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systemd.md

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Systemd (systemctl)

Has six units:

  1. .service
  2. .mount
  3. .device
  4. .socket
  5. .target
  6. .timer

Unit postfixes are attached magically for cetain types:

  • whatever === whatever.service
  • /home === home.mount
  • /dev/sda === dev-sda.device
  • @ = instance of a template unit

Check packaged units: pacman -Qql tlp | grep -Fe systemd/system

Unit descriptors:

  • /usr/lib/systemd/system/ → installed by packages
  • /etc/systemd/system/ → local (symlinks, folders, user saved unit files)

Systemctl (sctl) command:

  • Sctl: status, cat, list-unit-files, start, stop, restart, reload, enable, reenable, is-enabled, disable, mask, unmask, help
  • Sctl --failed
  • Sctl demon-reload (after editing a unit file)

Disabling a service just removes its symlink from etc (so it may not start during boot), but it may still start as a dependency (from usr/lib/systemd/system). Use mask to redirect the symlink to /dev/null.

Power management

  • Polkit needed (1 user at TTY = good, 1+ = ask for permission)
  • Sctl: reboot, poweroff, suspend, hibernate, hybrid-sleep

Certain actions may be needed on sleep start/stop (like wireless reinit or locking the screen, muting the audio). The sleep.target (see further below) will be called on all three states' changes (sleep, hibernate, hybrid).

Editing

  • Override locally (in etc?, not in usr lib)
  • Shortcut: systemctl edit --full unit (this is a copy + edit + reenable combo), but of course this is a full static copy, no magic

Targets

  • systemctl list-units --type=target
  • Targets are similar to sysv runlevels, but:
    • they are named (not numbered)
    • more than one may be active.
    • default target is graphic (~rl:5), while multi-user is like ~rl:3, and rescue is like ~rl:1
  • It has runlevel-like targets predefined (so telinit X "kinda" works)
  • Bootloader kernel param example: systemd.unit=rescue.target
  • Or set default target (all the time): systemctl set-default multi-user.target (symlinks to default.target "somewhere", use -f to override)

Services may be linked to targets. Create a directory with .wants postfix, for example: ls /etc/systemd/system/sleep.target.wants/ has three files for me (two added manually, one symlinked by the system, as of this writing): tlp-sleep.service wicd-sleep.service xlock.service (BUT all of these MUST be symlinks now)

Temp files

Certain packages (x11, screen, sudo, samba etc.) read/write from/to in /run or /var. These environments are set up and torn down on demand.

  • host tmp descriptors are in /usr/lib/tmpfiles, these are special .conf files (available commands are here)
  • linked (or devnulled) items are in /etc/tmpfiles.d

Timers

Timers are a bit like cron jobs, but are handled by systemd. The cron syntax is cleaner and simpler, and cron has access to the mailer daemon, but systemd offers greater control.

  • there are systemd cron implementations
  • systemctl list-timers --all
  • a .timer file controls a .service file
  • an example is here, a generic battery check and hibernate combo

Mounting

  • mounts from fstab
  • we can add x-systemd options to fstab
  • has an automounter (may be configurable via fstab or .automount)

Delayed/lazy mounting is possible with systemd: noauto,x-systemd.automount,x-systemd.idle-timeout=1min - which may or may not worth the hassle.

Journal

  • Syslog is not needed anymore, use journalctl
  • severity level is from 0 (emergency) to 7 (debug)
  • levels are incluseive. To show 0 and 1: journalctl -p 1
  • max journal size is 10% of partition, but max 4Gb (override w SystemMaxUse=XXXM in journald.conf, then restart with systemctl restart systemd-journald)
  • some useful options:
    • -b - since boot
    • -PID= - by pid
    • -k - kernel/dmesg
    • --disk-usage - du
    • --vacuum-time=2weeks - manual cleanup

Notes

  • read the wiki, read the forum
  • mkinitcpio will not be run on systemd update, do it manually or add a pacman hook

Troubleshooting

Power management: sleep target dependencies stopped working (lockscreen on lid close) - 2017.07.12.

  1. systemctl status sleep.target or journalctl | grep sleep.target - it says that from now on only symlinks shall be used
  2. mv sleep.target.wants/xlock.service /etc/systemd/system/_xlock.service - this is the location for the administrator's override scripts; while this is not an override script per se, it's still better here than in usr/lib or in /root/stuff
  3. ln -s /etc/systemd/system/_xlock.service ./_xlock.service - wiggle your big toe

Module loading during startup

  1. remove quiet from kernel boot params (press "e" in grub), or replace with debug - Useless: not verbose enough (or way too verbose), tty buffer fills up way too fast
  2. systemctl status systemd-modules-load.service - Useless: "Journal has been rotated since unit was started." (even after vacuuming)
  3. journalctl -u systemd-modules-load.service - Useless: "Transport endpoint is not connected"
  4. /usr/lib/systemd/systemd-modules-load (elf bin) - Helpful: "Failed to insert 'acpi_call': ..."