pipe and grep in Docker container seems to get blocked

Question

I have a simple bash script as follows that is part of a docker image.

test.sh,

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

run.sh,

#!/bin/sh /test.sh & # Run forever while true; do sleep 5; done

Dockerfile,

FROM openjdk:8u191-jre-alpine3.9 COPY files/run.sh / COPY files/test.sh / CMD ["/run.sh"]

If I run this container I only get the following output which leads me to believe somehow grep and "pipe" seem to get blocked.

[2019-03-06 11:10:45][INFO] Testing .... 1 Active Internet connections (servers and established) Proto Recv-Q Send-Q Local Address Foreign Address State PID/Program name tcp 0 0 172.17.0.2:58278 xxx.xxx.xx.xx:443 FIN_WAIT2 - [2019-03-06 11:10:45][INFO] Testing .... 2

Can someone please shed some light around this ?

It works fine If I comment out netstat -anpt | grep tcp. I would then see the subsequent log lines and it would also continue in the loop.

[2019-03-06 11:25:36][INFO] Testing .... 3 [2019-03-06 11:25:41][INFO] Testing .... 4

Can you give more logs, I ran the program on my machine, it seems to work fine. I want to compare what is there that you are not seeing on your console. — Van_Cleff
– Van_Cleff, Commented Mar 6, 2019 at 11:20
Your run.sh script launches a background process and then exits. Since that script is the main container process, once it exits, the container exits too. — David Maze
– David Maze, Commented Mar 6, 2019 at 11:21
@DavidMaze also i forgot to mention run.sh has few other things like running a elasticsearch instance with exec su-exec elasticsearch /bin/elasticsearch. So run.sh should never exit 0 — day0ops
– day0ops, Commented Mar 6, 2019 at 11:34
@vancleff check the description. will give you a more repeatable test now. — day0ops
– day0ops, Commented Mar 6, 2019 at 11:43

Andreas Lorenzen · Accepted Answer · 2019-03-06 12:35:37Z

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/bash set -e logit() { 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.

yeah what i thought about the EOF as well. Its strange that no one else has discovered this issue.
@Andrews i have marked your address. the main purpose of this grep was I wanted to pull the ip from inside the docker container. Theres number of ways to do it i know. But i was more curious about this problem than anything.
Yeah, sorry that I couldn't make the answer work with grep. I sure am wondering too, how noone else apparently has seen this issue.

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