bash signal trap will override the signal of its nohup sub commands?

Question

Today I've encountered one problem, the program that is started by a shell script can not receive INT signal. After some investigating, I will show my findings below.

This is the target program that I want to run, I compile it using gcc hello.c -o hello.out if you manually start this program, you can stop it by kill -2

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>


int main()
{
    while (1) {
        printf("--------Hello Wolrd!\n");
        sleep(2);
    }
    return 0;
}

Then I have a shell script which does some processing before start my program. Here is the concise version, let's call it trap.sh:

#!/bin/bash

pid=0

function singal_handler() {
    echo "pid is "$java_pid
}

trap "singal_handler" INT

pid=2

nohup ./hello.out &

while true; do
    echo "running...."
    sleep 2
done

Notice that I use trap to capture the INT signal to do my own work, and start my hello.out using nohup.

Now I start my program by bash trap.sh.

By issuing kill -2 to my trap.sh, the behavior is expected, that pid output is out.

What surprises me is that, at this time, when I issue kill -2 to my background hello.out, hello.out is still there, it does not vanish.

So I am writing this question to ask why this will happen. The bash trap will override the signal handler of its subcommands?

My platform is 64bit linux:

uname -r -----> 3.10.0-123.el7.x86_64

Thanks.

Answer 1

The bash manual says:

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. Unless otherwise noted, the values are inherited from the shell.

• the shell’s open files, plus any modifications and additions specified by redirections to the command
• the current working directory
• the file creation mode mask
• shell variables and functions marked for export, along with variables exported for the command, passed in the environment (see Environment)
• traps caught by the shell are reset to the values inherited from the shell’s parent, and traps ignored by the shell are ignored

man sigaction says:

A child created via fork(2) inherits a copy of its parent's signal dispositions. During an execve(2), the dispositions of handled signals are reset to the default; the dispositions of ignored signals are left unchanged.

So yes, the SIGINT get's ignored because the parent shell ignores it.