How to download an older version of xcode

Older versions of XCode can be found at the Apple Developer site (you will need to login with your AppleID).

Which version of Xcode should I download?

You can view the Version History for the XCode app in the app store, which will list the software requirements.
-OR-

This is a handy site that lists the major XCode releases as well as the macOS requirements.  There is a direct link to download the software from the Apple Developer website, but you need to be logged into developer.apple.com.

For simplicity, these are the XCode versions needed for different macOS versions:

Catalina (10.15.3): XCode 11.4 (Download)
Mojave (10.14.3):  Xcode 11.3.1 (Download) or XCode 10.3 (Download)
High Sierra (10.13.6): XCode 9.4.1 (Download)

For example, I am currently running macOS 10.14.3 (Mojave), so the XCode version I need is XCode 10.3.

For OSs other than Catalina you should also download the Command Line tools for the same version (this will replace the xcode-select –install step given in the handbook).

Mojave (10.14.3): Command Line Tools for Xcode 11.3.1.dmg

How do I install XCode from the Apple Developer site?

Once you have found the version of XCode that you need, you will download the application as an .xip file (the file is named XCode_<VERSION>.xip, and is ~6-7GB in size).
Then, navigate to your download, and double click it to extract the software.  Next, move the extracted XCode into your Applications folder, and you can now continue with the rest of the setup instructions in the biostar handbook (i.e. typing “xcode-select –install” ).

The Command Line Tool download is a .dmg file… simple double click on it and follow the install directions.