How can virtualenv create different python environments?

Question

My server has 3 virtualenv scripts:

$ ls -lt virtual*
lrwxrwxrwx 1 root root  4 Nov 22 06:19 virtualbox -> VBox
-rwxr-xr-x 2 root root 55 Aug 14 19:34 virtualenv
-rwxr-xr-x 1 root root 55 Aug 14 19:34 virtualenv2
-rwxr-xr-x 2 root root 55 Aug 14 19:34 virtualenv3
$ cat virtualenv
#!/usr/bin/python3
import virtualenv
virtualenv.main()
$ cat virtualenv2
#!/usr/bin/python2
import virtualenv
virtualenv.main()
$ cat virtualenv3
#!/usr/bin/python3
import virtualenv
virtualenv.main()

I know virtualenv2 is used to create Python 2 environment while virtualenv3 is for Python 3. But what confuses me is why just modify the /usr/bin/python2 to /usr/bin/python3 in shebang line can achieve the effect that create different environments? What is the magic behind it?

Answer 1

There are two pieces of "magic".

1) The shebang line informs the kernel of the correct interpreter to use. When you invoke virtualenv2, the kernel expands that to /usr/bin/python2 virtualenv2. Similarly, when you invoke vitualenv3, the kernel expands that to /usr/bin/python3 virtualenv3.

2) The virtualenv.main() creates a virtual environment based upon the currently running interpreter. For example, if virtualenv.main() is invoked by /usr/bin/python3, then it creates a virtual environment based upon /usr/bin/python3.

Hopefully you can see that these two "magic" items, taken together, perform the intended action.