Bitcoin's PoW in bash

pow.sh

#!/usr/bin/env bash
set -euo pipefail

usage() {
    echo "Usage: $0 <data> <difficulty>"
    echo "Example: $0 \"$(date -u +%N)\" 3"
    echo
    echo "Parameters:"
    echo "  data        - Input data to hash (typically a timestamp)"
    echo "  difficulty  - Number of leading zeros required in hash"
    exit 1
}

# calculate double SHA256 hash
calculate_hash() {
    local input="$1"
    echo -n "$input" | sha256sum | awk '{print $1}' | xxd -r -p | sha256sum | awk '{print $1}'
}

if [ "$#" -ne 2 ]; then
    usage
fi

data="$1"
difficulty="$2"

if ! [[ "$difficulty" =~ ^[0-9]+$ ]]; then
    echo "error: difficulty must be a positive number"
    exit 1
fi

target=$(printf '%0*d' "$difficulty" 0)

# proof of work loop
nonce=0
while true; do
    hash=$(calculate_hash "$data$nonce")
    if [[ $hash == "$target"* ]]; then
        printf "hash: %s\nnonce: %d\ndata: %s\n" "$hash" "$nonce" "$data"
        exit 0
    fi
    ((nonce++))
done