Use the when command, stop command, and trace command to create event handlers. (For detailed information, see when Command, stop Command, and trace Command.)
stop is shorthand for a common when idiom.
when event-specification { stop -update; whereami; }
An event-specification is used by the event management commands stop, when, and trace to specify an event of interest. (see Setting Event Specifications).
Most of the trace commands can be handcrafted using the when command, ksh functionality, and event variables. This is especially useful if you want stylized tracing output.
Every command returns a number known as a handler id (hid). You can access this number using the predefined variable $newhandlerid.