On both my work and home computers, I recently upgraded Ruby to 2.3.1, using ruby-install. I use chruby as my Ruby switcher.
I started seei
I've had this problem today too and it frustrated because I couldn't type because my whole editor flashed red with error messages.
I'm not entirely sure what exactly caused it but I believe it's because we have multiple ruby versions OR multiple ruby version managers installed and they overwrite each other and otherwise mess up your paths towards the gems.
The gem command also gets overwritten when you have rbenv and rvm.
See which ones you've installed by typing;
which rvm
which rbenv
which chruby
If one of those is installed it'll return a path. Then delete them, make sure to completely clean out all the directories and start with a clean install.
Remove
Here's what I did for rvm;
rvm implode
gem uninstall rvm
rm -rf ~/.rvm
rm -rf ~/.rvmrc
Here's what I did for rbenv
First installed https://github.com/meowsus/rbenv-clean, then
rbenv clean
sudo apt-get remove rbenv
rm -rf ~/.rbenv
Reinstall
Then you have a clean home directory to work from. I reïnstalled rbenv with How to install Ruby 2.1.4 on Ubuntu 14.04. And finally;
rbenv rehash