#!/bin/bash

if [ -z "$1" -o -z "$2" ] ; then
   echo "usage: $0 <offset in bit> <file>"
   echo "flip the bit of the file located at the provided offset"
   exit 1
fi

offbit=$1
file="$2"

offbyte=$(( $offbit / 8 ))
bitinbyte=$(( $offbit - ($offbyte * 8) ))
readbyte=`xxd -s $offbyte -p -l 1 "$file"`
mask=$(( 1 << $bitinbyte ))
newbyte=$(( 0x$readbyte ^ $mask ))
hexanewbyte=`printf "%.2x" $newbyte`

echo $hexanewbyte | xxd -p -l 1 -s $offbyte -r - "$file"