I am using the gcc compiler.

My project source tree looks like somewhat like this

test$~: tree .
.
|-- folder
|   |-- hello.cpp 
|   `-- hello.h
`-- main.cpp

1 directory, 3 files
test$~: 

The file main.cpp contains the main() function and all the functions invoked by main.cpp lie in the directory named folder

So far in all my little projects I never had to put some source code under a sub-directory.

What I am looking for, in short, is some gcc command for recursive compilation in sub-directories and their subdirectories and so on... This command should be invoked from the home directory of the code project.