#!/bin/bash
#
# Martin Lomas, Graham Dicker
#
Version='v0.01 2010-08-03'
#
# Made available under the GPLv3 license
#
ProgName='bb: "Byte Blip"'
#
# Very naive program using "dd" for modifying a single byte in a file.
# Modify default offset "os", change_from "a" and change_to "b" as required:

os="54"
a="$( echo -e "\001" )"
b="$( echo -e "\006" )"


# Run as follows:
#
# ./bb <file>
# ./bb <in file> <out file>
#
# For version and copyleft:
# ./bb -v
#
# If out file is not given, the input file will be overwritten. If it fails,
# you may lose data.
#
#
#
Copyright='   Copyright (C) 2010 martin Lomas'
#
#   This program is free software: you can redistribute it and/or modify
#   it under the terms of the GNU General Public License as published by
#   the Free Software Foundation, either version 3 of the License, or
#   (at your option) any later version.
#
#   This program is distributed in the hope that it will be useful,
#   but WITHOUT ANY WARRANTY; without even the implied warranty of
#   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
#   GNU General Public License for more details.
#
#   You should have received a copy of the GNU General Public License
#   along with this program.  If not, see <http://www.gnu.org/licenses/>.
#

if [ "X$1" == "X-v" ]
then
	echo
	echo "$ProgName"
	echo
	echo "$Version"
	echo
	echo "$Copyright"
	echo
	echo "This program comes with ABSOLUTELY NO WARRANTY."
	echo "This is free software, and you are welcome to redistribute it"
	echo "under certain conditions. For details, see:"
	echo "http://www.gnu.org/licenses/gpl-3.0.txt"
	echo
	exit
fi


f="$1"
fout="$2"

if [ -f "$f" ]
then
	if [ -n "$fout" ]
	then
		cp "$f" "$fout" || exit -1
		f="$fout"
	fi

	[ "$( dd bs=1 count=1 skip="$os" if="$f" )" == "$a" ] &&
		echo "$b" | dd bs=1 count=1 seek="$os" conv=notrunc of="$f"

	exit
fi

# Must be an error condition or help:

echo "Usage: bb -v | file [ outfile ]"