Learning Perl, 5th Edition

Fatal Errors with die

Let’s step aside for a moment. We need some stuff that isn’t directly related to (or limited to) I/O, but is more about getting out of a program earlier than normal.

When a fatal error happens inside Perl (for example, if you divide by zero, use an invalid regular expression, or call a subroutine that hasn’t been declared), your program stops with an error message telling why.^[‖] But this functionality is available to us with the die function, so we can make our own fatal errors.

The die function prints out the message you give it (to the standard error stream, where such messages should go) and makes sure that your program exits with a nonzero exit status.

You may not have known it, but every program that runs on Unix (and many other modern operating systems) has an exit status, telling whether it was successful or not. Programs that run other programs (like the make utility program) look at that exit status to see that everything happened correctly. The exit status is just a single byte, so it can’t say much; traditionally, it is 0 for success and a nonzero value for failure. Perhaps 1 means a syntax error in the command arguments, while 2 means that something went wrong during processing, and 3 means the configuration file couldn’t be found; the details differ from one command to the next. But 0 always means that everything worked. When the exit status shows failure, a program like make knows not to go on to the next step.

So we could rewrite the previous example, ...