34.5. Foreground and Background Process Groups

The controlling terminal maintains the notion of a foreground process group. Within a session, only one process group can be in the foreground at a particular moment; all of the other process groups in the session are background process groups. The foreground process group is the only process group that can freely read and write on the controlling terminal. When one of the signal-generating terminal characters is typed on the controlling terminal, the terminal driver delivers the corresponding signal to the members of the foreground process group. We describe further details in Section 34.7.

Note:

In theory, situations can arise where a session has no foreground process group. This could happen, for example, if all processes in the foreground process group terminate, and no other process notices this fact and moves itself into the foreground. In practice, such situations are rare. Normally, the shell is the process monitoring the status of the foreground process group, and it moves itself back into the foreground when it notices (via wait()) that the foreground process group has terminated.