Comment by pas

Mostly just zombies and signal handlers.

And your software can do it, if it's written with the assumption that it will be pid1, but most non-init software isn't. And rather than write your software to do so, it's easier to just reach for something like tini that does it already with very little overhead.

I'd recommend reading the tini readme[0] and its linked discussion for full detail.

[0]: https://github.com/krallin/tini

The main other problem is that the kernel doesn't register default signal handlers for signals like SIGTERM if the process is PID 1. So if your process doesn't register its own signal handlers, it's hard to kill (you have to use SIGKILL). I'm sure anyone who has used Docker a lot has run into containers that seem to just ignore signals -- this is the usual reason why.

> also, why can't the real pid1 do it? it sees all the processes after all.

How would the real PID 1 know if it _should_ reap the zombie? It's normal to have some zombie processes -- they're just processes whose exit statuses haven't been reaped yet. If you force-reaped a zombie you could break a program that just hasn't yet gotten around to checking the status of a subprocess it spawned.