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Systemd.md

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systemd

systemd is an init daemon for Linux that is used on many distributions (such as Arch Linux, Debian, and RHEL). It significantly changes the way that daemons have been started in the past. systemd introduces the concept of "unit files," which are declarative configurations with a standard syntax that are a significant departure from the previous practice of using arbitrary shell scripts to start daemons. It incorporates easy ways to do things like specifying the environment for a service or limiting the syscalls that can be made by processes started by the service. systemd also provides a ton of helpful CLI commands to analyze the state of your system.

CLI commands

Command Short Description
systemctl status [unit] Show the current status of all units or a single unit (includes processes and recent log messages if a unit is given)
systemctl restart unit Restart the unit
systemctl stop unit Stop the unit
systemctl start unit Start the unit
systemctl enable unit [--now] Enable the unit (run it on boot), and optionally start it now
systemctl disable unit [--now] Disable the unit, and optionally stop it now
systemctl cat unit Print the configuration for the unit
systemctl edit unit [--full] Create an override config for the unit, or just override the whole file
systemctl daemon-reload Load all the changes made to the unit files on disk
systemctl reboot Reboot the computer
systemctl poweroff Turn off the computer
systemctl suspend Put the computer to sleep

Service unit files

Service unit files end with .service and specify a service (or daemon) to run. Service files look something like the following:

[Unit]
Description=Do the Foo
Requires=network.target
After=network.target

[Service]
# Service doesn't double fork or use systemd integration
Type=simple
ExecStart=/usr/bin/foo
# Always re-run ExecStart if the command exits (can also be on-failure)
Restart=always

# Make it so we can enable and not just start the service
[Install]
# Ensure the service runs before the system boot is considered completed
WantedBy=multi-user.target

Documentation for all options can be found with man 5 systemd.service

Timer unit files

Timer unit files are a replacement for traditional crontab entries. They can be a dependency of services or can depend on services (or actually any type of unit files).

An example Timer unit file from Arch Linux is:

[Unit]
Description=Daily rotation of log files
Documentation=man:logrotate(8) man:logrotate.conf(5)

[Timer]
OnCalendar=daily
AccuracySec=1h
Persistent=true

[Install]
WantedBy=timers.target

This timer will run every day. If the system isn't on at exactly one day since the previous run, it will run as soon as possible. It will run within 23-25 hours (daily + AccuracySec=1h) of the last time it ran.

Mount unit files

Instead of specifying entries in fstab(5), you can instruct systemd to mount different filesystems. (Note: A well-configured systemd will automatically turn fstab entries into .mount units). These can be great when a service or timer depends on a particular file system to be mounted, including an NFS share. You can also declaratively state that your mount file depends on NFS and networking being ready! The automatically-generated mount unit for a /boot or EFI partition might look something like:

[Unit]
Description=EFI System Partition Automount
Documentation=man:systemd-gpt-auto-generator(8)
Requires=systemd-fsck@dev-nvme0n1p1.service
After=systemd-fsck@dev-nvme0n1p1.service

[Mount]
What=/dev/nvme0n1p1
Where=/boot
Type=vfat
Options=umask=0077,noauto,rw

Note that systemd will ensure that the file system is fsck(8)'d before it is mounted.

Other Unit Files

There are all kinds of other unit files far too complicated to start describing here. Some really interesting ones include:

  • slices
  • targets
  • devices
  • sockets