[ruby-on-rails-3] How do I fix the "You don't have write permissions into the /usr/bin directory" error when installing Rails?

I'm trying to install Rails 3 on a brand new MacBook Pro running OS X 10.6.3, Ruby 1.8.7, and Rails 2.3.5 and I'm wondering if I've hosed myself. So far, I've run these commands:

$ gem update --system
$ gem install arel tzinfo builder memcache-client rack rack-test rack-mount erubis mail text-format thor bundler i18n
$ gem install rails --pre

However, when I run the last command, I get this error:

ERROR:  While executing gem ... (Gem::FilePermissionError)
    You don't have write permissions into the /usr/bin directory.

I think it wants me to run the command with sudo so that it can write over /usr/bin/rails. But if I do that, won't I be overwriting my Rails 2.3.5 executable? I don't want to do that. Instead, I'd like to keep both versions of Rails. How do I do that?

On macOS High Sierra, this solved my issue:

sudo gem update --system -n /usr/local/bin/gem

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

Reference RailsCast's RVM Install tutorial.

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.

use -n parameter to install like for cocoapods:

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

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

This works for me.

For me, something different worked, that I found in on this answer from a similar question. Probably won't help OP, but maybe someone like me that had a similar problem.

You should indeed use rvm, but as no one explained to you how to do this without rvm, here you go:

sudo gem install tzinfo builder memcache-client rack rack-test rack-mount \
  abstract erubis activesupport mime-types mail text-hyphen text-format   \
  thor i18n rake bundler arel railties rails --prerelease --force

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

sudo gem install rails --pre