systemd is a new init system and system manager, that was adopted by most of Linux distributions now over the traditional SysVinit manager.
systemd is compatible with SysV and LSB init scripts.
It can work as a drop-in replacement for sysvinit system.
systemd is the first process started by the kernel and holds PID-1.
This is a parent process for everything and Fedora-15 is the first distribution that embraced the systemd init manager instead of Upstart.
systemctl is a command-line application and the primary tool for managing systemd daemons/services, allowing users to start, restart, stop, enable, disable, reload, and verify the service.
systemd uses .service files instead of bash scripts used by SysVinit.
systemd sorts all daemons in their own Linux cgroups and you can view the system hierarchy by examining the /cgroup/systemd file.
The following articles will help you to learn more about SysVinit and systemd.
- How To Check All Running Services In Linux
- How To Determine Which System Manager Is Running On Linux System
- How to Enable or Disable Services on Boot in Linux Using chkconfig and systemctl Command?
Many init systems have been released, but the three init systems below are widely used in Linux.
System V (Sys V):System V (Sys V) is one of the first and traditional init systems for the UNIX/Linux operating system.Upstart:Upstart is an event-based replacement for the /sbin/init daemon.systemd:systemd is a new init system and service manager for Linux operating systems.
What is System V (Sys V)
System V (Sys V) is one of the first and traditional init systems for the UNIX/Linux operating system. Init is the first process started by the kernel during system boot, and is a parent process for everything.
Most Linux distributions first started using the traditional init system, called System V (Sys V).
Over the years, several alternative init systems have been released to address design limitations in the stable versions such as launchd, Service Management, systemd and Upstart.
But the systemd has been adopted by many large Linux distributions over the traditional SysVinit manager.
What is systemd
systemd is a new init system and system manager that has become so popular that it has been widely transformed into the new standard init system by most Linux distributions.
Systemctl is a systemd application that allows you to manage the systemd system.
systemd Features:
- systemd provides aggressive parallelization capabilities
- Uses socket and D-Bus activation for starting services
- Offers on-demand starting of daemons
- Keeps track of processes using Linux cgroups
- Supports snapshotting and restoring of the system state
- Maintains mount and automount points
- Implements an elaborate transactional dependency-based service control logic
Service Commands:
These are the most commonly used service commands on a Linux system to manage services.
| Short Description | SysVinit Command | systemd Command |
| To Start a Service | service example start | systemctl start example |
| To Stop a Service | service example stop | systemctl stop example |
| Stop and then Start a Service (Restart a Service) | service example restart | systemctl restart example |
| Reload a Service (Reload the config file) | service example reload | systemctl reload example |
| Restarts if the service is already running | service example condrestart | systemctl condrestart example |
| How to check if a service is currently running | service example status | systemctl status example |
| How to enable a service on boot/startup | chkconfig example on | systemctl enable example |
| How to disable a service on boot/startup | chkconfig example off | systemctl disable example |
| How to check if a service is configured to start on boot or not | chkconfig example –list | systemctl is-enabled example |
| How to display a list of enabled or disabled services on boot with runlevels information | chkconfig | systemctl list-unit-files –type=service |
| Create a new service file or modify any configuration | chkconfig example –add | systemctl daemon-reload |
Runlevels/Targets:
The systemd has a concept of targets, which serves a similar purpose as runlevels, but operates slightly differently. Each target is named instead of numbers and is intended to serve a specific purpose.
| Short Description | SysVinit Command | systemd Command |
| To halt the system | 0, halt | runlevel0.target, poweroff.target, systemctl halt |
| Single user mode | 1, S, single | runlevel1.target, rescue.target |
| Multi User | 2 | runlevel2.target, multi-user.target |
| Multi User with Network | 3 | runlevel3.target, multi-user.target |
| Experimental (No User) | 4 | runlevel4.target, multi-user.target |
| Multi-user with Graphical & Network | 5 | runlevel5.target, graphical.target |
| To reboot a system | 6, reboot | runlevel6.target, reboot.target, systemctl reboot |
| Emergency shell | emergency | emergency.target |
Other systemd Commands:
There are other commands in systemd that are very useful.
| Short Description | systemd Command |
| Control the system hostname | hostnamectl |
| Team daemon control tool | teamdctl |
| Control the system time and date | timedatectl |
| Query the systemd journal | journalctl |
| How to check system boot time | systemd-analyze or systemd-analyze time |
| How to check each service time consumption at startup | systemd-analyze blame |
| Kill all processes associated with a service | systemctl kill example |
| List all running services | systemctl |
| Show system status | systemctl status |
| How to run the systemd command on a remote machine | systemctl status example -H user@remotehost |
systemd is NOT new. It came out 6 years ago.
Yes but still some of the major Distribution not yet adopted (Mint)