Shell script to execute a command once (while running)

once.sh

#!/bin/bash

pidfile=$1
cmd=$2

if [ -z "$pidfile" ] || [ -z "$cmd" ]; then
  echo "Usage: once <pidfile> \"<command>\""
  exit 0
fi

if [ -f "$pidfile" ] && kill -0 `cat $pidfile` 2>/dev/null; then
    echo Still running [`cat $pidfile`]
    exit 1
fi

# echo $$ > $pidfile

$cmd&

echo Started [$!]: $cmd
echo $! > $pidfile