Why can't I use gem install command (linux/ ruby / rails)?

I'm having problems when issuing 'gem' command. I have both rubygems1.8 and rubygems1.9. My rails version is rails-2.3.3. My ruby ruby 1.8.7. And I'm on Linux.

My problem is that I can't issue

gem开发者_JAVA技巧 install <gem name>

Instead I have to give:

gem1.8 install <gem name>

Can anyone tell me why is that so?

Create a symbolic link to gem1.8

ln /usr/bin/gem1.8 /usr/bin/gem

Because you installed two different versions and they can't both be named "gem."