urldecode the contents of a file and print it in standard output.

urldecodefile.sh

#!/usr/bin/env bash

urldecode() {
    local encoded="${*//+/ }"
    printf '%b\n' "${encoded//%/\\x}"
}

main() {
  local file="$1"
  local line

  if [[ -z "$file" ]]; then
    file='/dev/stdin'
    echo "WARNING: no filename given, reading from stdin." >&2
  elif [[ ! -f "$file" ]]; then
    echo "ERROR: invalid file '$file'" >&2
    echo "USAGE: $0 [filename]" >&2
    exit 1
  fi

  while IFS= read -r line || [[ -n "$line" ]]; do
    urldecode "$line"
  done < "$file"
}

main "$@"