CODEWITHSUNDEEP
linuxbashshellpipeline
Zhani Baramidze
Zhani Baramidze

Reputation: 1497

bash running nc with ampersand terminates program

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.

Upvotes: 1

Views: 916

Answers (2)

neuhaus
neuhaus

Reputation: 4104

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

Upvotes: 1

Sanket Patel
Sanket Patel

Reputation: 386

If I get your purpose right, you want to feed data manually to nc which will be sent to client.

You can use named pipes for this purpose.

cat /tmp/f | ./parser.sh 2>&1 | nc -lvk 127.0.0.1 1234 > /tmp/f

where /tmp/f is a pipe made using mkfifo /tmp/f

whatever you want to feed to nc can be echo-ed in parser.sh

Upvotes: 1

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