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muru
Asked: 2015-08-09 19:31:49 +0800 CST

How do I override or configure systemd services?

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Many sysv init scripts used a corresponding file in /etc/default to allow the administrator to configure it. Upstart jobs can be modified using .override files. How do I override or configure systemd units, now that systemd is the default in Ubuntu?

configuration

1 Answers

  1. Best Answer

    systemd units need not obey files in /etc/default. systemd is easily configurable, but requires that you know the syntax of systemd unit files.

    Packages ship unit files typically in /lib/systemd/system/. These are not to be edited. Instead, systemd allows you to override these files by creating appropriate files in /etc/systemd/system/.

    For a given service foo, the package would provide /lib/systemd/system/foo.service. You can check its status using systemctl status foo, or view its logs using journalctl -u foo. To override something in the definition of foo, do:

    sudo systemctl edit foo

    This creates a directory in /etc/systemd/system named after the unit, and an override.conf file in that directory (/etc/systemd/system/foo.service.d/override.conf). You can add or override settings using this file (or other .conf files in /etc/systemd/system/foo.service.d/). This is also applicable to non-service units - you could do systemctl edit foo.mount, systemctl edit foo.timer, etc.

    Overriding command arguments

    Take the getty service for example. Say I want to have TTY2 autologin to my user (this is not advisable, but just an example). TTY2 is run by the getty@tty2 service (tty2 being an instance of the template /lib/systemd/system/getty@service). To do this, I have to modify the getty@tty2 service.

    $ systemctl cat getty@tty2
# /lib/systemd/system/[email protected]
#  This file is part of systemd.
#
#  systemd is free software; you can redistribute it and/or modify it
#  under the terms of the GNU Lesser General Public License as published by
#  the Free Software Foundation; either version 2.1 of the License, or
#  (at your option) any later version.

[Unit]
Description=Getty on %I
Documentation=man:agetty(8) man:systemd-getty-generator(8)
Documentation=http://0pointer.de/blog/projects/serial-console.html
After=systemd-user-sessions.service plymouth-quit-wait.service
After=rc-local.service

# If additional gettys are spawned during boot then we should make
# sure that this is synchronized before getty.target, even though
# getty.target didn't actually pull it in.
Before=getty.target
IgnoreOnIsolate=yes

# On systems without virtual consoles, don't start any getty. Note
# that serial gettys are covered by [email protected], not this
# unit.
ConditionPathExists=/dev/tty0

[Service]
# the VT is cleared by TTYVTDisallocate
ExecStart=-/sbin/agetty --noclear %I $TERM
Type=idle
Restart=always
RestartSec=0
UtmpIdentifier=%I
TTYPath=/dev/%I
TTYReset=yes
TTYVHangup=yes
TTYVTDisallocate=yes
KillMode=process
IgnoreSIGPIPE=no
SendSIGHUP=yes

# Unset locale for the console getty since the console has problems
# displaying some internationalized messages.
Environment=LANG= LANGUAGE= LC_CTYPE= LC_NUMERIC= LC_TIME= LC_COLLATE= LC_MONETARY= LC_MESSAGES= LC_PAPER= LC_NAME= LC_ADDRESS= LC_TELEPHONE= LC_MEASUREMENT= LC_IDENTIFICATION=

[Install]
WantedBy=getty.target
DefaultInstance=tty1

    In particular, I have to change the ExecStart line, which currently is:

    $ systemctl cat getty@tty2 | grep Exec     
ExecStart=-/sbin/agetty --noclear %I $TERM

    To override this, do:

    sudo systemctl edit getty@tty2

    And add:

    [Service]
ExecStart=
ExecStart=-/sbin/agetty -a muru --noclear %I $TERM

    Note that:

    1. I had to explicitly clear ExecStart before setting it again, as it is an additive setting, similar to other lists like Environment (as a whole, not per-variable) and EnvironmentFile; and opposed to overriding settings like RestartSec or Type. ExecStart can have multiple entries only for Type=oneshot services. Note that dependency settings like Before, After, Wants, etc. are also lists, but cannot be cleared using this way. You'll have to override/replace the entire service for that (see below).
    2. I had to use the proper section header. In the original file, ExecStart is in the [Service] section, so my override has to put ExecStart in the [Service] section as well. Often, having a look at the actual service file using systemctl cat will tell you what you need to override and which section it is in.

    Usually, if you edit a systemd unit file, for it to take effect, you need to run:

    sudo systemctl daemon-reload

    However, systemctl edit automatically does this for you.

    Now:

    $ systemctl cat getty@tty2 | grep Exec
ExecStart=-/sbin/agetty --noclear %I $TERM
ExecStart=
ExecStart=-/sbin/agetty -a muru --noclear %I $TERM

$ systemctl show getty@tty2 | grep ExecS
ExecStart={ path=/sbin/agetty ; argv[]=/sbin/agetty -a muru --noclear %I $TERM ; ... }

    And if I do:

    sudo systemctl restart getty@tty2

    and press CtrlAltF2, presto! I'll be logged into my account on that TTY.

    As I said before, getty@tty2 is an instance of a template. So, what if I wanted to override all instances of that template? That can be done by editing the template itself (removing the instance identifier - in this case tty2):

    systemctl edit getty@

    Overriding the environment

    A common use case of /etc/default files is setting environment variables. Usually, /etc/default is a shell script, so you could use shell language constructs in it. With systemd, however, this is not the case. You can specify environment variables in two ways:

    Via a file

    Say you have set the environment variables in a file:

    $ cat /path/to/some/file
FOO=bar

    Then, you can add to the override:

    [Service]
EnvironmentFile=/path/to/some/file

    In particular, if your /etc/default/grub contains only assignments and no shell syntax, you could use it as the EnvironmentFile.

    Via Environment entries

    The above could also be accomplished using the following override:

    [Service]
Environment=FOO=bar

    However, this can get tricky with multiple variables, spaces, etc. Have a look at one of my other answers for an example of such an instance.

    Variations in editing

    Replacing the existing unit entirely

    If you want to make massive changes to the existing unit, such that you're effectively replacing it altogether, you could just do:

    systemctl edit --full foo

    Temporary edits

    In the systemd file hierarchy, /run takes precedence over /etc, which in turn takes precedence over /lib. Everything said so far also applies to using /run/systemd/system instead of /etc/systemd/system. Typically /run is a transient filesystem whose contents are lost on reboot, so if you want to override a unit only until reboot, you can do:

    systemctl edit --runtime foo

    Undoing changes

    You can simply remove the corresponding override file, and do systemctl daemon-reload to have systemd read the updated unit definition.

    You can also revert all changes:

    systemctl revert foo

    Further Reading

    Via this mechanism, it becomes very easy to override systemd units, as well as to undo such changes (by simply removing the override file). These are not the only settings which can be modified.

    The following links would be useful:

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