Difference between revisions of "Systemd"

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*[http://fedoraproject.org/wiki/SysVinit_to_Systemd_Cheatsheet Fedora's SysVinit to systemd cheatsheet]
*[http://fedoraproject.org/wiki/SysVinit_to_Systemd_Cheatsheet Fedora's SysVinit to systemd cheatsheet]
*[[Allow Users to Shutdown|Configuring systemd to allow normal users to shutdown]]
*[[Allow Users to Shutdown|Configuring systemd to allow normal users to shutdown]]
*[http://wiki.gentoo.org/wiki/Systemd Gentoo Wiki systemd page]

Revision as of 00:53, 1 March 2014

zh-CN:Systemd zh-TW:Systemd

From the project web page:

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 control groups, supports snapshotting and restoring of the system state, maintains mount and automount points and implements an elaborate transactional dependency-based service control logic.
Note: For a detailed explanation as to why Arch has moved to systemd, see this forum post.

Basic systemctl usage

The main command used to introspect and control systemd is systemctl. Some of its uses are examining the system state and managing the system and services. See man 1 systemctl for more details.

Tip: You can use all of the following systemctl commands with the -H user@host switch to control a systemd instance on a remote machine. This will use SSH to connect to the remote systemd instance.
Note: systemadm is the official graphical frontend for systemctl. It is provided by the systemd-ui-gitAUR package from the AUR.

Analyzing the system state

List running units:

$ systemctl


$ systemctl list-units

List failed units:

$ systemctl --failed

The available unit files can be seen in /usr/lib/systemd/system/ and /etc/systemd/system/ (the latter takes precedence). You can see a list of the installed unit files with:

$ systemctl list-unit-files

Using units

Units can be, for example, services (.service), mount points (.mount), devices (.device) or sockets (.socket).

When using systemctl, you generally have to specify the complete name of the unit file, including its suffix, for example sshd.socket. There are however a few short forms when specifying the unit in the following systemctl commands:

  • If you do not specify the suffix, systemctl will assume .service. For example, netcfg and netcfg.service are equivalent.
  • Mount points will automatically be translated into the appropriate .mount unit. For example, specifying /home is equivalent to home.mount.
  • Similar to mount points, devices are automatically translated into the appropriate .device unit, therefore specifying /dev/sda2 is equivalent to dev-sda2.device.

See man systemd.unit for details.

Tip: Most of the following commands also work if multiple units are specified, see man systemctl for more information.

Activate a unit immediately:

# systemctl start unit

Deactivate a unit immediately:

# systemctl stop unit

Restart a unit:

# systemctl restart unit

Ask a unit to reload its configuration:

# systemctl reload unit

Show the status of a unit, including whether it is running or not:

$ systemctl status unit

Check whether a unit is already enabled or not:

$ systemctl is-enabled unit

Enable a unit to be started on bootup:

# systemctl enable unit
Note: Services without an [Install] section are usually called automatically by other services. If you need to install them manually, use the following command, replacing foo with the name of the service.
# ln -s /usr/lib/systemd/system/foo.service /etc/systemd/system/graphical.target.wants/

Disable a unit to not start during bootup:

# systemctl disable unit

Show the manual page associated with a unit (this has to be supported by the unit file):

$ systemctl help unit

Reload systemd, scanning for new or changed units:

# systemctl daemon-reload

Power management

polkit is necessary for power management as an unprivileged user. If you are in a local systemd-logind user session and no other session is active, the following commands will work without root privileges. If not (for example, because another user is logged into a tty), systemd will automatically ask you for the root password.

Shut down and reboot the system:

$ systemctl reboot

Shut down and power-off the system:

$ systemctl poweroff

Suspend the system:

$ systemctl suspend

Put the system into hibernation:

$ systemctl hibernate

Put the system into hybrid-sleep state (or suspend-to-both):

$ systemctl hybrid-sleep

Writing custom .service files

The syntax of systemd's unit files is inspired by XDG Desktop Entry Specification .desktop files, which are in turn inspired by Microsoft Windows .ini files.

See systemd/Services for more examples.

Handling dependencies

With systemd, dependencies can be resolved by designing the unit files correctly. The most typical case is that the unit A requires the unit B to be running before A is started. In that case add Requires=B and After=B to the [Unit] section of A. If the dependency is optional, add Wants=B and After=B instead. Note that Wants= and Requires= do not imply After=, meaning that if After= is not specified, the two units will be started in parallel.

Dependencies are typically placed on services and not on targets. For example, network.target is pulled in by whatever service configures your network interfaces, therefore ordering your custom unit after it is sufficient since network.target is started anyway.


