Helping a new dev setup their machine today with rbenv when trying to bundle I ran into this error:

ERROR:  While executing gem ... (Gem::FilePermissionError)
You don't have write permissions for the /Library/Ruby/Gems/2.0.0 directory.

Thankfully I already dealt with this before and referenced it as a comment to a question on StackOverflow. For my own future reference I will list the solution here as well.

You need to correct your paths

To determine if this fix will work run the following:

which gem

This should output a directory you do not have permissions to:

/usr/bin/gem

To fix this perform the following steps.

  1. Determine the path you need to copy to your profile:
    rbenv init -
    

    The first line of the output is the line you need to copy over to your profile:

      export PATH="/Users/justin/.rbenv/shims:${PATH}" #path that needs to be copied
      source "/usr/local/Cellar/rbenv/0.4.0/libexec/../completions/rbenv.zsh"
      rbenv rehash 2>/dev/null
      rbenv() {
        typeset command
        command="$1"
        if [ "$#" -gt 0 ]; then
          shift
        fi
    
        case "$command" in
        rehash|shell)
          eval `rbenv "sh-$command" "$@"`;;
        *)
          command rbenv "$command" "$@";;
        esac
      }
    
  2. Copy the path to your profile and save it
  3. Reload your profile (source ~/.zshenv for me)
  4. Run rbenv rehash

Now when you run which gem you should get a local path that you have permissions to:

/Users/justin/.rbenv/shims/gem