Convert a hexadecimal string to bytes.

hex_string_to_bytes.bash

#!/bin/bash

# hex_string_to_bytes.bash: convert a hexadecimal string to bytes.
# Examples:
# $ bash hex_string_to_bytes FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFEBAAEDCE6AF48A03BBFD25E8CD0364140
# $ bash hex_string_to_bytes FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFEBAAEDCE6AF48A03BBFD25E8CD0364140 > output_file.bin

export LC_ALL=C
hex_string="$1"

#######################################
# Input validation - only hexadecimal characters permitted
#######################################
case ${hex_string} in
  ''|*[^[:xdigit:]]*) echo "The string contains characters that don't belong to hexadecimal set. The program execution is terminated."; exit 1;;
  *) ;;
esac

#  The equivalent of the above case command except less portable.
#  if ! [[ "${hex_string}" =~ ^[[:xdigit:]]+$ ]]; then
#    continue
#  else
#    exit 1
#  fi

#######################################
# Input validation - even number of characters is required
#######################################
if (( ${#hex_string} % 2 )); then
  echo "The number of characters is invalid and doesn't translate to whole bytes."
  exit 1
fi

for ((i=0; i<${#hex_string}; i+=2)); do
  printf "%b" "\x${hex_string:i:2}"
done