Using systemctl edit via bash script?

Question

I install Debian a lot. To do this I have a fully-automated preseed.cfg; at the end of the preseed, it downloads and runs a postinstall.sh script from my TFTP server, which does some additional customization.

I'm in the process of switching from GNOME to LXQTE, and using SDDM instead of GDM. However, SDDM tries to start too quickly for my hardware. To get around this, I've been using systemctl edit sddm to add the following:

[Service]
ExecStartPre=/bin/sleep 5

This works great, and I'd like to automate this process by adding it to the postinstall.sh script. However, I can't figure out how to pass the file contents to systemctl edit via a bash script. How can I do this?

Answer 1

You can override the $SYSTEMD_EDITOR environment variable to use a different command other than your editor when running systemctl edit.

For instance, using something like SYSTEMD_EDITOR='cp /path/to/source.file' seems to work OK (even though it's pretty ugly, expecting the last argument to be appended there by systemd!)

For your particular case, you could use:

$ { echo "[Service]"; 
    echo "ExecStartPre=/bin/sleep 5";
  } >~/tmp/sddm-override.conf
$ sudo env SYSTEMD_EDITOR="cp $HOME/tmp/sddm-override.conf" systemctl edit sddm

But all that systemctl edit really does is create an override file (in its case, named override.conf) under the /etc/systemd/system/<service>.service.d/ directory, which is created if it does not exist... So doing that directly is also a totally accepted approach. (See mentions of "drop-in" and "override" in the man page for systemd.unit for more details.)

So, in your case, this would be an appropriate solution:

$ sudo mkdir -p /etc/systemd/system/sddm.service.d/
$ { echo "[Service]"; 
    echo "ExecStartPre=/bin/sleep 5";
  } | sudo tee /etc/systemd/system/sddm.service.d/10-startup-delay.conf
$ sudo systemctl daemon-reload

Which drops a file with the expected contents in the "drop-in" directory for your unit, in which case you can also name it appropriately after what it tries to accomplish.

UPDATED: As @GracefulRestart points out, you need a systemctl daemon-reload after adding a drop-in directly.

Answer 2

Since I have not found how to use systemctl edit in a script yet, best practice would be to simulate the systemctl edit sddm command and place the override in the /etc/systemd/system directory, as service units in /usr/lib/systemd/system could be changed when packages are upgraded:

UNIT='sddm.service'
DIR="/etc/systemd/system/${UNIT}.d"
mkdir $DIR
echo -e "[Service]\nExecStartPre=/bin/sleep 5" > ${DIR}/override.conf
systemctl daemon-reload

That should be roughly equivalent to what systemctl edit sddm is doing.

Answer 3

You can change what editor systemctl(8) uses for its edit subcommand with the SYSTEMD_EDITOR variable. This essentially constructs a command whose final argument is always a systemctl(8) created temporary file that, once written to, is then moved to override.conf.

Because of this, we can use tee(1) as the SYSTEMD_EDITOR and then redirect any text we want from the shell to the entire systemctl(8) command on stdin. For example, redirecting a Here-Document:

SYSTEMD_EDITOR=tee systemctl edit sddm <<EOF
[Service]
ExecStartPre=/bin/sleep 5
EOF

This creates the drop-in file /etc/systemd/system/sddm.service.d/override.conf with the contents between the "words" EOF.

Under-the-hood the edit subcommand looks something like this:

# Create drop-in directory for service
mkdir -p /etc/systemd/system/sddm.service.d

# Create empty temporary file
touch /etc/systemd/system/sddm.service.d/.

# Pass temporary file as last argument of SYSTEMD_EDITOR
tee /etc/systemd/system/sddm.service.d/.\#override.conf846580011dbe64db/.\#override.conf846580011dbe64db<<EOF
[Service]
ExecStartPre=/bin/sleep 5
EOF

# Rename temporary file
mv /etc/systemd/system/sddm.service.d/{.\#override.conf846580011dbe64db,override.conf}

After the unit has been edited, the systemd configuration is reloaded automatically (in a way that is equivalent to daemon-reload).

Answer 4

I want to second the answer given in 3 but I do it this way using tee

env SYSTEMD_EDITOR=tee  sudo -E systemctl edit --system [your_unit_name] < [your_content_file]

When doing it this way you can feed the content via stdin rather than from a file, which can be useful when invoking systemctl from a script.

Answer 5

I'd attack the file directly:

sed -i 's/\[Service]/ a\
ExecStartPre=/bin/sleep 5' /usr/lib/systemd/system/sddm.service