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Perl for Web Site Management by John Callender

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Running External Commands with system and Backticks

After a user had created a bunch of student and leader pages, I wanted him to be able to run the make_cf.plx script, which re-creates all the links at the bottom of all the pages, without my having to do it for him. For that reason, I created the Regenerate Site Links choice on the default page delivered by make_page.cgi. Let’s look now at what happens when the &regenerate_site_links subroutine is invoked.

eval { system('./make_cf.plx') == 0 or die $? };

This looks pretty scary, but it’s not so bad once we pick it apart. As usual, we’ll start on the inside of the statement and work our way out. First comes system('./make_cf.plx'). This uses the system function to have our script start up a separate process and run the function’s argument in that process. Our script will wait for that other process to finish, then continue on. This is a lot like running an external command inside backticks (`), except that backticks return the output of that external command so that we can capture it into our program and do interesting things with it. system just runs the external command without returning its output.

It is important to check that nothing went wrong when the external command ran, which is what that == 0 or die $? part is doing. Unlike the way we normally check for success or failure in a Perl function, where the function returns a true value for success and a false value for failure, the system function does just the opposite. ...

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