Resetting Traps

Another "special case" of the trap command occurs when you give a dash (-) as the command argument. This resets the action taken when the signal is received to the default, which usually is termination of the process.

As an example of this, let's return to Task 8-2, our mail program. After the user has finished sending the message, the temporary file is erased. At that point, since there is no longer any need to clean up, we can reset the signal trap to its default state. The code for this, apart from function definitions, is:

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trap abortmsg INT
trap cleanup TERM
    
msgfile=/tmp/msg$$
cat > $msgfile
# send the contents of $msgfile to the specified mail address...
rm $msgfile
    
trap - INT TERM

The last line of this code resets the handlers for the INT and TERM signals.

At this point you may be thinking that you could get seriously carried away with signal handling in a shell script. It is true that "industrial strength" programs devote considerable amounts of code to dealing with signals. But these programs are almost always large enough so that the signal-handling code is a tiny fraction of the whole thing. For example, you can bet that the real UNIX mail system is pretty darn bullet-proof.

However, you will probably never write a shell script that is complex enough, and that needs to be robust enough, to merit lots of signal handling. You may write a prototype for a program as large as mail in shell code, but prototypes by definition do not need to be bullet-proofed.

Therefore, you shouldn't worry about putting signal-handling code in every 20-line shell script you write. Our advice is to determine if there are any situations in which a signal could cause your program to do something seriously bad and add code to deal with those contingencies. What is "seriously bad"? Well, with respect to the above examples, we'd say that the case where HUP causes your job to terminate is seriously bad, while the temporary file situation in our mail program is not.

 

[12] This is what usually happens. Sometimes the command currently running will abort (sleep acts like this, as we'll see soon); at other times it will finish running. Further details are beyond the scope of this book.

[13] This command is the same as the built-in shell no-op command ":".

[14] disown is not available in versions of bash prior to 2.0.