Learning Perl, 5th Edition

Reopening a Standard Filehandle

We mentioned earlier that if you were to reopen a filehandle (that is, if you were to open a filehandle FRED when you’ve already got an open filehandle named FRED, say), the old one would be closed for you automatically. And we said that you shouldn’t reuse one of the six standard filehandle names unless you intended to get that one’s special features. And we also said that the messages from die and warn, along with Perl’s internally generated complaints, go automatically to STDERR. If you put those three pieces of information together, you now have an idea about how you could send error messages to a file, rather than to your program’s standard error stream:^[*]

# Send errors to my private error log
if ( ! open STDERR, ">>/home/barney/.error_log") {
  die "Can't open error log for append: $!";
}

After reopening STDERR, any error messages from Perl will go into the new file. But what happens if the die is executed—where will that message go, if the new file couldn’t be opened to accept the messages?

The answer is that if one of the three system filehandles—STDIN, STDOUT, or STDERR—fails to reopen, Perl kindly restores the original one.^[†] That is, Perl closes the original one (of those three) only when it sees that opening the new connection is successful. Thus, this technique could be used to redirect any (or all) of those three system filehandles from inside your program,^[‡] almost as if the program had been run with that I/O redirection from the shell ...