To add to this, Google spent a lot of time and effort on building a custom build system (Bazel) to make this sort of dependency tracking in a mono-repo easy. It

To add to this, Google spent a lot of time and effort on building a custom build system (Bazel) to make this sort of dependency tracking in a mono-repo easy. It's pretty great, and I'd rather write BUILD files than Makefiles/shell scripts any day, but the OP appears to have been focusing on extremely simple projects, where shell scripts are okay.