BASH script to catch changes to a file greater than 150MB

Question

Need some ideas here. I was using something along the lines of if $[$x-$y>1500] then etc.... to detect the size of the changes to a file. I've realized that won't work as the files sizes could increase or decrease which could result in a negative number. Is there a way to use absolute value of x-y?

Answer 1

I have done this. I create a new file (with at ".new" suffix) but before replacing the old I would check the size difference of the files, and abort (sending some type of notification, such as mail) if there are too many changes.

I generally do this in perl, but bash would be similar.

$file="file_being_updated";
$new=".new";

if ( -f $file ) {
  my $percent_diff = abs( 100 - 100*(-s "$file$new")/(-s $file) );
  if (  $percent_diff > 20 ) {   # more that this to different!
    printf STDERR "File \"$file$new\" differs by more that 20%%! (%.1f%%)\n",  $percent_diff;
    printf STDERR "-------------- ABORTING REPLACMENT -----------\n";
    exit 10;
  }
}

Answer 2

diff=$((x - y)
diff=$(( (diff > 0) ? $diff : -1*$diff ))
if [ $diff -gt 1500 ]
then
  echo do something
fi