pipe and grep in Docker container seems to get blocked

This one has me puzzled! But I have a solution for you:

Use awk instead of grep

In test.sh use this instead:

netstat -anpt | awk /tcp/

So that the file looks like this:

#!/bin/bashset -elogit() {    log_date=`date +"%F %T"`    echo "[$log_date][INFO] $1"}waitForServerToStart() {    while true; do        logit "Testing .... 1"        netstat -anpt        logit "Testing .... 2"        netstat -anpt | awk /tcp/        logit "Testing .... 3"        sleep 5        logit "Testing .... 4"    done}waitForServerToStart

For a reason that I cannot explain - grep will not return when reading from the pipe when invoked from the script. I created your container locally, ran it and entered it - and the command netstat -anpt | grep tcp runs just fine and exits. If you replace it with netstat -anpt | cat in your test.sh script, then it will also pass just fine.

I looked all over the place for the someone with an identical issue with grep in a container from the distro you are using, the version etc. - but came up empty handed.

I believe that it may have to do with grep waiting for a EOF character that never lands - but I am not sure.