bash running nc with ampersand terminates program

Question

I want to run nc via & and then manually feed data in stdin from /proc filesystem whenever I want. so the problem is:

if I run nc 127.0.0.1 1234 &

program runs in background and I can write in stdin whatever I want. but, if I create test.sh and add

#!/bin/bash
nc 127.0.0.1 1234 &
sleep 20

it connects to 1234 and terminates immediately (doesn't even wait for 20 seconds). why? I was suspecting it gets it's stdin written from somewhere.

Answer 1

Interesting question.

The bash manpage states:

   If  a  command  is  followed  by a & and job control is not active, the
   default standard input for the command is  the  empty  file  /dev/null.
   Otherwise,  the  invoked  command  inherits the file descriptors of the
   calling shell as modified by redirections.

If you call nc 127.0.0.1 1234 < /dev/null outside a shell script (with job control) it will result in the same.

You can change your bash script like this to make it work:

#!/bin/bash
nc 127.0.0.1 1234 < /dev/stdin &
sleep 20