#!/bin/bash

PLACEHOLDER=$1
FILES="${@:2}"

function printInputError() {
  echo "Usage: $0 PLACEHOLDER FILE…" >&2
  exit 1
}

function timestamp() {
  date -u +"%Y-%m-%d %H:%M:%S"
}

if [ "$#" -le 1 ]; then
  printInputError
fi
for TARGET in ${FILES}; do
  if ! [ -f "${TARGET}" ]; then
    printInputError
  fi
done

sed -i "s/${PLACEHOLDER}/$(timestamp)/g" ${FILES}