There are several different start-up types to consider when writing a custom service file. This is set with the Type= parameter in the [Service] section. See man systemd.service for a more detailed explanation.

  • Type=simple (default): systemd considers the service to be started up immediately. The process must not fork. Do not use this type if other services need to be ordered on this service, unless it is socket activated.
  • Type=forking: systemd considers the service started up once the process forks and the parent has exited. For classic daemons use this type unless you know that it is not necessary. You should specify PIDFile= as well so systemd can keep track of the main process.
  • Type=oneshot: this is useful for scripts that do a single job and then exit. You may want to set RemainAfterExit=yes as well so that systemd still considers the service as active after the process has exited.
  • Type=notify: identical to Type=simple, but with the stipulation that the daemon will send a signal to systemd when it is ready. The reference implementation for this notification is provided by libsystemd-daemon.so.
  • Type=dbus: the service is considered ready when the specified BusName appears on DBus's system bus.

Editing provided unit files

To edit a unit file provided by a package, you can create a directory called /etc/systemd/system/unit.d/ for example /etc/systemd/system/httpd.service.d/ and place *.conf files in there to override or add new options. systemd will parse these *.conf files and apply them on top of the original unit. For example, if you simply want to add an additional dependency to a unit, you may create the following file:

Requires=new dependency
After=new dependency

As another example, in order to replace the ExecStart directive for a unit that is not of type oneshot, create the following file:

ExecStart=new command

One more example to automatically restart a service:


Then run the following for your changes to take effect:

# systemctl daemon-reload
# systemctl restart unit

Alternatively you can copy the old unit file from /usr/lib/systemd/system/ to /etc/systemd/system/ and make your changes there. A unit file in /etc/systemd/system/ always overrides the same unit in /usr/lib/systemd/system/. Note that when the original unit in /usr/lib/ is changed due to a package upgrade, these changes will not automatically apply to your custom unit file in /etc/. Additionally you will have to manually reenable the unit with systemctl reenable unit. It is therefore recommended to use the *.conf method described before instead.

Tip: You can use systemd-delta to see which unit files have been overridden and what exactly has been changed.

As the provided unit files will be updated from time to time, use systemd-delta for system maintenance.

Syntax highlighting for units within Vim

Syntax highlighting for systemd unit files within Vim can be enabled by installing vim-systemd from the official repositories.


systemd uses 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 with the possibility of having multiple ones active at the same time. Some targets are implemented by inheriting all of the services of another target and adding additional services to it. There are systemd targets that mimic the common SystemVinit runlevels so you can still switch targets using the familiar telinit RUNLEVEL command.

Get current targets

The following should be used under systemd instead of running runlevel:

$ systemctl list-units --type=target

Create custom target

The runlevels that are assigned a specific purpose on vanilla Fedora installs; 0, 1, 3, 5, and 6; have a 1:1 mapping with a specific systemd target. Unfortunately, there is no good way to do the same for the user-defined runlevels like 2 and 4. If you make use of those it is suggested that you make a new named systemd target as /etc/systemd/system/your target that takes one of the existing runlevels as a base (you can look at /usr/lib/systemd/system/graphical.target as an example), make a directory /etc/systemd/system/your target.wants, and then symlink the additional services from /usr/lib/systemd/system/ that you wish to enable.

Targets table

SysV Runlevel systemd Target Notes
0 runlevel0.target, poweroff.target Halt the system.
1, s, single runlevel1.target, rescue.target Single user mode.
2, 4 runlevel2.target, runlevel4.target, multi-user.target User-defined/Site-specific runlevels. By default, identical to 3.
3 runlevel3.target, multi-user.target Multi-user, non-graphical. Users can usually login via multiple consoles or via the network.
5 runlevel5.target, graphical.target Multi-user, graphical. Usually has all the services of runlevel 3 plus a graphical login.
6 runlevel6.target, reboot.target Reboot
emergency emergency.target Emergency shell

Change current target

In systemd targets are exposed via target units. You can change them like this:

# systemctl isolate graphical.target

This will only change the current target, and has no effect on the next boot. This is equivalent to commands such as telinit 3 or telinit 5 in Sysvinit.

Change default target to boot into

The standard target is default.target, which is aliased by default to graphical.target (which roughly corresponds to the old runlevel 5). To change the default target at boot-time, append one of the following kernel parameters to your bootloader:

Tip: The .target extension can be left out.
  • systemd.unit=multi-user.target (which roughly corresponds to the old runlevel 3),
  • systemd.unit=rescue.target (which roughly corresponds to the old runlevel 1).

