Last week I mentioned that systemd has replaced the traditional init program in Linux. Let’s see some capabilities it adds!
Systemd organizes tasks into units which can include initializing hardware, mounting file systems, and starting services that will daemonize and run in the background. The active units are those which are enabled (that is, configured to be started within the current run state) and which should have successfully run to completion or still be running as a daemon.
You can list all the active units with this command:
$ systemctl list-units
Some of the units are targets. These are analogous to the run levels in SVR4, but you can tell the system to go to a state combining arbitrarily many targets of arbitrary complexity. The basic ones are rescue.target, multi-user.target, and graphical.target, analogous to the traditional run levels 1, 3, and 5, respectively. These will require (or “want”) other targets to have happened first. Maybe multi-user.target wants nfs-server.target, which in turns needs network.target, and before that, basic.target to do the basic system initialization, local-fs.target to mount the local file systems, and more. You can view a list of the available targets:
$ systemctl --type=target --all UNIT LOAD ACTIVE SUB JOB DESCRIPTION basic.target loaded active active Basic System cryptsetup.target loaded active active Encrypted Volumes emergency.target loaded inactive dead Emergency Mode final.target loaded inactive dead Final Step getty.target loaded active active Login Prompts graphical.target loaded active active Graphical Interface local-fs-pre.target loaded active active Local File Systems (Pre) local-fs.target loaded active active Local File Systems multi-user.target loaded active active Multi-User network.target loaded active active Network nfs.target loaded active active Network File System Client and nss-lookup.target loaded active active Host and Network Name Lookups nss-user-lookup.target loaded inactive dead User and Group Name Lookups printer.target loaded active active Printer remote-fs-pre.target loaded inactive dead Remote File Systems (Pre) remote-fs.target loaded active active Remote File Systems rescue.target loaded inactive dead Rescue Mode rpcbind.target loaded active active RPC Port Mapper shutdown.target loaded inactive dead Shutdown sockets.target loaded active active Sockets sound.target loaded active active Sound Card swap.target loaded active active Swap sysinit.target loaded active active System Initialization syslog.target loaded active active Syslog time-sync.target loaded active active System Time Synchronized umount.target loaded inactive dead Unmount All Filesystems LOAD = Reflects whether the unit definition was properly loaded. ACTIVE = The high-level unit activation state, i.e. generalization of SUB. SUB = The low-level unit activation state, values depend on unit type. JOB = Pending job for the unit. 26 loaded units listed. To show all installed unit files use 'systemctl list-unit-files'.
One of the more impressive commands has a simple directive. Try it on your system, ask about the fundamental network service:
$ systemctl show network
Wow! What a lot of information that command will show! It would take a lot of commands to track down as much of this as really practical on a traditional init system, and you would have to carefully analyze all the output.
You would still end up way short of what you can get with systemctl because, among many other things, systemd has integrated the restriction of resource use through control groups or cgroups into the service control system. We have just started showing people how to control resource utilization with cgroups in Learning Tree’s new Linux optimization and troubleshooting course.
It’s an advanced course and it just started running, I’ve taught the second North American run in New York in January. While many organizations are holding off on moving the Red Hat Enterprise Linux 7 (or the corresponding CentOS 7), I found it interesting how many people wanted to do some bonus exercise work in setting up systemd services controlled by cgroups.
The change from init to systemd is an enormous one. It completely changes system configuration, service control, hardware detection, and more. I have tried to capture the basics in what has turned into a rather large web page. Check out my “phrase book” comparing the syntax of classic init (RHEL 5), Upstart (RHEL 6), and systemd (RHEL 7, Ubuntu, Mageia, SuSE, and most of the other mainstream distros).
And, I hope to see you in one of the Linux courses soon!
