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Revision as of 20:18, 10 January 2014 by Sundaram (talk | contribs) (Clean up intro. Recommend .conf method to override unit file. Add tip on using systemd-delta)

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. For more information, watch the video at http://linuxconfau.blip.tv/file/4696791/ or http://www.youtube.com/watch?v=TyMLi8QF6sw

For system administrators
System administrators can visit this page, to understand how to use the native systemctl calls that replace their old workflow in SysVinit. Note that the service and chkconfig commands will continue to work as expected in the systemd world.

Why systemd?

http://0pointer.de/blog/projects/why.html

systemd documentation

systemd has very comprehensive documentation. Refer to

http://0pointer.de/blog/projects/systemd-docs.html

Tools

  • 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 page. This is a early version and needs more work. Do not use it for now unless you are a developer.

View the man pages for more details.

Boot Kernel Command Line

On boot systemd activates (by default), the target unit default.target whose job is to activate services and other units by pulling them in via dependencies.

To override the unit to activate, systemd parses its own kernel command line arguments via the systemd.unit= command line option. This may be used to temporarily boot into a different boot unit. The classical run-levels are replaced as following:

systemd.unit=rescue.target is a special target unit for setting up the base system and a rescue shell (similar to run level 1); systemd.unit=emergency.target, is very similar to passing init=/bin/sh but with the option to boot the full system from there; systemd.unit=multi-user.target for setting up a non-graphical multi-user system; systemd.unit=graphical.target for setting up a graphical login screen.

For details about these special systemd boot units, view the man systemd.special page. it scripts

What is the tool to manage services with systemd?

systemctl is the primary tool to use. It combines the functionality of both service and chkconfig into a single tool that you can use for instance to enable/disable services permanently or only for the current session.

list all running services etc: 

 systemctl

Refer to man systemctl for more details. systemd-cgls lists the running process in a tree format. It can recursively show the content of any given control group. Refer to man systemd-cgls for more details.

How do I start/stop or enable/disable services?

Activates a service immediately: 

  systemctl start foo.service

Deactivates a service immediately: 

  systemctl stop foo.service

Restarts a service: 

 systemctl restart foo.service

Shows status of a service including whether it is running or not: 

 systemctl status foo.service

Enables a service to be started on bootup: 

 systemctl enable foo.service

Disables a service to not start during bootup: 

 systemctl disable foo.service

Check whether a service is already enabled or not: 

 systemctl is-enabled foo.service; echo $?

0 indicates that it is enabled, and 1 indicates that it is disabled. In Fedora 17, in addition to the return code, "enabled" or "disabled" will be printed to stdout.

Refer to man systemctl for more details.

How do I change the runlevel?

systemd has the concept of targets which is a more flexible replacement for runlevels in sysvinit.

Run level 3 is emulated by multi-user.target. Run level 5 is emulated by graphical.target. runlevel3.target is a symbolic link to multi-user.target and runlevel5.target is a symbolic link to graphical.target.

You can switch to 'runlevel 3' by running 

 systemctl isolate multi-user.target (or) systemctl isolate runlevel3.target

You can switch to 'runlevel 5' by running 

 systemctl isolate graphical.target (or) systemctl isolate runlevel5.target

How do I change the default runlevel?

Symlinks can be still used, but they were obsoleted by a built-in command to set the default target. For example, default graphical target can be set by: 

 systemctl enable graphical.target --force

Symlinks:

systemd can also use symlinks to point to the default runlevel. You have to delete the existing symlink first before creating a new one 

 rm /etc/systemd/system/default.target

Switch to runlevel 3 by default 

 ln -sf /lib/systemd/system/multi-user.target /etc/systemd/system/default.target

Switch to runlevel 5 by default 

 ln -sf /lib/systemd/system/graphical.target /etc/systemd/system/default.target

systemd does not use /etc/inittab file.

How do I know the current run level?

runlevel command still works with systemd. You can continue using that however runlevels is a legacy concept in systemd and is emulated via 'targets' and multiple targets can be active at the same time. So the equivalent in systemd terms is 

 systemctl list-units --type=target

How to power off the machine ?

You can use 

poweroff

Some more possibilities are: halt -p, init 0, shutdown -P now

Note that halt used to work the same as poweroff in previous Fedora releases, but systemd distinguishes between the two, so halt without parameters now does exactly what it says - it merely stops the system without turning it off.

Does service command work with systemd?

Yes. It has been modified to call systemctl automatically when dealing with systemd service files. So either of the following commands does the same thing 

 service NetworkManager stop

(or) 

 systemctl stop NetworkManager.service

Does chkconfig command work with systemd?