Alternatively, you may leave the bootloader alone and change default.target. This can be done using systemctl:

# systemctl enable multi-user.target

The effect of this command is output by systemctl; a symlink to the new default target is made at /etc/systemd/system/default.target. This works if, and only if:


is in the target's configuration file. Currently, multi-user.target and graphical.target both have it.

Temporary files

"systemd-tmpfiles creates, deletes and cleans up volatile and temporary files and directories." It reads configuration files in /etc/tmpfiles.d/ and /usr/lib/tmpfiles.d/ to discover which actions to perform. Configuration files in the former directory take precedence over those in the latter directory.

Configuration files are usually provided together with service files, and they are named in the style of /usr/lib/tmpfiles.d/program.conf. For example, the Samba daemon expects the directory /run/samba to exist and to have the correct permissions. Therefore, the samba package ships with this configuration:

D /run/samba 0755 root root

Configuration files may also be used to write values into certain files on boot. For example, if you used /etc/rc.local to disable wakeup from USB devices with echo USBE > /proc/acpi/wakeup, you may use the following tmpfile instead:

w /proc/acpi/wakeup - - - - USBE

See the systemd-tmpfiles and tmpfiles.d(5) man pages for details.

Note: This method may not work to set options in /sys since the systemd-tmpfiles-setup service may run before the appropriate device modules is loaded. In this case you could check whether the module has a parameter for the option you want to set with modinfo module and set this option with a config file in /etc/modprobe.d. Otherwise you will have to write a udev rule to set the appropriate attribute as soon as the device appears.


Systemd can replace cron functionality to a great extent. For further information, please refer to systemd/cron functionality.


systemd has its own logging system called the journal; therefore, running a syslog daemon is no longer required. To read the log, use:

# journalctl

In Arch Linux, the directory /var/log/journal/ is a part of the systemd package, and the journal (when Storage= is set to auto in /etc/systemd/journald.conf) will write to /var/log/journal/. If you or some program delete that directory, systemd will not recreate it automatically; however, it will be recreated during the next update of the systemd package. Until then, logs will be written to /run/systemd/journal, and logs will be lost on reboot.

Tip: If /var/log/journal/ resides in a btrfs file system, you should consider disabling Copy-on-Write for the directory. See the main article for details: Btrfs#Copy-On-Write (CoW).

Filtering output

journalctl allows you to filter the output by specific fields. Be aware that if there are many messages to display or filtering of large time span has to be done, the output of this command can be delayed for quite some time.


Show all messages from this boot:

# journalctl -b

However, often one is interested in messages not from the current, but from the previous boot (e.g. if an unrecoverable system crash happened). This is possible through optional offset parameter of the -b flag: journalctl -b -0 shows messages from the current boot, journalctl -b -1 from the previous boot, journalctl -b -2 from the second previous and so on. See man 1 journalctl for full description, the semantics is much more powerful.

Follow new messages:

# journalctl -f

Show all messages by a specific executable:

# journalctl /usr/lib/systemd/systemd

Show all messages by a specific process:

# journalctl _PID=1

Show all messages by a specific unit:

# journalctl -u netcfg

Show kernel ring buffer:

# journalctl _TRANSPORT=kernel

See man 1 journalctl, man 7 systemd.journal-fields, or Lennert's blog post for details.

Journal size limit

If the journal is persistent (non-volatile), its size limit is set to a default value of 10% of the size of the respective file system. For example, with /var/log/journal located on a 50 GiB root partition this would lead to 5 GiB of journal data. The maximum size of the persistent journal can be controlled by SystemMaxUse in /etc/systemd/journald.conf, so to limit it for example to 50 MiB uncomment and edit the corresponding line to:


Refer to man journald.conf for more info.

Journald in conjunction with syslog

Compatibility with classic syslog implementations is provided via a socket /run/systemd/journal/syslog, to which all messages are forwarded. To make the syslog daemon work with the journal, it has to bind to this socket instead of /dev/log (official announcement). The syslog-ng package in the repositories automatically provides the necessary configuration.

# systemctl enable syslog-ng

A good journalctl tutorial is here.

Forward journald to /dev/tty12

In /etc/systemd/journald.conf enable the following:


Restart journald with sudo systemctl restart systemd-journald.


