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.

Possible duplicate of CLI: Write byte at address (hexedit/modify binary from the command line) — eadmaster
– eadmaster, Commented Jan 13, 2018 at 3:49
What is the question? It is stated as homework or a work order. — Peter Mortensen
– Peter Mortensen, Commented May 20, 2022 at 19:17

hansaplast · Accepted Answer · 2022-05-24 04:01:17Z

11

# 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

edited May 24, 2022 at 4:01

answered Jan 23, 2017 at 13:07

hansaplast

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9 Comments

Dario Over a year ago

I have upvoted your answer because I like it, but the OP asked for two different files, so cp A.bin B.bin should be put somewhere before the final dd, which would operate on B.bin; and, more importantly, this doesn’t work if $b equals ASCII NUL ...

hansaplast Over a year ago

@Dario thanks for pointing out that it should operate on a new file, added that in

Dario Over a year ago

If I replace the last line of your script with printf "00040c: %02x" $b | xxd -r - B.bin it works in all cases, because if $b is NUL (or, equivalently for my shell, an empty string) it gets expanded by %02x as 00. xxd is a standard command line tool as dd is.

hansaplast Over a year ago

@Dario thanks for the xxd suggestion. I altered the last line and also the reading. Now I don't need the ord function anymore so the code is now stripped down to 4 lines. Can you proofread that I didn't add any errors?

Peter Mortensen Over a year ago

Re "take the line number": But it is a binary file(?).

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