Move a file which is constantly in use - bash

Question

I need to move the whole content of a file (test.log) which is constantly in use. Moving the file could result in an error for the application which is writing to the file.
My approach is to redirect the output to test.log_bck, copy the original file (test.log) to test.log_cp, clear it and then check if there was any data added in the same time the test.log file was cleared. If any data was missing from the _cp file then merge it with the _bck file without redundant data. It is a lot of effort for such an easy task and my question is: is there another, easier / more efficient way to do it.
#/usr/bin/bash
#redirect the output to /tmp/logging/test.log_bck
bck(){
`tail -f /tmp/logging/test.log &> /tmp/logging/test.log_bck`
}
#run it in background
bck &
#copy the original file to test.log_cp
`cp /tmp/logging/test.log /tmp/logging/test.log_cp` | echo "Copped"
#clear the original file
echo "" > /tmp/logging/test.log | echo "Cleared"
#get the PID of the redirection process and check if there are other running and kill them
bck_pid=`ps -ef | grep "tail -f /tmp/logging/test.log" | grep -v grep | awk '{print$2}' | head -1`
while [ "$bck_pid" != "" ]
do
echo $bck_pid
kill $bck_pid | echo "Killed"
bck_pid=`ps -ef | grep "tail -f /tmp/logging/test.log" | grep -v grep | awk '{print$2}' | head -1`
done
date=$(date '+%Y_%m_%d_%H_%M')
cat /tmp/logging/test.log_cp /tmp/logging/test.log_bck > /tmp/logging/test.log_$date
cat -n /tmp/logging/test.log_$date | sort -uk2 | sort -nk1 | cut -f2-