Systemd is new service manager for Linux. It is a replacement for init system and can manage system startup and services. It starts up and supervises the entire system. In the article we are using centos 7.0 installed with systemd 216 version and the latest version is available for download from freedesktop.org.
PID 1 is occupied by "systemd" and can be seen from pstree command as well:
[[email protected] ~]# pstree
Let's explore what systemd is capable of and what possibilities we have with the new replacement for sysVinit.
1) Faster startup
The sysvinit starts the processes serially, one at a time. Systemd starts services in parallel and starts only those services which are actually required, reducing the boot time significantly.
You can get the boot process duration with the following command:
[[email protected] ~]# systemd-analyze
The command systemd-analyze time also shows the same information.
[[email protected] ~]# systemd-analyze time
If you want to print a list of all running units, the blame option to systemd-analyze command can provide you with that, ordered by the time taken to initialize.
[[email protected] ~]# systemd-analyze blame
The above screen shows only a small number of processes, you can scroll through the list with arrows just like in less pager.
2) systemctl command
The systemctl command is the most talked command that comes with systemd. You can manage a whole lot of your system with this command. Let's explore this command before going any further:
List Units
systemctl command without any option lists all the running units. The list-units switch also does the same.
[[email protected] ~]# systemctl
or
[[email protected] ~]# systemctl list-units
Listing failed units
The failed units can be listed with --failed switch.
[[email protected] ~]# systemctl --failed
You will see the use of systemctl command at many places in this article.
3) Managing services
Let us now see how services can be managed with systemd.
Active services
All the active services can be checked with the following command:
[[email protected] ~]# systemctl list-units -t service
Service status
In the sysvinit, we could use the "service" command to manage the services, but with systemd, the systemctl command is used to manage services. In ordwer to see whether a service is running or not, we can use the systemctl command like this:
[[email protected] ~]# systemctl status dnsmasq
Start a service
To start a service, again we use the systemctl command as:
[[email protected] ~]# systemctl start dnsmasq
As opposed to service command, this command does not give any output. But of course, we can check the status of the service once again to confirm that its started successfully:
Stopping a service
Now you are smart enough and already know the command to stop a service with systemd:
[[email protected] ~]# systemctl stop dnsmasq
Restart a service
Similarly, restarting a service is managed using 'systemctl restart ':
[[email protected] ~]# systemctl restart dnsmasq
Reload a service
In case we need to reload the configuration of service (say ssh), without restarting it, we can use the command:
[[email protected] ~]# systemctl reload sshd
Although all of the above syntax is working, the official documentation suggests that these command be run with the following syntax:
[[email protected] ~]# systemctl status dnsmasq.service
4) Managing services at boot
The chkconfig command was used to manage services at boot. The same command systemd is used with systemd to manage services at boot.
Checking service status at boot
In order to check if a service is enabled on boot or not:
[[email protected] ~]# systemctl is-enabled dnsmasq.service
Enable a service at boot
systemctl command can be used like this to enable a service at boot (this corresponds to sysvinit 'chkconfig on')
[[email protected] ~]# systemctl enable dnsmasq.service
Disable a service at boot
Similarly, the services can be disabled at boot with systemctl command:
[[email protected] ~]# systemctl disable dnsmasq.service
5) Managing Remote systems
Typically, all of the above systemctl commands can be used to manage a remote host with systemctl command itself. This will use ssh for communication with the remote host. All you need to do is add the user and host to systemctl command like this:
[[email protected] ~]# systemctl status sshd -H [email protected]
6) Managing targets
Systemd has a concept of targets having a similar purpose to runlevels in sysVinit.
The runlevels in sysVinit were mostly numeric (0,1,2,...). Here are the runlevels in sysVinit with their systemd counterparts:
0 runlevel0.target, poweroff.target
1, s, single runlevel1.target, rescue.target
2, 4 runlevel2.target, runlevel4.target, multi-user.target
3 runlevel3.target, multi-user.target
5 runlevel5.target, graphical.target
6 runlevel6.target, reboot.target
emergency emergency.target
Changing current target
The current target(runlevel) can be changed with the command:
[[email protected] ~]# systemctl isolate graphical.target
List current target
If you want to see what target you are in, you need to list all the corresponding units. It might not feel at home with this new way, but its the way systemd works.
[[email protected] ~]# systemctl list-units --type=target
You can see "graphical.target" listed here. This is what we changed our target into. Now let's change the runlevel again to multi-user.target and then analyze this output:
[[email protected] ~]# systemctl isolate multi-user.target
[[email protected] ~]# systemctl list-units --type=target
List default target
To list the default target, we use systemctl command like this:
[[email protected] ~]# systemctl get-default
Change default target
The default target can be set with set-default command with systemctl:
[[email protected] ~]# systemctl set-default graphical.target
7) Logging in systemd
The systemd has its own logging system called journald. It replaces the syslog daemon from sysVinit. The command journalctl is used to read the logs.
[[email protected] ~]# journalctl
Boot messages
To see all boot messages, run the command "journalctl -b".
[[email protected] ~]# journalctl -b
Follow logs
The following command follows the system logs in real time (similar to tail -f).
[[email protected] ~]# journalctl -f
Service specific logs
To check logs specific to a particular service or executable, use journalctl like this:
[[email protected] ~]# journalctl /usr/sbin/dnsmasq
8) Power management
The systemctl command can be used to put the system down, or reboot or hibernate.
To poweroff, reboot, suspend and hibernate, use the following commands respectively:
[[email protected] ~]# systemctl poweroff [[email protected] ~]# systemctl reboot [[email protected] ~]# systemctl suspend [[email protected] ~]# systemctl reboot
9) Bonus Tip
The systemd brings out the whole new approach to interacting with your operating system. The systemd is so full of features. For example, you can get the hostname and other useful features about your Linux machine, you can use hostnamectl command
[[email protected] ~]# hostnamectl
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