Systemd Concepts
External
- man systemd.service
Internal
Overview
systemd is a init system, a system and user space service manager for the Linux operating system. The purpose of an init system is to manage - initialize, start, stop - components in the user space, which are the programs that run after the kernel has booted. Some of those components are services and daemons that provide services.
systemd runs as the first process at boot, with PID 1, and acts as the init system that brings up and maintains the user-space services.
Unlike System V init, which starts services one at a time, moving to the next service after running dependency checks and waiting on the daemon to start, systemd can start services in parallel. It takes advantage of the daemon to daemon relationship in that dependent daemons do not actually need to be running, they only need the correct domain sockets to be available. systemd creates all sockets first, all the daemons second and any client requests to daemons not yet running are cached in the socket buffer and filled when the daemons come online.
systemd interprets its main configuration configuration file /etc/systemd/system.conf and the files in system.conf.d directories.
Differences between System V and systemd
systemctl
systemctl is the central management tool for systemd. More operational details:
Units
The resources managed by systemd are known as units. systemd provides a dependency system between units.
Units represent different types of resources. The units are defined in configuration files known as unit files. The unit type is given by the suffix of the corresponding unit file. There are 12 types of resources:
.service: A service unit describes how to manage a service or an application. This including starting and stopping the service, under which circumstance it should be automatically started and its dependencies. For more details, see the Services section, below..socket: A socket unit describes a network or IPC socket, or a FIFO buffer systemd uses for socket-based activation..device: A device unit describes a device that needs systemd management..mount: A mount unit defines a systemd-managed mountpoint..automount: Units that configure a mountpoint to be automatically mounted..swap: Units that describe the swap space on the system..target: A unit that provides a synchronization point for other units when booting up or changing state. For more details, see Targets section below..path: Units defining paths that can be used for path-based activation..timer: A timer unit defines a timer managed by systemd, similar to a cron job for delayed or scheduled activation. For more details see Timers section below..snapshot.slice.scope
A unit is in a "loaded" state if the unit's configuration has been parsed by systemd. The configuration of loaded units is kept in memory.
Query Known Units
To list all units known to systemd, use:
systemctl list-units
The command will list the units and their state.
To query units matching a certain pattern, use:
systemctl list-units PATTERN
Unit Files
Unit files are configuration files for units. They are not executable scripts. The type of each unit can be inferred from the suffix at the end of the file, as described in the Units section.
Unit File Location
The package maintainer unit files reside in /usr/lib/systemd/system (or /lib/systemd/system, as /lib is a symbolic link to /usr/lib).
The user-created unit files conventionally reside in /etc/systemd/system.
If a user wants to modify one of the default unit files, they should copy them in /etc/systemd/system, because systemd will give priority to /etc.
For more details see
man systemd-user.conf
Unit File Structure
A typical unit file is similar to:
[Unit]
Description=My Service
After=network.target
[Service]
Type=forking
ExecStart=/usr/local/bin/myserviced
PIDFile=/var/run/myservice.pid
[Install]
WantedBy=multi-user.target
[Unit] Section
The [Unit] section contains a description of the service and its dependency declarations.
In the example above, we declare that systemd must not attempt to start the service until the network is not running on the host. For more details on dependencies, see the Dependencies section, below. For more practical details on how to declare dependencies, see systemd Declaring a Service Dependency.
[Service] Section
Type can be set to one of simple, forking, oneshot, dbus, notify or idle.
Type=forking
ExecStart represents the path to the binary to execute when the service is started. The binary is expected to fork.
If set to "forking", it is expected that the process configured with ExecStart will call fork() as part of its start-up. The parent process is expected to exit when start-up is complete and all communication channels are set up. The child continues to run as the main daemon process. This is the behavior of traditional UNIX daemons. If this setting is used, it is recommended to also use the PIDFile option, so that systemd can identify the main process of the daemon. systemd will proceed with starting follow-up units as soon as the parent process exits.
