systemd and boot cheat sheet

The chapter on one page: what to remember, the commands to have at your fingertips, and flashcards for quick revision.

The chapter in eight sentences

systemd is PID 1; it manages units (services, targets, sockets, timers, mounts), and systemctl is its control tool.
A service has two independent properties: whether it is running now (start, stop) and whether it starts at boot (enable, disable); enable --now does both.
mask makes a unit impossible to start at all; unmask undoes it.
systemctl status shows the state and the last log lines; journalctl -u NAME shows its whole log; systemctl --failed lists trouble.
Your own units belong in /etc/systemd/system; change packaged ones with a drop-in, and always systemctl daemon-reload after editing.
A service unit needs ExecStart= with a full path, a restart policy as wanted, and WantedBy=multi-user.target to start at boot.
Boot order: firmware, GRUB, kernel, initramfs, systemd, default target; set it with systemctl set-default, switch now with isolate.
With console access you can recover: rd.break resets a lost root password (remount /sysroot, chroot, passwd, touch /.autorelabel), and findmnt --verify, mount -a and nofail protect against a bad /etc/fstab.

Command	Does
`systemctl start	stop
`systemctl enable	disable [--now] NAME`
`systemctl mask	unmask NAME`
`systemctl status NAME`	State and recent log
`systemctl is-active	is-enabled NAME`
`systemctl list-units --type=service`	What is loaded
`systemctl list-unit-files`	What is installed
`systemctl --failed`	Failed units
`journalctl -u NAME`	The service's log

Item	Detail
Your units	`/etc/systemd/system/NAME.service`
Packaged units	`/usr/lib/systemd/system/` (do not edit)
Drop-in	`/etc/systemd/system/NAME.service.d/*.conf` or `systemctl edit NAME`
Show what applies	`systemctl cat NAME`
After any change	`systemctl daemon-reload`
Check the file	`systemd-analyze verify FILE`
`[Service]`	`ExecStart=`, `Restart=`, `User=`, `Environment=`
`[Install]`	`WantedBy=multi-user.target`
Status 203/EXEC	Program not found or not executable

Command / key	Does
`systemctl get-default`, `set-default T`	Show / set the boot target
`systemctl isolate T`	Switch the running system
`systemd-analyze`, `blame`, `critical-chain`	Boot timing
`cat /proc/cmdline`	Arguments of the running kernel
`grubby --update-kernel=ALL --args=X`	Add a kernel argument (`--remove-args` removes)
GRUB: `e`, then `Ctrl+X`	Edit one boot
`rd.break`	Early shell for a root-password reset
`findmnt --verify`, `mount -a`	Test `/etc/fstab`
`nofail`	Optional file system

Start, stop, restart and reload a service?: systemctl start | stop | restart | reload NAME
Boot policy versus current state?: enable/disable change boot; start/stop change now. is-enabled and is-active ask.
Make a service start now and at boot?: systemctl enable --now NAME
Forbid a service completely?: systemctl mask NAME (undo with unmask)
See a service in detail, and its log?: systemctl status NAME · journalctl -u NAME
List failed units?: systemctl --failed
Where are your unit files, and what after editing?: /etc/systemd/system/ · systemctl daemon-reload
Three sections of a service unit?: [Unit] description and ordering · [Service] what to run · [Install] what enable does
Show and change the default target?: systemctl get-default · systemctl set-default multi-user.target
Boot order in six words?: Firmware, GRUB, kernel, initramfs, systemd, target.
Add a kernel argument permanently, and for one boot?: grubby --update-kernel=ALL --args=... · press e in GRUB
Reset root password with console access?: rd.break, remount /sysroot rw, chroot, passwd root, touch /.autorelabel, exit twice.