How to run xcode from terminal?

Xcode should be the default application for .xcodeproj files, so this should work:

$ open a.xcodeproj

If that opens a different application, you can force it to use xcode:

$ open -a Xcode a.xcodeproj

If you want the command xcode to work, you can just alias it:

$ alias xcode="open -a Xcode"

then you can just xcode a.xcodeproj (and add this to ~/.bash_profile)

You could also simply run xed . in the project's root directory, apparently it will try to load a project in a hierarchical manner, i.e. the first that exists:

1. the folder, if it's a Package (Xcode 11+)
2. xcworkspace
3. xcodeproj
4. playground

which means you don't need to verify yourself the existing file structure in order to choose the best one to open.