execute arbitrary bash code/variable substitution in systemd units

systemd-prblm.service

[Unit]
Description=Demonstrate Bash

[Service]
ExecStartPre=/usr/bin/bash -c "/usr/bin/systemctl set-environment MYVAR=$(( 2 + 2 ))"
ExecStart=/usr/bin/echo "2 + 2 = ${MYVAR}"

The scope of this variables is at systemd manager, so they are available to all systemd units, you can clean it with ExecStop directive invoking systemctl unset-environment. Also, you can use templated units, and use the variable %i to populate a different variable for every different instance thus avoiding variables collision without copypasting unit files.

https://gist.github.com/tzkmx/0ae0d2686ee5da5f245f330e3a6a3329

Must you use ExecStartPre=/usr/bin/bash or ExecStart=/usr/bin/echo ?
Could you use ExecStartPre=/bin/sh?

👍

@katcaola Yes. The binary just has to be given with an absolute path.

thx 👍

Thanks! Used it to dynamically set a USB Ethernet dongle to a separate namespace:

https://gist.github.com/PhilipSchmid/55e40dba4e323e9236a1101a7d0d6abb

Note it won't work if user is not root (i.e. if you define "User" in [Service] section). It can be fixed with "PermissionsStartOnly=true", so all commands will be executed as root, and ExecStart command will be run as the defined user as intended.

add --user to systemctl and it works.

nice! I have a question why does env variables generated by ExecStartPre can be used by ExecStart?

I prefer to write the variable to a file specific to this unit:

[Service]
EnvironmentFile=-/dev/shm/demo.env
ExecStartPre=/usr/bin/bash -c "echo MYVAR=$(( 2 + 2 )) >/dev/shm/demo.env"
ExecStart=/usr/bin/echo "2 + 2 = ${MYVAR}"

Thanks. I use it to finally make tigervnc work with SDDM:
https://gist.github.com/mjtiempo/8662e88da74236c58115bed5cc78ceaf

Beware that there's a pitfall if you have to use %s with your command.
echo (date +%s)
SystemD sees this as user shell and resolve to /bin/sh instead. You need to use %%s to avoid that.

EnvironmentFile=/tmp/timestamp.txt
ExecStartPre=/bin/bash -c "echo TIMESTAMP=$(date +%%s) > /tmp/timestamp.txt;"

Although this seems simple, it was quite hard to find this info. ChatGPT was not yielding a correct result either.