Debugging internal server errors (premature end of script headers) part 8

Die statements

If your script opens files like this:

open (F, "myfile.txt"); @lines=<F>; close (F);

then you need to do something about it! If at any stage the script can't open the file, this can lead to problems. Either a script can sit there hanging if it can't open and write to a file, or you can spend ages trying to work out exactly why your script isn't doing what it should be.

Change the above code to:

open (F, "myfile.txt") || die "cannot open myfile.txt"; @lines=<F>; close (F);

The script will then error with the message:

cannot open myfile.txt at test.cgi line 31.

If you use '$!' in your statement:

open (F, "myfile.txt") || die "cannot open myfile.txt $!"; @lines=<F>; close (F);

...it will print out why it died

cannot open myfile.txt No such file or directory at test.cgi line 31.

$! contains the error returned by the server and you use it in your die statement to print the error out.

Use the same die statement with sendmail as well - if there's problems, it will stop the script hanging and return the error.

open(MAIL, "| /usr/bin/sendmail -t") || die "Unable to open sendmail. $!";

If there's a problem with sendmail, or the path to sendmail is wrong - it will end the script and return the error:

Unable to open sendmail. No such file or directory at test.cgi line 311.

It is recommended that you go right through your script and put 'die' statements in wherever the script tries to open up a file. It does two things: lets you know that there's a problem, as well as killing off the execution of the script instead of letting it keep trying forever to open a file that doesn't exist.