Learning to use the httpd error log to debug CGI programs
In all of the above examples, we call die only after having already sent the MIME header to the browser (with the built-in CGI.pm “header” method). Failing to do so can create all sorts of odd problems, and results in the dreaded message:
Server Error. The server encountered an internal error or misconfiguration and was unable to complete your request.
For example, running the following CGI program:
#!/usr/local/bin/perl5 -w use strict; # Check our syntax strictly use diagnostics; # Tell us how to fix mistakes use CGI; # Import the CGI module use CGI::Carp; # Create an instance of CGI my $query = new CGI; # Don't do very much die "This program crashed on purpose ";The above program dies upon reaching the last line, and inserts an appropriate message into the httpd error log. What will the user see on his or her browser? In the above example, the user will get the dreaded:
The server encountered an internal error or misconfiguration and was unable to complete your request.You can alleviate this problem by sending a MIME header before performing any serious computations:
#!/usr/local/bin/perl5 -w use strict; # Check our syntax strictly use diagnostics; # Tell us how to fix mistakes use CGI; # Import the CGI module use CGI::Carp; # Create an instance of CGI my $query = new CGI; print $query->header("text/html"); # Don't do very much die "This program crashed on purpose ";Note: while the documentation for CGI::Carp claims to alleviate this problem, experiments with the version installed on my system (from early September 1996) didn't seem to work quite this way. I thus suggest that you output MIME headers as close to the beginning of your programs as possible, simply to alleviate such problems.
Despite the colorful language that might occur to you when your program produces an error message, good warnings and logs are extremely helpful when writing and debugging code. This is especially true for CGI programs, where much of the work takes place behind the scenes and the only output to the user comes via a web browser. Learn to use the httpd error log effectively, and while your programming problems certainly won't disappear, your mistakes should be easier to identify and correct.
Reuven M. Lerner has been playing with the Web since early 1993, when it seemed more like a fun toy than the world's Next Great Medium. He currently works from his apartment in Haifa, Israel as an independent Internet and Web consultant. When not working on the Web or informally volunteering with school-age children, he enjoys reading (on just about any subject, but especially computers, politics, and philosophy—separately and together), cooking, solving crossword puzzles, and hiking. You can reach him at firstname.lastname@example.org or email@example.com.
Practical Task Scheduling Deployment
July 20, 2016 12:00 pm CDT
One of the best things about the UNIX environment (aside from being stable and efficient) is the vast array of software tools available to help you do your job. Traditionally, a UNIX tool does only one thing, but does that one thing very well. For example, grep is very easy to use and can search vast amounts of data quickly. The find tool can find a particular file or files based on all kinds of criteria. It's pretty easy to string these tools together to build even more powerful tools, such as a tool that finds all of the .log files in the /home directory and searches each one for a particular entry. This erector-set mentality allows UNIX system administrators to seem to always have the right tool for the job.
Cron traditionally has been considered another such a tool for job scheduling, but is it enough? This webinar considers that very question. The first part builds on a previous Geek Guide, Beyond Cron, and briefly describes how to know when it might be time to consider upgrading your job scheduling infrastructure. The second part presents an actual planning and implementation framework.
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With all the industry talk about the benefits of Linux on Power and all the performance advantages offered by its open architecture, you may be considering a move in that direction. If you are thinking about analytics, big data and cloud computing, you would be right to evaluate Power. The idea of using commodity x86 hardware and replacing it every three years is an outdated cost model. It doesn’t consider the total cost of ownership, and it doesn’t consider the advantage of real processing power, high-availability and multithreading like a demon.
This ebook takes a look at some of the practical applications of the Linux on Power platform and ways you might bring all the performance power of this open architecture to bear for your organization. There are no smoke and mirrors here—just hard, cold, empirical evidence provided by independent sources. I also consider some innovative ways Linux on Power will be used in the future.Get the Guide