I have no problem following the procedure replacing my macbook bash to oh_my_zsh. https://github.com/robbyrussell/oh-my-zsh
But for Ubuntu, the procedure a little different.
1. sudo apt-get install zsh
2. curl -L https://github.com/robbyrussell/oh-my-zsh/raw/master/tools/install.sh | sudo sh
can skip this… if point 2 is done correctly.
3. sudo chsh -s /bin/zsh
check whether your bash is correct
You should have root and ubuntu (for ec2 user) set properly
if yours is still bin/bash, then you need to manually change for ubuntu user
#chsh -s /bin/zsh ubuntu