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2.8. Getting User Input

At this point, you're probably wondering how to get a value from the keyboard into a Perl program. Here's the simplest way: use the line-input operator, <STDIN> .[61] Each time you use <STDIN> in a place where a scalar value is expected, Perl reads the next complete text line from standard input (up to the first newline), and uses that string as the value of <STDIN>. Standard input can mean many things, but unless you do something uncommon, it means the keyboard of the user who invoked your program (probably you). If there's nothing waiting to be read (typically the case, unless you type ahead a complete line), the Perl program will stop and wait for you to enter some characters followed by a newline (return).[62]

[61]This is actually a line-input operator working on the filehandle STDIN, but we can't tell you about that until we get to filehandles (in Chapter 11, "Filehandles and File Tests").

[62]To be honest, it's normally your system that waits for the input; Perl waits for your system. Although the details depend upon your system and its configuration, you can generally correct your mistyping with a backspace key before you press return -- your system handles that, not Perl itself. If you need more control over the input, get the Term::ReadLine module from CPAN.

The string value of <STDIN> typically has a newline character on the end of it.[63] So you could do something like this:

[63]The exception is if the standard input stream somehow runs out in the middle of a line. But that's not a proper text file, of course!

$line = <STDIN>;
if ($line eq "\n") {
  print "That was just a blank line!\n";
} else {
  print "That line of input was: $line";
}

But in practice, you don't often want to keep the newline, so you need the chomp operator.



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