Clean way to launch the web browser from shell script?

Question

In a bash script, I need to launch the user web browser. There seems to be many ways of doing this:

• $BROWSER
• xdg-open
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Answer 1

Taking the other answers and making a version that works for all major OS's as well as checking to ensure that a URL is passed in as a run-time variable:

#!/bin/bash
if [ -z $1 ]; then
  echo "Must run command with the url you want to visit."
  exit 1
else
  URL=$1
fi
[[ -x $BROWSER ]] && exec "$BROWSER" "$URL"
path=$(which xdg-open || which gnome-open) && exec "$path" "$URL"
if open -Ra "safari" ; then
  echo "VERIFIED: 'Safari' is installed, opening browser..."
  open -a safari "$URL"
else
  echo "Can't find any browser"
fi