How do I fix the "You don't have write permissions into the /usr/bin directory" error when installing Rails?

Question

use -n parameter to install like for cocoapods:

sudo gem install cocoapods -n /usr/local/bin

Using the -n /usr/local/bin flag does work, BUT I had to come back to this page every time I wanted to update a package again. So I figured out a permanent fix for this.

For those interested in fixing this permanently:

Create a ~/.gemrc file

vim .gemrc

With the following content:

:gemdir:
   - ~/.gem/ruby
install: -n /usr/local/bin

Now you can run your command normally without the -n flag.

Enjoy!

sudo gem install cocoapods --pre -n /usr/local/bin

This works for me.

You can use sudo gem install -n /usr/local/bin cocoapods

This works for me.

I'd suggest using RVM it allows you have multiple versions of Ruby/Rails installed with gem profiles and basically keep all your gems contained from one another. You may want to check out a similar post How can I install Ruby on Rails 3 on OSX

To fix your specific error you need to run that command as sudo, ie:

sudo gem install rails --pre

This Error hit me after installing RVM correctly. Solution: re-boot Terminal.

Reference RailsCast's RVM Install tutorial.