Background process in shell script receives EOF

Question

It's hard to explain this behavior, so here's a reproducible example (tested on macOS).

First, I have the following C file. The details aren't important, but I essentially use the read system call to read 16 bytes from standard input, or until an EOF is encountered. Note that read will return 0 on an EOF.

// test.c

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

int main() {
    char buf[1];
    for (int i = 0; i < 16; i++) {
        if (read(STDIN_FILENO, buf, 1) == 0) {
            printf("EOF encountered! Number of bytes read: %d\n", i);
            return 0;
        }
    }
    printf("Read all 16 bytes\n");
    return 0;
}

Suppose I compile this file into a binary file called test. Here's the program in action:

$ echo 'sixteen bytes!!' | ./test
Read all 16 bytes

$ echo '' | ./test
EOF encountered! Number of bytes read: 1

$ ./test # Waits for user input

Makes sense right? I can even take the last command and run it as a background process (although this is quite useless):

$ ./test &
[1] 19204

[1]  + suspended (tty input)  ./test

Suppose I take this command and put it in the following shell script (called huh.sh):

#!/bin/sh

./test &

When I run this shell script, here's the output:

$ ./huh.sh

EOF encountered! Number of bytes read: 0

This means read immediately encountered an EOF, and this only happens in the context of a shell script.

I see similar behavior if I replace test with another program that is sensitive to EOFs. For instance, if I instead run node & directly in a terminal, I'll be able to see the node process in the output of ps. However, if I run node & in a shell script, it immediately exits.

Can someone explain this?

Answer 1

Job control is enabled by default in interactive shells, but disabled by default in shell scripts. Here's a relevant quote from POSIX:

If job control is disabled (see set, -m), the standard input for an asynchronous list, before any explicit redirections are performed, shall be considered to be assigned to a file that has the same properties as /dev/null. This shall not happen if job control is enabled.