Yes, for turning on/off services, compatibility has been provided both ways. chkconfig has been modified to call systemctl when dealing with systemd service files. Also systemctl automatically calls chkconfig when dealing with a traditional sysv init file.

So either of the following commands does the same thing 

 chkconfig NetworkManager off

(or) 

 systemctl disable NetworkManager.service

chkconfig --list doesn't list systemd services, only Sys V services. The output of chkconfig takes note of this, along with supplying additional information.

Does system-config-services work with systemd?

Yes. It does.

How do I change the number of gettys running by default?

To add another getty:

Simply place another symlink for instantiating another getty in the getty.target.wants/ directory: 

ln -sf /lib/systemd/system/getty@.service /etc/systemd/system/getty.target.wants/getty@tty9.service
systemctl daemon-reload
systemctl start getty@tty9.service

To remove a getty:

Simply remove the getty symlinks you want to get rid of in the getty.target.wants/ directory: 

rm /etc/systemd/system/getty.target.wants/getty@tty5.service /etc/systemd/system/getty.target.wants/getty@tty6.service
systemctl daemon-reload
systemctl stop getty@tty5.service getty@tty6.service

systemd does not use /etc/inittab file.

How do I set automatic login on a virtual console terminal?

First create a new service similar to getty@.service: 

# cp /lib/systemd/system/getty@.service \
     /etc/systemd/system/autologin@.service
# ln -s /etc/systemd/system/autologin@.service \
        /etc/systemd/system/getty.target.wants/getty@tty8.service

then edit ExecStart, Restart and Alias values, like this: 

...
ExecStart=-/sbin/mingetty --autologin USERNAME %I
Restart=no
...
Alias=getty.target.wants/getty@tty8.service

and finally reload daemon and start the service: 

systemctl daemon-reload
systemctl start getty@tty8.service

Note that if you exit tty8 session, you wont be able to use it until next reboot or manual start by systemctl, except if you leave Restart as ‘always’, but I highly recommend to avoid this according to security reasons.

How do I customize a unit file/ add a custom unit file?

The best way to customize unit files is to add /etc/systemd/system/foobar.service.d/*.conf where foobar.service is the name of the service you want to customize. If a directory doesn't already exist, create one and drop a conf file with the settings you want to override. For example,

/etc/systemd/system/httpd.service/restart.conf

[Service] Restart=always RestartSec=30

Refer to man systemd.unit page for more details.

Alternatively, you can copy the distribution provided unit file from /lib/systemd/system to /etc/systemd/system since the latter has higher precedence. If a line starts with .include followed by a file name, the specified file will be parsed at this point. Make sure that the file that is included has the appropiate section headers before any directives.You should use .include statement instead of copying the whole unit file from /lib/systemd/system to /etc/systemd/system if possible. This will enable to update the unchanged directives correctly during future package updates.

Be careful when using .include together with directives that can be defined multiple times (like EnvironmentFile=), since we can only add new directives, but we can't remove already defined ones. We have to copy the whole file from /lib/systemd/system to /etc/systemd/system in this case.

Let's say we use a lighttpd server and we want to lower its niceness value. All we need to do is to add Nice=-5 to the lighttpd.service file. We can do this by either copying the whole file from /lib/systemd/system/lighttpd.service to /etc/systemd/system/lighttpd.service or creating the following file in /etc/systemd/system/lighttpd.service: 

.include /lib/systemd/system/lighttpd.service 
[Service]
Nice=-5
Don't forget to reload systemd daemon using systemctl daemon-reload and systemctl restart foobar after editing a unit file where foobar is the name of the unit. Also note that you can systemd-delta to list the unit files which have been customized and also the precise differences
When modifying an unit that has an symlink pointing to that unit for example like the display-manager.service -> prefdm.service does, the symlink should be copied instead of the actual unit as in display-manager.service should be copied to the /etc/systemd/system directory or a new unit created with .includes that bears that name .

How do I debug systemd issues?

Refer to How_to_debug_Systemd_problems

Readahead

systemd has a built-in readahead implementation which is not enabled on upgrades. It should improve bootup speed but your mileage may vary depending on your hardware. To enable it: 

systemctl enable systemd-readahead-collect.service
systemctl enable systemd-readahead-replay.service

Warnings on separate /usr partition

Refer to http://freedesktop.org/wiki/Software/systemd/separate-usr-is-broken and http://cgit.freedesktop.org/systemd/tree/README for details.

Man pages

systemd comes with extensive documentation including several man pages. The web version is at

http://www.freedesktop.org/software/systemd/man/

References

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