Ruby Gem on Mac

# install hombrew ruby, so no mess-up with ruby come with Mac

brew install ruby

# add below path to your environment

export PATH="/usr/local/opt/ruby/bin:$PATH"

export PATH="/usr/local/lib/ruby/gems/2.7.0/bin:$PATH"

# install gem fastlyctl, you will find installed bin file in /usr/local/lib/ruby/gems/2.7.0/bin

gem install fastlyctl

# try it

fastlyctl version

Reference

https://stackoverflow.com/questions/51126403/you-dont-have-write-permissions-for-the-library-ruby-gems-2-3-0-directory-ma