What is the most minimalistic way to run a program inside an OS? - process

Question

Generally, when I start a program from bash, it forks bash and inherits many things from it, like stdin, stdout. Is there some other way to run a program, with no such setup? Maybe it explicitly opens fd 1, writes something and closes it?
I came across nohup and disown. But both of those detaches a running process from bash, but initially still the process inherits from bash. Maybe is there a way to start a process that inherits from nothing?
I am asking this just out of curiosity, and have no practical purposes. When a program is ran in a microcontroller, it is just our program that is running with no additional setup (if setup is required, user have to prepend it). Similarly, is there a way even in the presence of an operating system, to run just what is programmed, without any setups?

Answer 1

I assume you are using Linux.
Printing top -u root you see for eg on my system (Ubuntu 20.04 x86_64):
PID PPID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
1 0 root 20 0 170380 11368 6500 S 0.0 0.0 0:23.52 systemd
2 0 root 20 0 0 0 0 S 0.0 0.0 0:00.61 kthreadd
3 2 root 0 -20 0 0 0 I 0.0 0.0 0:00.00 rcu_gp
4 2 root 0 -20 0 0 0 I 0.0 0.0 0:00.00 rcu_par_gp
5 2 root 0 -20 0 0 0 I 0.0 0.0 0:00.00 netns
10 2 root 0 -20 0 0 0 I 0.0 0.0 0:00.00 mm_percpu_wq
11 2 root 20 0 0 0 0 S 0.0 0.0 0:00.00 rcu_tasks_rude_
12 2 root 20 0 0 0 0 S 0.0 0.0 0:00.00 rcu_tasks_trace
13 2 root 20 0 0 0 0 S 0.0 0.0 0:04.48 ksoftirqd/0
14 2 root 20 0 0 0 0 I 0.0 0.0 1:28.82 rcu_sched
You see that all processes inherit ultimately from PPID (parent process ID) zero.
This process is not a process, it represents the Linux scheduler. Then systemd (PID 1) is launched by the kernel and every other process in the system is launched by systemd.
At a user level top -u madfred
3371 1 madfred 20 0 19928 7664 6136 S 0.0 0.0 0:11.62 systemd
3372 3371 madfred 20 0 170404 2460 0 S 0.0 0.0 0:00.00 (sd-pam)
3379 3371 madfred 39 19 659828 16348 12500 S 0.0 0.0 0:02.38 tracker-miner-f
3402 3371 madfred 20 0 8664 5112 3412 S 0.0 0.0 0:00.94 dbus-daemon
3407 3371 madfred 20 0 239712 6740 6064 S 0.0 0.0 0:00.03 gvfsd
There is one user systemd that is launched by the root systemd and runs as the user. This user systemd is in charge of launching every process for that user.
That is necessary for assuring all the guarantees that the Linux OS provides as security, memory protection, file resources etc.
What you want would be to replace the kernel with something else, which is very possible. Check for example:
https://wiki.osdev.org/Bare_bones
https://github.com/contiki-ng/contiki-ng
It is pretty easy to replace systemd (or the old /sbin/init) with your own custom initializer. Check this answer:
Writing my own init executable