fluent.conf

<source>
  @type tail
  path /fluentd/log/main.log
  format none
  tag x
</source>

<match **>
  @type file
  path /fluentd/log/output.log
</match>

run.sh

#!/bin/bash
set -e

mkdir -p log
rm -rf log/*


gen_x_lines() {
  N=$1; shift
  yes | head -n $N > log/main.log 
}


ID=$(sudo docker run  -d -v $PWD/fluent.conf:/fluentd/etc/fluent.conf -v $PWD/log:/fluentd/log fluent/fluentd:latest)
trap "sudo docker stop $ID >/dev/null; sudo docker rm $ID >/dev/null;" EXIT
echo Started container $ID
sleep 1

C=1;
LIMIT=10
echo Generating logs
while gen_x_lines 10000 && ((C++ < $LIMIT)) ; do sleep 2;  mv log/main.log log/main.log.${C} ; done

echo Comparing results
GOAL=$(wc -l log/main.log*|awk '/total/ {print $1}')
echo original output: $GOAL

FNUM=
I=$(wc -l log/output.log* |awk '/total/ {print $1}')
while (( FNUM != I )); do 
  echo fluent produced: $I; FNUM="$I"
  sleep 1
  I=$(wc -l log/output.log* |awk '/total/ {print $1}')
done