PIDFile takes an absolute file name pointing to the PID file of the service's daemon. PIDFile is recommended for services where Type is set to forking. systemd will read the PID of the main process of the daemon after start-up of the service. systemd will not write to the file configured here, although it will remove the file after the service has shut down if it still exists. systemd uses this file to identify the main process of the daemon, but the file will need to be written by other means.
Type=oneshot
If the Type is set to oneshot, it is expected that the process configured with ExecStart is the main process of the service and will exit before systemd starts follow-up units. However, if RemainAfterExit is configured with true, systemd will consider the service active even after the ExecStart process had exited. You may want to specify the pair ExecStop wrapper so systemd knows how to cleanly shut down the process. This is a typical pattern for a Java server started by a wrapper script.
Typical oneshot [Service] section:
...
[Service]
Type=oneshot
ExecStart=/usr/lib/systemd/scripts/activemq start
ExecStop=/usr/lib/systemd/scripts/activemq stop
RemainAfterExit=true
...
Environment
Environment variables can be declared with Environment="variable_name=variable_value" as follows:
[Service]
...
Environment="JAVA_HOME=/opt/java"
Also see:
[Install] Section
WantedBy=multi-user.target means that the service should be started while in multi-user mode (equivalent with System V runlevel 3).
Create a Custom Unit File for a Service
Services
For service management tasks. use service units. The associated files have a .service extension.
Start (Enable) a Service Automatically at Boot
To configure a service to start automatically at boot it must be enabled:
systemctl enable <application>.service
This command creates a symbolic link from the service unit file, which, if it is a standard system service, it usually maintained in /usr/lib/systemd/system, into a location on disk where systemd looks for autostart files. This location usually is a /etc/systemd/system/<some-target>.target.wants directory. See Target for more details.
Example:
systemctl enable docker
# ln -s '/usr/lib/systemd/system/docker.service' '/etc/systemd/system/multi-user.target.wants/docker.service'
Also see:
Starting a Service
Starting a service means using systemctl to execute instructions in the service's unit file. It must be executed as root:
systemctl start <application>.service
The short form also works:
systemctl start <application>
See also:
Reload Configuration without a Restart
If the service supports reloading configuration without actually restarting the service, use:
systemctl reload <application>.service
Restart a Service
systemctl restart <application>.service
Reload or Restart a Service
If you are unsure whether the service has the configuration reload functionality, use the reload-or-restart command:
systemctl reload-or-restart <application>.service
Stop (Disable) a Service
systemctl disable <application>.service
Status of a Service
Status Report
To check the status of a service run:
systemctl status <application>.service
Active
A service can be active (running) or not active. To check whether a service is active (running), execute:
systemctl is-active <application>.service
This will return the current unit state as a string ("active" or "inactive"). The exit code will be "0" if it is active.
Enabled
A service can be configured to start at boot (enabled) or not enabled. To check whether a service is enabled, execute:
systemctl is-enabled <application>.service
This will return the current unit enabled state as a string ("enabled" or "disabled"). The exit code will be "0" if it is enabled.
Failed
systemctl can be used to check to see whether the service is in a failed state:
systemctl is-failed <application>.service
This will return "active" if it is running properly or "failed" if an error occurred. If the unit was intentionally stopped, it may return "unknown" or "inactive". An exit status of "0" indicates that a failure occurred and an exit status of "1" indicates any other status.
Create a Custom Unit File for a Service
Dependencies
The dependencies are declared in the [Unit] section of the unit file.
See
After
Before
Wants
Requires
Targets
A .target is the configuration file for a unit that provides a synchronization point for other units when booting up or changing state. Target units are used for grouping units and as well-known synchronization points during start-up.
systemd and network status
Daemon Reload
systemctl has a daemon-reload command, which reloads all systemd manager configuration. This will rerun all generators, reload all unit files and recreate the entire dependency tree. While the daemon is being reloaded, all sockets systemd listens on behalf of user configuration will stay accessible.
systemctl daemon-reload
journald
Timers
systemctl list-timers
Timer Unit File
Timer unit files contain information about a timer controlled and supervised by systemd. By default, a service with the same name as the timer is activated. Also see service unit file.
For an example, see: