On UNIX, script agents use one of the different agent binaries that are shipped with VCS. The agent binaries are located at:

$VCS_HOME/bin/

See Script based agent binaries.

You can implement entry points using C++ or scripts. If you are implementing even one entry point in c++ then you must implement the VCSAgStartup function. If you do not implement any entry points in C++, then you do not need to implement the VCSAgStartup function since the default implementation of VCSAgStartup is present in the script agent binary provided by VCS as mentioned above.

See About the VCSAgStartup routine.