How to convert a sequence of bits (bitmask) into the corresponding hex number

Question

Let's say I have the bitmask 1000000000. I would like to convert it to its equivalent hexadecimal number, i.e. 0x200 (specifically, I only want the 200 part, but that's easy to take care of)

I know I can do this in Python or using various bash features and functions. Examples:

python -c "print format(0b1000000000, 'x')"
200

printf '%x\n' "$((2#1000000000))"
200

echo 'ibase=2;obase=10000;1000000000'|bc
200

But, I wanna do this using only functions available in sh (i.e. Shell, not Bash). More specifically, I want it to work with sh in an initrd image that I'm putting together. AFAIK, none of the examples above would work in an initramfs / busybox context.

Answer 1

It seems busybox sh has enough features ("substring" parameter substitution and arithmetic evaluation) to be useful enough for this:

$ busybox sh


BusyBox v1.22.1 (Ubuntu 1:1.22.0-15ubuntu1) built-in shell (ash)
Enter 'help' for a list of built-in commands.

~ $ bitstr=1000000000
~ $ n=0
~ $ i=0
~ $ while [ $i -lt ${#bitstr} ]; do
> n=$(( 2*n + ${bitstr:$i:1} ))
> i=$((i+1))
> done
~ $ echo $n
512
~ $ printf "%x\n" $n
200

Encapsulate into a function:

b2h() {
  local bitstr=$1 n=0 i=0
  while [ $i -lt ${#bitstr} ]; do
    n=$(( 2*n + ${bitstr:$i:1} ))
    i=$(( i + 1 ))
  done
  printf "%x\n" "$n"
}
b2h 1000000000   # => 200