Converting from binary to hex and back

Question

Given a binary file, how do you convert it to a hex string and back, using only standard tools like sed and cut, on a minimal system with busybox installed?

These tools are not available:

• perl
• python
• xxd (comes with vim)
• gcc

A hexdump command comes with busybox, but it is different from the one that comes with util-linux.

I'm looking for a script or command to convert a file to a hex string, and a corresponding one for converting it back to binary. The intermediate format doesn't have to be hex, it can be base64 or something else.

This is for an embedded device with limited disk space.

Answer 1

Here's what I came up with (based on several online sources and some experimentation).

Converting from hex to bin (hex2bin):

#!/bin/sh
sed 's/\([0-9A-F]\{2\}\)/\\\\\\x\1/gI' "$1" | xargs printf

Converting from bin to hex (bin2hex):

#!/bin/sh
hexdump -v -e '1/1 "%02x"' "$1"

Example use:

./bin2hex binary_file_1 | ./hex2bin - > binary_file_2
diff -s binary_file_1 binary_file_2

This works with busybox, but hex2bin is unfortunately limited by the maximum length of the argument given to xargs, so this method will only work for small files (less than 32 KiB on my desktop system).

Answer 2

POSIXly (and only using a common subset compatible with busybox (or at least the busybox as built for the current busybox Debian package):

• bin2hex:

  (
    export LC_ALL=C
      od -An -vtx1 |
      tr -s ' \t\n' '\n\n\n' |
      grep .
  )
  

  (one hex per line)

• hex2bin:

  (
    export LC_ALL=C
      awk '
        BEGIN{
          for (i = 0; i < 256; i++)
            c[sprintf("%02x", i)] = sprintf("%o", i)
        }
        NR % 200 == 1 {printf "%s", end "printf '\''"; end = "'\''\n"}
        {printf "\\%s", c[$0]}
        END {print end}' |
      sh
  )
  

If your busybox, contrary to Debian's one has been built without the DESKTOP option, then the -An and -tx1 option to od won't be available. You can use od -b instead which will give a one-byte octal dump with octal offsets. od -b is Unix but not POSIX however so won't work on every Unix-like system.

bin2hex would become:

(
  export LC_ALL=C
    od -b |
    awk '
      BEGIN{
        for(i = 0; i < 256; i++)
          hex[sprintf("%03o", i)] = sprintf("%02x", i)
      }
      NF > 1 {for (i = 2; i<= NF; i++) print hex[$i]}'
)

Again, tested only with Debian's busybox, I can't tell how much of that is dependant on some busybox compile-time option or another. You'd have to test on the target system.

Answer 3

Your busybox may have been built with the applet base64 so you could do

busybox base64 <bin >txt
busybox base64 -d <txt >bin

Or if not you may have the venerable uuencode and uudecode, which may also have the -m option to encode in hex. Eg

echo -n abcABC | busybox uuencode -m - | busybox uudecode - 

List the built-in applets with busybox --list.