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'who' is the normal way.

An alternative is the following which shows a list of userids that are active:
ps --no-headers -eo euser,ruser,suser,fuser | sort -u

both of these can be piped through...

The wait requites a mutex parameter and it does the following:
unlock mutex
wait for signal on condition
lock mutex

If you don't lock the mutex before calling wait, the wait will...