specifying file descriptor number?

Question

My understanding was that one could not control the file descriptor (integer) assigned by the OS when opening a new file using open(). How then is it possible in a bash shell to assign a specific file descriptor using a command like

exec 5>&1

(I suppose I could find out by reading the bash sources...)

Answer 1

See the dup2 Unix system call.

Answer 2

Also, file descriptors are assigned sequentially, so if you know 0, 1, 2, ..., n are already opened and none of them have been closed, the next will be n+1.