Edit binary files with dd: replacing, deleting, and copying bytes

Replace bytes with dd (hex editor, edit bytes)

# The file you want to edit:
$ echo 'My name is dark' > file.txt
$ cat file.txt
My name is dark

# Replace 1 byte:
$ echo -n 'M' |
  dd \
    of=file.txt  \
    bs=1 \
    seek=11 \
    count=1 \
    conv=notrunc

1+0 records in
1+0 records out
1 byte copied, 5.1489e-05 s, 19.4 kB/s

# Inpect file:
$ cat file.txt
My name is Mark
$ xxd file.txt
00000000: 4d79 206e 616d 6520 6973 204d 6172 6b0a  My name is Mark.Copy

Notes:

• stdin = input file = ‘if’, in other words, what you want to add to the new file
• of is the file you want to edit.
• bs, read and write up to BYTES bytes at a time (default: 512); overrides ibs and obs
• seek, The decimal address you want to start replacing. If you want to add a hex address, wrap in double quotes and use arithmetic substitution$((0x30)).
• count, how many bytes from the ‘if’ you want to add to the ‘of’.
• conv=notrunc, do not truncate the file, which is the default.

Delete bytes with dd

It seems that you can’t remove individual bytes. If you think about it, there isn’t a system call that does this.

What you can do though is to write the whole file again, without those bytes:

 $ echo 'helllo doge world ' |
  { dd bs=1 count=4 ; dd skip=1 bs=1 count=2 ; dd skip=5 bs=1 ; }  2>/dev/nullCopy

Explanation:

• 1. print hell
• 2. skip 3rd l, print ‘o’ and space.
• 3. delete doge and space and print world.

See https://unix.stackexchange.com/questions/128871/dd-remove-range-of-bytes

Copy a part of a file with dd

$ dd if=input.txt iflag=skip_bytes,count_bytes skip=5 count=3 of=part.txt
# or
$ dd if=input.txt bs=1 skip=5 count=3 of=part.txtCopy

There are 2 versions because if you use the skip/count combo without a block size (bs) skip=N and count=N, the N is a block and not bytes. So, if you want to skip bytes, and you don’t want to change the block size, use the 1st option (set the block size to 1 byte). Otherwise, you can use the second option.