps with systemd-delta
• start/stop = now / enable/disable = from next boot / mask = fully locked; emergency is even more minimal than rescue