Beginning Web Development With Perl : From Novice to ... - Nabo

CHAPTER 4 ■ SYSTEM INTERACTION 85When executed, the variable $uptime would contain the output from the uptime command:21:20:07 up 202 days, 5:52, 3 users, load average: 0.01, 0.02, 0.00Obviously, the output from your uptime command probably will be different.System Processes As FilehandlesYou learned about filehandles earlier in this chapter. You can also use system processes as filehandlesby using the same open() function as you use to create a filehandle. Like file-flavoredfilehandles, you can open process filehandles for reading and writing.The syntax of the open() function is largely the same for processes as it is for files, exceptthat the pipe character (|) is used instead of the greater-than/less-than characters. The locationof the pipe character determines whether the process handle is opened for reading or writing.Creating a process handle for reading looks like this:open(UPTIME, "uptime|");Creating a handle for writing looks like this:open(PRINTER, "|lpr ");Then you could print to that handle, similar to how you print to a filehandle:print PRINTER "Printing something from this handle\n";Using System Processes Within a CGI ProgramUsing a system call from within a Perl-based CGI script is no different from using the samesystem call from within another Perl program. The one exception is that you must pay particularattention to the environment within which the script will be run.Many times, the environment will be that of the web server user. This means that the Perlprogram might not have access to the same commands, files, and other environmental bits asyour user on the same system. A common symptom is that the script will appear to run finewhen you execute it from your shell, but when you attempt to access the script through theweb browser, it won’t work, possibly giving the infamous Internal Server Error message thatPerl programmers everywhere have come to love.Some web servers use Apache’s suEXEC feature for the execution of CGI scripts. WithsuEXEC, CGI scripts are executed as the user who owns the program, which is frequently thesame as your userid for many installations.Security Considerations with System InteractionWhen working with the system—files or processes—from within a Perl program, you must payadditional attention to your surroundings. It’s quite possible and unfortunately easy to overwriteexisting and essential files by opening a file for writing instead of appending.Whenever you work with anything outside your Perl program, there is always a risk of introducingunknown data into your Perl program or doing something unintentional to the systemitself. The former is an important concern for developers; the latter is primarily a concern for thesystem administrator. For those of us who frequently wear both hats, it’s important to perform