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?

  • remove the hyphen, it'll then be taken as positive
    – gwillie
    May 10, 2017 at 1:40
  • Do any of the provided answers solve your problem? If so, don't forget to mark one with the checkmark; thank you!
    – Jeff Schaller
    May 21, 2017 at 11:25

2 Answers 2


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.


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",  $percent_diff;
    printf STDERR "-------------- ABORTING REPLACMENT -----------\n";
    exit 10;
diff=$((x - y)
diff=$(( (diff > 0) ? $diff : -1*$diff ))
if [ $diff -gt 1500 ]
  echo do something

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