Bash function to only install Vagrant plugins if they're not already installed.

use_it_like.sh

# This would install vagrant-aws, if it's not already installed
# And do nothing if it was already installed.
vagrant_plugin_install vagrant-aws

vagrant_plugin_install.sh

# Don't reinstall Vagrant plugins, if they're already installed
# We can always call `vagrant plugin update` explicitly.
vagrant_plugin_install () {
  set +v
  installed_packages=$(vagrant plugin list | cut -f1 -d' ')
  if ! [[ $installed_packages =~ (^|[[:space:]])"$1"($|[[:space:]]) ]]; then
    vagrant plugin install $1
  else
    echo $'\e[4m''Notice'$'\e[0m'": Vagrant plugin '$1' is already installed.  Use 'vagrant plugin update $1' to force an upgrade."
  fi
  set -v
}