Systemd is at the time of writing this article is the de-facto init system for Linux. Systemd has three primary aspect:
- System and service manager
- Software platform for developing other applications
- “Glue” between the kernel and user applications
Systemd manages units. Units have dependency and systemd handles that dependency. Location of unit file paths:
- /lib/systemd/system :; for package-installed units
- /etc/systemd/system :: for administrator-configured units
- /run/gystemd/system :; for non-persistent runtime modification
Unit files are named in following convention name.type. For example: ssh.socket, example.service. The unit file follows toml like syntax.
[Unit] Description=Consul Requires=network-online.target After=network-online.target [Service] Restart=on-failure ExecStart=/usr/local/bin/example ExecReload=/bin/kill -HUP $MAINPID killSignal=SIGTERM User=consul Group=consul [Install] WantedBy=mulit-user.target
Service Unit Types
- Simple (default):: for executables which run without daemonising
- Forking:: for executables which daemonise themselves
- Oneshot:; ususally short lived programs which run to completion and terminate
- Notify:; for executables which will notify systemd when htey are started and available for works
- systemctl enable example.service :: starts at boot
- systemctl start example.service
- systemctl stop example.service
- systemctl restart example.service
- systemctl status consul.service
Modification of default service configuration can be done through following location /etc/systemd/system/example.service.d/example.conf. We can use systemd-delta command to see final configuration of unit file of any service.
Common Bootstrapping Pattern
- Install software from a generic package
- Install bootstrapping script and drop-in configuration an environment-specific package.
Replacement for runlevel. Targets is reached when all the services are started successfully.
Enables separating service and socket. Socket does not need to die when service die.
Some kind of replacement for corn.
[Unit] Description="example script/service that needs to be run" [Service] Type=oneshot ExecStart="bash example.sh"
[Unit] Description="run example.service every 10 minutes" [Timer] OnCalendar:*:0/10