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}"

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.