Check if a #ruby gem is installed from bash script

If you are adding customizations to your zshrc, such as adding auto completion for a certain gem, you want to make sure that the gem is installed before performing any action.

The gem list [gemname] -i command returns a boolean representing whether a gem is installed or not.

if `gem list lunchy -i`; then 
  echo "Lunchy gem is installed!"; 
  # do some configuration here
fi

If you wish to be more granular you can check whether a specific version is installed by adding the --version [version] flag.

if `gem list lunchy -i --version 0.10.4`; then 
  echo "Lunchy v0.10.4 is installed!"; 
  # do some configuration here
fi