Bash – Write hexadecimal values to binary file with bash

Your way is adding line breaks to every thing that it write in space of whatever separator ($IFS) is using to split up the read. Instead of breaking it up into newlines just take the whole thing and pass it along. You can reduce the entire bit of code above to this:

 cat - > $file

You don't need the truncate bit, this will truncate and write the whole STDIN stream out to it.

Edit: If you are using zsh you can just use > $file in place of the cat. You are redirecting to a file and truncating it, but if there is anything hanging out there waiting for something to accept STDIN it will get read at that point. I think you can do something like this with bash but you would have to set some special mode.

Testing with this file:

$ echo hello world > test.txt
$ echo -n $'\x1b\x1f' >> test.txt
$ echo whatever >> test.txt
$ hexdump -C test.txt 
00000000  68 65 6c 6c 6f 20 77 6f  72 6c 64 0a 1b 1f 77 68  |hello world...wh|
00000010  61 74 65 76 65 72 0a                              |atever.|
$ grep -a -b --only-matching $'\x1b\x1f' test.txt 
12:

So in this case the 1B 1F is at position 12.

• Convert to integer (there is probably an easier way)

  $ echo 'ibase=16; '`xxd -u -ps -l 2 -s 12 test.txt`  | bc
  6943
  

• And the reverse:

  $ printf '%04X' 6943 | xxd -r -ps | hexdump -C
  00000000  1b 1f                                             |..|
  $ printf '%04X' 4242 | xxd -r -ps | hexdump -C
  00000000  10 92                                             |..|
  

• And putting it back in the file:

  $ printf '%04X' 4242 | xxd -r -ps | dd of=test.txt bs=1 count=2 seek=12 conv=notrunc
  2+0 records in
  2+0 records out
  2 bytes (2 B) copied, 5.0241e-05 s, 39.8 kB/s
  

• Result:

  $ hexdump -C test.txt
  00000000  68 65 6c 6c 6f 20 77 6f  72 6c 64 0a 10 92 77 68  |hello world...wh|
  00000010  61 74 65 76 65 72 0a                              |atever.|