Showing posts with label systemd. Show all posts
Showing posts with label systemd. Show all posts

Friday, September 26, 2014

transition from sysV to systemd, from chkconfig to systemctl

If you have just been installed CentOS 7.0 and as usual, command chkconfig is executed
to list what processes will be start on boot. As seen below:
[root@localhost ~]# chkconfig

Note: This output shows SysV services only and does not include native
systemd services. SysV configuration data might be overridden by native
systemd configuration.

If you want to list systemd services use 'systemctl list-unit-files'.
To see services enabled on particular target use
'systemctl list-dependencies [target]'.

iprdump 0:off 1:off 2:on 3:on 4:on 5:on 6:off
iprinit 0:off 1:off 2:on 3:on 4:on 5:on 6:off
iprupdate 0:off 1:off 2:on 3:on 4:on 5:on 6:off
network 0:off 1:off 2:on 3:on 4:on 5:on 6:off
tomcat 0:off 1:off 2:off 3:off 4:off 5:off 6:off

That's odd, something has changed. For your information, sysV has been replaced in favor of systemd and today we are going to learn what is systemd is. So what is systemd ?

systemd is a system and service manager for Linux, compatible with SysV and LSB init scripts. 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 and implements an elaborate transactional dependency-based service control logic. It can work as a drop-in replacement for sysvinit. 

That is a very lengthy definition. If you are still not so sure, perhaps take a moment to watch a video here.

Because there are a lot of documentations in the google to explain what is systemd in details, but this article will target busy people who need the solution right now. As such, if you want more details solutions, you should google or read a few helpful links below.

So why replace sysV with systemd? What have been improved?

Lennart Poettering and Kay Sievers, the software engineers who initially developed systemd,[1] sought to surpass the efficiency of the init daemon in several ways. They wanted to improve the software framework for expressing dependencies; to allow more processing to be done concurrently or in parallel during system booting; and to reduce the computational overhead of the shell.

Systemd's initialization instructions for each daemon are recorded in a declarative configuration file rather than a shell script. For inter-process communication, systemd makes Unix domain sockets and D-Bus available to the running daemons. Systemd is also capable of aggressive parallelization.

There are several tools to manage systemd.

  • systemctl:
    used to introspect and control the state of the systemd system and service manager

  • systemd-cgls:
    recursively shows the contents of the selected Linux control group hierarchy in a tree

  • systemadm:
    a graphical frontend for the systemd system and service manager that allows introspection and control of systemd. Part of the systemd-gtk package. This is an early version and needs more work. Do not use it for now unless you are a developer.

Below are a table to summarize what you usually done in chkconfig and in systemd, what command you can use as a replacement.

Sysvinit CommandSystemd CommandNotes
service frobozz startsystemctl start frobozz.serviceUsed to start a service (not reboot persistent)
service frobozz stopsystemctl stop frobozz.serviceUsed to stop a service (not reboot persistent)
service frobozz restartsystemctl restart frobozz.serviceUsed to stop and then start a service
service frobozz reloadsystemctl reload frobozz.serviceWhen supported, reloads the config file without interrupting pending operations.
service frobozz condrestartsystemctl condrestart frobozz.serviceRestarts if the service is already running.
service frobozz statussystemctl status frobozz.serviceTells whether a service is currently running.
ls /etc/rc.d/init.d/systemctl list-unit-files --type=service (preferred)
ls /lib/systemd/system/*.service /etc/systemd/system/*.service
Used to list the services that can be started or stopped Used to list all the services and other units
chkconfig frobozz onsystemctl enable frobozz.serviceTurn the service on, for start at next boot, or other trigger.
chkconfig frobozz offsystemctl disable frobozz.serviceTurn the service off for the next reboot, or any other trigger.
chkconfig frobozzsystemctl is-enabled frobozz.serviceUsed to check whether a service is configured to start or not in the current environment.
chkconfig --listsystemctl list-unit-files --type=service(preferred)
ls /etc/systemd/system/*.wants/
Print a table of services that lists which runlevels each is configured on or off
chkconfig frobozz --listls /etc/systemd/system/*.wants/frobozz.serviceUsed to list what levels this service is configured on or off
chkconfig frobozz --addsystemctl daemon-reloadUsed when you create a new service file or modify any configuration


Systemd has a concept of targets which serve a similar purpose as runlevels but act a little different. Each target is named instead of numbered and is intended to serve a specific purpose.

Sysvinit RunlevelSystemd TargetNotes, poweroff.targetHalt the system.
1, s,, rescue.targetSingle user mode.
2,,, multi-user.targetUser-defined/Site-specific runlevels. By default, identical to 3., multi-user.targetMulti-user, non-graphical. Users can usually login via multiple consoles or via the network., graphical.targetMulti-user, graphical. Usually has all the services of runlevel 3 plus a graphical login., reboot.targetReboot
emergencyemergency.targetEmergency shell

Below are a summarize the command you will (hopefully) use.

  • systemctl isolate
    To change the target/runlevel, to switch to runlevel 3

  • systemctl set-default <name of target>.target is the default. You might want for the equivalent of non graphical (runlevel 3) from sysv init.

  • systemctl get-default
    to show the currentl target/runlevel

Note, there are several changes you should keep in mind.
* systemd does not use /etc/inittab file.
* change number of gettys in /etc/systemd/logind.conf
* unit files are now store in /usr/lib/systemd/system/

That's it, I hope you get a basic understanding and will be able to start using systemd.