Systemd is a system and service manager for Linux. It can run tasks in parallel (unlike SysVinit) which helps reducing boot time with tasks executed simultaneously. These tasks are called units and their configs are stored in files in the format unit_name.type_extension in the /etc/systemd/system location.

The type_extention identifies the unit type, which may be a service, a mount, etc.

A unit service has a further type definition that configures the unit startup functionality. The “types” are listed as follows:

• simple: It’s the default value if no type is defined. The process started with ExecStart is the main process of the service. This is the preferred type for a service that remains attached to the shell after execution.
• forking: The process started with ExecStart spawns a child process that becomes the main process of the service. The parent process exits when the startup is complete. This is the preferred type for services that would run in background.
• oneshot: This type is similar to simple, but the process exits before starting consequent units. It’s useful for running commands sequentially.
• dbus: This type is similar to simple, but consequent units are started only after the main process gains a D-Bus name.
• notify: This type is similar to simple, but consequent units are started only after a notification message is sent via the sd_notify() function.

• idle: similar to simple, but the actual execution of the service binary is delayed until all jobs are finished, which avoids mixing the status output with shell output of services.

  [Unit]
  Description=My Program 123
  After=syslog.target network.target
  
  [Service]
  Type=forking
  WorkingDirectory=/opt/my_progam
  User=user123
  ExecStart=/opt/my_program/program123
  PIDFile=/var/run/program123.pid
  TimeoutStartSec=3600
  
  [Install]
  WantedBy=multi-user.target
  

In the above example, I defined the type as forking, therefore the program will run detached from the main execution process.

The After directive indicates that syslog and network services should have been started prior to starting “My Program 123”.

More information about systemd configuration can be found in the RHEL 7 System Administrator’s Guide.