I want to build an Android Studio app (the Gradle build system), but I want to do this via the command line.
Official Documentation is here:
To build a debug APK, open a command line and navigate to the root of your project directory. To initiate a debug build, invoke the assembleDebug task:
gradlew assembleDebug
This creates an APK named module_name-debug.apk in project_name/module_name/build/outputs/apk/.