Rails requires rubygems> = 1.3.1 (You have 1.2.0). Please 'gem Update-system' and try agai

After installing Ror, you plan to run a demo.

 

Question 1,

$ Ruby script/Server

Rails requires rubygems> = 1.3.1 (You have 1.2.0). Please 'gem update -- system' and try again.

 

Question 2,

Execute as prompted:
$ Sudo gem update -- System
But it prompts that the usage of -- system has been disabled:
Error: While executing gem... (runtimeerror)
Gem update -- system is disabled on Debian. rubygems can be updated using the official...

Question 3,
Update with APT-Get as prompted. No version is updated:
$ Sudo apt-Get install rubygems
(It is found that many of the commands for installing gem on the Internet are incorrect. The Gem package in Ubuntu is not a ruby gem. The Gem package in Ubuntu is explained as follows:
Graphics environment for multimedia-puredata Library
GEM is a loadable library for puredata, which adds OpenGL graphics rendering
And animation to PD. PD is a graphical programming language and computer
Music system.
This package has nothing to do with Ruby. The correct one is the rubygems package .)

Update method,
New VersionUpdate MethodInstall rubygems-update with gem:
$ Sudo gem install rubygems-Update
Successfully installed rubygems-update-1.3.7
Then run the update_rubygems update command:
$ Sudo/var/lib/gems/1.8/bin/update_rubygems

Rubygems installed the following executables:
/Usr/bin/gem1.8


Upgrade successful:
$ Gem-V
1.3.7

Note: After the upgrade, the gem list is empty and you need to reinstall rails.
(I personally want to install sudo gem install-V = 2.3.5 rails)