To fix this, it seems that you'll need to explicitly agree to the license agreement.
If you end up with multiple versions of Xcode, you can select the version you want with:
xcode-select --switch /Applications/Xcode.app
Or if you want to run command line tools without Xcode:
xcode-select --switch /Library/Developer/CommandLineTools
See this article for more information.
If you've ever tried running a shell script, only to get a command not found error, it probably means your shell script isn't executable: sudo ./install.