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

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.