Process groups

The nohup performs the following tricks:

• Changes the stdin fd to /dev/null..
• Redirects the stdout and stderr to a file on disk..
• Set an ignore SIG_IGN flag for SIGHUP signal. The interesting moment here is

that the SIG_IGN is preserved after the execve() syscall..

• Run the execve()..

  All the above make the program immune to the SIGHUP signal and can’t fail due to writing to or reading from the closed terminal.

When a controlling process loses its terminal connection, the kernel sends a SIGHUP signal to inform it of this fact. Shell processes (which are usually control terminals) have a handler to catch SIGHUP signals. Receiving a signal starts a fan-out process of sending SIGHUP signals to all jobs it has created and know about (remember the fg, bg and waitpid()).

The default action for the SIGHUP is terminate. Inside a shell, a process group is usually called a job. In its turn, each process group belongs to a session

cat /var/log/nginx.log | grep string | head # a process group with 3 processes

signal can be sent to all members of a process group by using killpg() or kill() with a negative PGID parameter.

https://biriukov.dev/docs/fd-pipe-session-terminal/3-process-groups-jobs-and-sessions/