Table of Contents

• Overview
• Reference
• Units
  • Syntax
  • Types
  • Paths
• Special units
• Sections
  • Unit
  • Service
  • Install
• Commands
  • systemctl
  • journalctl
• Topics
  • Is it expected for systemd to start disabled services?
  • What is STATE from 'systemctl list-unit-files'?
• How-to
• Links

Overview

Reference

Units

Syntax

• case-sensitive
• X-<name> for non-standard sections
• 1, yes, on, true, 0, no, off, false for booleans

[Section]
Directive1=value
Directive2=value
...

Types

Target

• Target units do not offer any additional functionality on top of the generic functionality provided by units.
• They exist merely to group units via dependencies

Paths

Special units

Sections

Unit

• The first section found in most unit files
• Defines metadata for the unit and configuring the relationship of the unit to other units.

Description

Documentation

Requires

• activate dependenceis and fail if they fail

Wants

• activate dependencies but don't care whether they fail or not

Requisite

• fails if the dependency is not active

Conflicts

• deactivate the dependency if it is active

Before

After

Condition…

Assert…

Service

• Defines configuration that is only applicable for services

Type

• how to manage services. simple is default, forking for services which fork, oneshot for shot-lived process

RemainAfterExit

• with oneshot type. active after process exits

PIDFile

• with forking type |

BusName

• with dbus type |

NotifyAccess

• with notify type |

ExecStart

• if path to command starts with -, systemd considers active even if it's failed

ExecStartPre

ExecStartPost

ExecReload~

ExecStop=

• if not given, the process will be killed when stopped

ExecStopPost

RestartSec

Restart

• always, on-success, on-failure, on-abnormal, on-abort, on-watchdog

TimeoutSec

• When stopping, after waiting n seconds, systemd kills the process

Install

• The last section is often
• optional
• Defines the behavior or a unit if it is enabled or disabled
• Enabling a unit marks it to be automatically started at boot.

WantedBy

For sshd.service,

[Unit]
...
[Service]
...
[Install]
WantedBy=multi-user.target

When enabling sshd.service, systemd adds it to multi-user.target by following:

ln -s '/usr/lib/systemd/system/sshd.service' '/etc/systemd/system/multi-user.target/wants/sshd.service'

RequiredBy

Alias

Also

DefaultInstance

Commands

systemctl

systemctl start <name>.service
systemctl stop <name>.service
systemctl restart <name>.service
systemctl reload <name>.service      # reload some configs while running
systemctl enable <name>.service      # make service start at boot
systemctl disable <name>.service

systemctl status <name>.service      # overview
systemctl show <name>.service        # low-level details
systemctl list-dependencies <name>.service

systemctl list-units                 # only active
systemctl list-units --all           # all loaded
systemctl list-unit-files            # all installed

systemctl cat <name>.service         # contents of unit file
systemctl edit <name>.service        # edit override for unit file
systemctl edit --full <name>.service # edit unit file

systemctl daemon-reload              # reload after edit

systemctl list-unit-files --type=target
systemctl get-default

journalctl

journalctl                   # all
journalctl -b                # current boot
journalctl -k                # only kernel messages
journalctl -u <name>.service # unit
journalctl -f                # follow, like tail -f

Topics

Is it expected for systemd to start disabled services?

The systemctl enable and systemctl disable operations configure auto-starting of a unit.

So, starting a unit without enabling it is valid.

What is STATE from 'systemctl list-unit-files'?

$ systemctl list-unit-files --type=service
UNIT FILE              STATE
[...]
chronyd.service        enabled
clamd@.service         static
clamd@scan.service     disabled

enabled

it has a symlink in a .wants directory.

disabled

it does not.

static

the service is missing the [Install] section in its init script, so you cannot enable or disable it. Static services are usually dependencies of other services, and are controlled automatically.

How-to

Links