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Trapping Errors

This last topic on Bourne shell programming deals with the way programs can handle interruptions. An interruption can be anything from a terminal being disconnected to the computer running out of memory. What happens if an interruption occurs during execution of a script? This is not an easy question to answer as the answer is basically that it depends on what kind of interruption has occured. If, for example, the power to the system was lost, there is really nothing that can happen since the computer will no longer be operational. If however, the user presses an interrupt (or break) key, the question is still not answered. Will the program stop immediately or will it finish executing the current loop, or perhaps nothing will happen. What actually happens depends on a command called trap. The trap command allows the user to have the program carry out a command string of one or more commands prior to exiting the script. If more than one command is contained in the string, the commands should be contained in quotes as described in the section on shell special characters, and command execution. The syntax of the trap command is as follows:

trap command(s) signal(s)

One situation where this command is extremely useful is where information is being written to a file (to be removed at normal exit of the program) and an interruption (or using correct Unix terminology, a signal) is sent which would by default cause the program to terminate. If program termination occurred prior to normal exit from the program, the default action would terminate the program and leave the mess behind. Adding the following to the script would allow the clean-up to occur before termination due to a user break (control-C):

trap `rm tmp/*; exit 1;`

Care must be taken for a couple of reasons when trapping signals. First, the signals which may be trapped vary from machine to machine, and second, the definite kill signal (9) cannot be trapped on any machine. What follows is a typical table of signals and the event which causes them:

Table 2.9: Signals and the event which causes them.

The details of this list are not really important as there are only a few which will be trapped in ordinary day-to-day activities: 0, 1, 2 and 15. One should also note that the default (ie. not using trap) is always immediate termination of the script.

next up previous contents
Next: Dot Files Up: Advanced Programming Topics Previous: Functions

Douglas M Gingrich
Mon Apr 27 15:25:49 MDT 1998