Investigating systemd errors

As an example, we will investigate an error with systemd-modules-load service:

1. Lets find the systemd services which fail to start:

$ systemctl --state=failed
systemd-modules-load.service   loaded failed failed  Load Kernel Modules

2. Ok, we found a problem with systemd-modules-load service. We want to know more:

$ systemctl status systemd-modules-load
systemd-modules-load.service - Load Kernel Modules
   Loaded: loaded (/usr/lib/systemd/system/systemd-modules-load.service; static)
   Active: failed (Result: exit-code) since So 2013-08-25 11:48:13 CEST; 32s ago
     Docs: man:systemd-modules-load.service(8).
  Process: 15630 ExecStart=/usr/lib/systemd/systemd-modules-load (code=exited, status=1/FAILURE)

If the Process ID is not listed, just restart the failed service with systemctl restart systemd-modules-load

3. Now we have the process id (PID) to investigate this error in depth. Enter the following command with the current Process ID (here: 15630):

$ journalctl -b _PID=15630
-- Logs begin at Sa 2013-05-25 10:31:12 CEST, end at So 2013-08-25 11:51:17 CEST. --
Aug 25 11:48:13 mypc systemd-modules-load[15630]: Failed to find module 'blacklist usblp'
Aug 25 11:48:13 mypc systemd-modules-load[15630]: Failed to find module 'install usblp /bin/false'

4. We see that some of the kernel module configs have wrong settings. Therefore we have a look at these settings in /etc/modules-load.d/:

$ ls -Al /etc/modules-load.d/
-rw-r--r--   1 root root    79  1. Dez 2012  blacklist.conf
-rw-r--r--   1 root root     1  2. Mär 14:30 encrypt.conf
-rw-r--r--   1 root root     3  5. Dez 2012  printing.conf
-rw-r--r--   1 root root     6 14. Jul 11:01 realtek.conf
-rw-r--r--   1 root root    65  2. Jun 23:01 virtualbox.conf

5. The Failed to find module 'blacklist usblp' error message might be related to a wrong setting inside of blacklist.conf. Lets deactivate it with inserting a trailing # before each option we found via step 3:

# blacklist usblp
# install usblp /bin/false

6. Now, try to start systemd-modules-load:

$ systemctl start systemd-modules-load.service

If it was successful, this shouldn't prompt anything. If you see any error, go back to step 3. and use the new PID for solving the errors left.

If everything is ok, you can verify that the service was started successfully with:

$ systemctl status systemd-modules-load
systemd-modules-load.service - Load Kernel Modules
   Loaded: loaded (/usr/lib/systemd/system/systemd-modules-load.service; static)
   Active: active (exited) since So 2013-08-25 12:22:31 CEST; 34s ago
     Docs: man:systemd-modules-load.service(8)
 Process: 19005 ExecStart=/usr/lib/systemd/systemd-modules-load (code=exited, status=0/SUCCESS)
Aug 25 12:22:31 mypc systemd[1]: Started Load Kernel Modules.

Often you can solve these kind of problems like shown above. For further investigation look at Diagnosing boot problems.

Diagnosing boot problems

Boot with these parameters on the kernel command line: systemd.log_level=debug systemd.log_target=kmsg log_buf_len=1M

More Debugging Information

Shutdown/reboot takes terribly long

If the shutdown process takes a very long time (or seems to freeze) most likely a service not exiting is to blame. systemd waits some time for each service to exit before trying to kill it. To find out if you are affected, see this article.

Short lived processes do not seem to log any output

If journalctl -u foounit does not show any output for a short lived service, look at the PID instead. For example, if systemd-modules-load.service fails, and systemctl status systemd-modules-load shows that it ran as PID 123, then you might be able to see output in the journal for that PID, i.e. journalctl -b _PID=123. Metadata fields for the journal such as _SYSTEMD_UNIT and _COMM are collected asynchronously and rely on the /proc directory for the process existing. Fixing this requires fixing the kernel to provide this data via a socket connection, similar to SCM_CREDENTIALS.

Disabling application crash dumps journaling

Run the following in order to overwrite the settings from /lib/sysctl.d/:

# ln -s /dev/null /etc/sysctl.d/50-coredump.conf
# sysctl kernel.core_pattern=core

This will disable logging of coredumps to the journal.

Note that the default RLIMIT_CORE of 0 means that no core files are written, either. If you want them, you also need to "unlimit" the core file size in the shell:

$ ulimit -c unlimited

See sysctl.d and the documentation for /proc/sys/kernel for more information.

Error message on reboot or shutdown

cgroup : option or name mismatch, new: 0x0 "", old: 0x4 "systemd"

See this thread for an explanation.

watchdog watchdog0: watchdog did not stop!

See this thread for an explanation.

See also