Bash script for finding the percentage of Byte difference between two files. Uses wc, cmp, grep and bc.

percent_different_bash.sh

#! /bin/bash
echo Reports the percentage of matching bytes between two files.

LENGTH_OF_A=$( wc -c $1 | grep -Eo [0-9]+)
LENGTH_OF_B=$( wc -c $2 | grep -Eo [0-9]+)
SAME_SIZE="false"

if [ "$LENGTH_OF_B" -gt "$LENGTH_OF_A" ]
  then
  LONGER=$2
  SHORTER=$1
  let LONGER_BYTE_NUMBER=$LENGTH_OF_B
  let SHORTER_BYTE_NUMBER=$LENGTH_OF_A
else
  LONGER=$1
  SHORTER=$2
  let LONGER_BYTE_NUMBER=$LENGTH_OF_A
  let SHORTER_BYTE_NUMBER=$LENGTH_OF_B
fi


if [ "$LENGTH_OF_A" -eq "$LENGTH_OF_B" ]
  then
  let SAME_SIZE="true"
  echo The files lengths were equal.
else
  let DIFF=$LONGER_BYTE_NUMBER-$SHORTER_BYTE_NUMBER
  echo "Difference in file lengths: $DIFF byte(s)"
fi  

let DIFF_COUNT=$(cmp -l $LONGER $SHORTER 2> /dev/null | wc -w)  # Runs byte by byte comparison sends it for a word count

let WRONG=$(((DIFF_COUNT)/3))                                   # Each line of 'cmp -l' is three 'words'
let CORRECT=$SHORTER_BYTE_NUMBER-$WRONG
ANSWER=$(echo "scale=9; $CORRECT / $SHORTER_BYTE_NUMBER * 100 " | bc )
echo "$1 length: $LENGTH_OF_A"
echo "$2 length: $LENGTH_OF_B"
echo "Unmatched bytes: $WRONG"
echo "Matched bytes: $CORRECT"
echo "Similarity is $ANSWER % of $SHORTER_BYTE_NUMBER bytes."  
if [ "$SAME_SIZE" == "true" ]
  then
  exit
fi
ANSWER=$(echo "scale=9; $CORRECT / $LONGER_BYTE_NUMBER * 100" | bc )
echo "Similarity is $ANSWER % of the longer $LONGER_BYTE_NUMBER bytes."