Write byte at address (hexedit/modify binary from the command line)

Solution 1:

printf '\x31\xc0\xc3' | dd of=test_blob bs=1 seek=100 count=3 conv=notrunc 

dd arguments:

• of | file to patch
• bs | 1 byte at a time please
• seek | go to position 100 (decimal)
• conv=notrunc | don't truncate the output after the edit (which dd does by default)

One Josh looking out for another ;)

Solution 2:

Here's a Bash function replaceByte, which takes the following parameters:

• the name of the file,
• an offset of the byte in the file to rewrite, and
• the new value of the byte (a number).

#!/bin/bash

# param 1: file
# param 2: offset
# param 3: value
function replaceByte() {
    printf "$(printf '\\x%02X' $3)" | dd of="$1" bs=1 seek=$2 count=1 conv=notrunc &> /dev/null
}

# Usage:
# replaceByte 'thefile' $offset 95

Solution 3:

The printf+dd based solutions do not seem to work for writing out zeros. Here is a generic solution in python3 (included in all modern distros) which should work for all byte values...

#!/usr/bin/env python3
#file: set-byte

import sys

fileName = sys.argv[1]
offset = int(sys.argv[2], 0)
byte = int(sys.argv[3], 0)

with open(fileName, "r+b") as fh:
    fh.seek(offset)
    fh.write(bytes([byte]))

Usage...

set-byte eeprom_bad.bin 0x7D00 0
set-byte eeprom_bad.bin 1000 0xff

Note: This code can handle input numbers both in hex (prefixed by 0x) and dec (no prefix).

Solution 4:

xxd tool, which comes with vim (and thus is quite likely to be available) allows to hex dump a binary file and construct a new binary file from a modified hex dump.