Comment by kazinator

Also, if we are going to involve the shell, we could also just make .env a shell fragment and do this:

   sh -c '. .env; <cmd>'

There is a way to pass commands to it which are reliably executed, like thisL

   sh -c '. .env; "$@"' -- command arg1 arg2 arg3.

The non-option arguments passed to the shell are available as `"$@"`. A command consisting of nothing but `"$@"` basically executes the arguments. We can use `exec`, speaking of which:

   sh -c '. .env; exec "$@"' -- command arg1 arg2 arg3.

What I'm getting at is that this form is fairly easily exec-able;

   execl("/bin/sh", "/bin/sh", ". .env; exec \"$@\"", "--", "command",
         "arg1", "arg2", "arg3", (char *) 0);

The command and arguments can be arbitrary strings, not subject to any shell mangling.