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}"

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https://www.freedesktop.org/software/systemd/man/systemd.exec.html#EnvironmentFile=

The files listed with this directive will be read shortly before the process is executed (more specifically, after all processes from a previous unit state terminated. This means you can generate these files in one unit state, and read it with this option in the next).

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?

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@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.