bash - How to continually process last lines of two files when the files change randomly? -

i have following simple snippet:

#!/bin/bash  tail -f "data/top.right.log" | while read val1   val2=$(tail -n 1 "data/top.left.log")       echo $(echo "$val1 - $val2" | bc) done 

top.left.log , top.right.log files other processes continually write. bash script subtracts last lines of both files , show result.

i make script more efficient. in pseudo-code this:

#!/bin/bash  magiccommand "data/top.right.log" "data/top.left.log" | while read val1 val2   echo $(echo "$val1 - $val2" | bc) done  

so whenever top.left.log or top.right.log changes echo command called.

i have tried various snippets stackoverflow rely on fact files not change or both files contain same amount of lines not case.

if have inotify-tools can use following command:

inotifywait -q -e modify file1 file2 

description:

   inotifywait  efficiently waits changes files using linux's inotify(7) interface.    suitable waiting changes files shell scripts.    can either exit once event occurs, or continually execute , output events occur. 

an example:

while : ;     inotifywait -q -e modify file1 file2     echo `tail -n1 file1`     echo `tail -n1 file2` done