Shell – How to show a terminal shell’s process tree including children

To background itself, that application forks a child process and exits the parent. The shell knows of the parent process as that's the one it has forked itself before executing the command itself, but it has no visibility on the children or grand-children that that process may have spawned.

A simpler example of such a backgrounding command would be:

$ sh -c 'ps -j; sleep 10 &'
  PID  PGID   SID TTY          TIME CMD
 6562  6562 14469 pts/13   00:00:00 sh
 6563  6562 14469 pts/13   00:00:00 ps
14469 14469 14469 pts/13   00:00:00 zsh

My zsh knows about that sh process and its process group.

However, all it sees is that 6562 process exiting. It has no way to know that 6562 process has spawned a 6563 one.

$ ps -j
  PID  PGID   SID TTY          TIME CMD
 6564  6562 14469 pts/13   00:00:00 sleep
 6565  6565 14469 pts/13   00:00:00 ps
14469 14469 14469 pts/13   00:00:00 zsh

However, you can see the process running sleep is also in that 6562 process group (though, there's nothing stopping commands start new process groups, or even sessions (as daemons typically do)).

Those process groups are only created when the shell is interactive though.

Another thing you could do is:

cmd | cat &
wait

If the processes that cmd spawns don't close their stdout, then cat won't die until they have all died.

This outcome cannot be due to the code block you showed in your question. There must be something else in other parts of your overall setup.

Given only the usage scenario you described, the most likely reason is that you actually set SIGINT to be ignored in your code somewhere before the forkpty, or before the execve in forkpty’s child.

This would yield the outcome you describe because then your forkpty’ed bash -i would inherit such SIGINT-ignored setup, and while it would set it otherwise for its own internal purposes (thus not ignoring SIGINT) it would also reset it to the inherited ignored state for each and every command it would spawn.

This is documented Bash’s behavior, see Bash’s man-page at “COMMAND EXECUTION ENVIRONMENT” chapter, specifically the paragraph saying:

When a simple command other than a builtin or shell function is to be executed, it is invoked in a separate execution environment that consists of the following. [...]

[...]

o traps caught by the shell are reset to the values inherited from the shell's parent, and traps ignored by the shell are ignored

HTH