CMPS 335 Web Publishing

Debugging Perl Scripts

Finding and correcting errors in a program can be very time consuming and frustrating. There are two types of errors: syntax errors and logic errors.

Syntax Errors

The most common errors are typing-related errors. Some of the most frequently repeated syntax errors:

Missing semicolons
Missing quotation marks
Missing commas, parentheses, curly braces, brackets, etc.
Misspelling keywords
Bare words

Bare words are character strings without surrounding quotation marks and that do not begin with a variable designator. Variable designators for scalar, array, and hash variables are $, @, and %, respectively. Some fo the techniques and tools for finding syntax errors are:

Turning on the perl switch -c or -w
Examples: perl -c filename or perl -w filename
Checking the shebang line: #!/usr/bin/perl
Checking the MIME content-type statement for the script that generates output to the browser

Logic Errors

Some fo the techniques and tools for finding syntax errors are:

Checking the access permissions
Using print statements to follow a variable's progress
Using a Perl debugger
Example: perl -d filename

Examples of -w and -c switches


   [jhu@cs cgi-bin]$ cat hello.cgi
   #!/usr/bin/perl
   $date = `date`;  # date is a Unix date function
   $time = time;    # time is a Perl function
   print "\n";
   print "Hello, world.\n";
   print "Today is $date\n";
   print "Today is $time\n";
   # End of hello.cgi

   [jhu@cs cgi-bin]$ perl -c hello.cgi
   hello.cgi syntax OK
   [jhu@cs cgi-bin]$ perl -w hello.cgi

   Hello, world.
   Today is Mon Jan 28 13:46:37 CST 2002

   Today is 1012247197

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