RedHat RHEL/CentOS 7 Service Start Up Changes
RHEL/CentOS 7 Has A New Initialisation Daemon
The init daemon is the initialisation daemon first implemented in System V UNIX systems and has been around for over 40 years!
RedHat’s RHEL 7 and CentOS 7 introduces a replacement for init called systemd.
Startup & Run Levels
RHEL 6 (and CentOS 6) supported SysV init and Upstart with start and kill scripts created as links to scripts in /etc/rc.d/init.d scripts in the appropriate run level directory e.g. /etc/rc2.d.
RHEL 7 (and CentOS 7) implements system and service units, managed with the systemctl command.
The service command can still be used to bring services up and down as in RHEL and earlier, but is only provided in RHEL 7 for backwards compatibility.
| service Command (6) | systemctl Command (7) | Description |
|---|---|---|
|
service name start |
systemctl start name.service |
Starts a service. |
|
service name stop |
systemctl stop name.service |
Stops a service. |
|
service name restart |
systemctl restart name.service |
Restarts a service. |
|
service name cond restart |
systemctl try-restart name.service |
Restarts a service only if it is running. |
|
service name reload |
systemctl reload name.service |
Reloads configuration. |
|
service name status |
systemctl status name.service, systemctl is-active name.service |
Checks if a service is running. |
|
service --status-all |
systemctl list-units –type service –all |
Displays the status of all services. |
Changing Run Levels
The traditional init based systems implement run levels. The new systemd based systems work with targets.
| Run Level (6) | Target (7) | Description |
|---|---|---|
|
0 runlevel |
0. target, poweroff.target |
Shut down and power off the system. |
|
1 runlevel |
1. target, rescue.target |
Set up a rescue shell. |
|
2 runlevel |
2.target, multi-user.target |
Set up a non-graphical multi-user system. |
|
3 runlevel |
3. target, multi-user.target |
Set up a non-graphical multi-user system. |
|
4 runlevel |
4 .target, multi-user.target |
Set up a non-graphical multi-user system. |
|
5 runlevel |
5.target, graphical.target |
Set up a graphical multi-user system. |
|
6 runlevel |
6.target, reboot.target |
Shut down and reboot the system. |
Shortcut Commands To Change Run Levels and Targets
The following commands are quick ways to change the run level or target of a Linux system.
| Command(6) | Command(7) | Description |
|---|---|---|
|
halt |
systemctl halt |
Halts the system. |
|
poweroff |
systemctl poweroff |
Powers off the system. |
|
reboot |
systemctl reboot |
Restarts the system. |
|
pm-suspend |
systemctl suspend |
Suspends the system. |
|
pm-hibernate |
systemctl hibernate |
Hibernates the system. |
|
pm-suspend -hybrid |
systemctl hybrid-sleep |
Hibernates and suspends the system. |
Working With Targets
Check the Default Run Level or Target
For init based systems the default run level is defined through an entry in the /etc/inittab file. The line with a label initdefault defines the run level the system will boot to by default.
The new systemd based systems use the systemctl command to set and check the default target to boot to.
systemctl – get-defaultList the Active Targets
systemctl list-units –type targetChange the Default Target
systemctl set-default multi-user.target
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