Hi all,
i'm writing a simple init program only for study purpose. Now, how can i fastly test it? I tried to use systemd container but even if I replace the init symlink making it point to my init program, it is ignored, starts systemd anyway.
What is the best and fastest way to do that?
Regards

Are you writing an init script to start a service under initd, runit, s6, or systemd? Or are you writing an init service to start the system services REPLACING systemd, init.d, runit, or s6? This is unclear form your statement of the question, and critical to answering it.

There are a couple of dozen distributions out there that do not use SystemD. Seek one out and use it to create a virtual machine or full guest container. Then you can play with the system init features all you want without having systemd start behind your back. No matter what the answer to the question I asked, this is far safer than trying things in your native install.

Strongly agree with wpeckham ... grab a distro that still uses 'init,' put it in a virtual machine, and hack away to your heart's content.

Everybody still uses init - just ask the kernel.

But I agree light-weight containers ain't the environment to test in.

Thanks all for the answers.
I know that i can put all in a virtual machine but that means that i have to transfer the compiled code from host to guest.
Or I should to develop it directly in virtual machine. That means that for each modification (even a simple printf), I'll have to make a reboot machine passing init on the command line as parameter.
This solution doesn't looks like fast imo.
For this reason I wanted to understand if a lightweight container offered by systemd can help me.
A probably solution could be this

It works!
systemd-nspawn -D /path/my/cont myinit
it runs myinit program as PID 1.
Mark as solved.