modify a byte in a binary file using standard linux command line tools

Question

I need to modify a byte in a binary file at a certain offset.

Example:

• Input file: A.bin
• Output file: B.bin

I need to read a byte at the offset 0x40c from A.bin, clear to 0 least significant 2 bits of this byte, and then write file B.bin equal to A.bin, but with the calculated byte at offset 0x40c.

• I can use Bash and standard commands like printf and dd.
• I can easily write a byte into a binary file, but I don't know how to read it.

Modify a byte in a binary file using standard Linux command line tools.

Answer 1

# Read one byte at offset 40C
b_hex=$(xxd -seek $((16#40C)) -l 1 -ps A.bin -)

# Delete the three least significant bits
b_dec=$(($((16#$b_hex)) & $((2#11111000))))
cp A.bin B.bin

# Write one byte back at offset 40C
printf "00040c: %02x" $b_dec | xxd -r - B.bin

It was tested in Bash and Z shell (zsh) on OS X and Linux.

The last line explained:

• 00040c: is the offset xxd should write to
• %02x converts $b from decimal to hexadecimal
• xxd -r - B.bin: reverse hexadecimal dump (xxd -r) — take the byte number and the hexadecimal value from standard input (-) and write to B.bin