systemd units need not obey files in /etc/default, and they may or may not. systemd is easily configurable, but require that you know the syntax of a systemd unit file.
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/).
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/getty@.service
# 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 serial-getty@.service, 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:
- I had to clear
ExecStart before setting it again. This has to be done for any units which are not of type oneshot (similar to Upstart task jobs), which means most services. This is not necessary for many options (like Environment, for example).
- 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 -a muru
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.
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:
