[systemd-devel] Howto detect a program/service is started by systemd?

[Mantas Mikulėnas]

No, it really shouldn't detect "started by systemd" at all. These are
several independent things and you shouldn't mix them all together.

Regarding startup, systemd services have several distinct Type= settings
with different expectations and the program has no way to reliably detect
which one is configured. And other init systems exist which also support
handling non-forking services. For example, OpenRC can just as well start
services that don't fork/demonize, and it even supports the systemd-style
"Type=notify" nowadays.

So you shouldn't have a "systemd" mode, but rather have a "--(no-)daemon"
or "--(no-)fork" or "--foreground" option that describes the behavior
expected.

If Type=notify is offered, this can be detected from the specific
$NOTIFY_SOCKET, again separately from which init system is offering it.

On Mon, Jun 30, 2025, 13:26 Stef Bon <stefbon at gmail.com> wrote:

> Hi,
>
> it's important for a program to detect it has been started by systemd.
> For example it does not have to fork since it has already been forked.
>
> I've written a test script, which calls env, and there are a few
> unique environment variables which are an indication it has been
> started by systemd: SYSTEMD_EXEC_PID and INVOCATION_ID.
>
> Is it a good way to test these environment vars (to detect started by
> systemd) or is there a better way?
>
> S. Bon
>
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