Why are PIDs in new PID namespace not contiguous

Question

I am running Ubuntu 20.04. According to https://lwn.net/Articles/531419/ and https://stackoverflow.com/questions/3446727/how-does-linux-determine-the-next-pid/3457108#3457108, assigned PIDs should be sequential. However, that is not the case with my newly created PID namespace where the first process has PID 1 (as it should) and the next process has PID 8. What is strange is that every PID after PID 8 is sequentially assigned.

$ sudo unshare --fork --pid --mount-proc bash
root@anon:/home/anon# ps aux
USER         PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root           1  0.0  0.0   9844  3840 pts/2    S    12:13   0:00 bash
root           8  0.0  0.0  11500  3364 pts/2    R+   12:13   0:00 ps aux

Other processes after are assigned 9, 10, 11 ...

I'm curious why the initial jump from 1 to 8? If there are additional processes being created and killed before ps aux can be run that are occupying PIDs 2-7, how do I view what those are? I tried viewing /var/log/syslog but couldn't find anything useful.

Answer 1

I'm curious why the initial jump from 1 to 8?

When you start Bash in interactive mode, rc files (~/.bashrc, ~/.profile if login shell, and corresponding system-wide files in /etc) are read and executed before giving you a prompt.

If you execute bash --norc those rc files won't be executed and your ps command will have PID 2 as expected.

If there are additional processes being created and killed before ps aux can be run that are occupying PIDs 2-7, how do I view what those are?

Execute bash -x