Learning Perl, Fourth Edition

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>.^[57]

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; unless you do something uncommon, it means the keyboard of the user who invoked your program (probably you). If there’s nothing waiting for <STDIN> to 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).^[58]

The string value of <STDIN> typically has a newline character on the end of it.^[59] So, you could do something like this:

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

In practice, you don’t often want to keep the newline, so you need the chomp operator.

^[57] This is a line-input operator working on the filehandle STDIN, but we can’t tell you about that until we get to filehandles (in Chapter 5).

^[58] To be honest, it’s normally your system that waits for the input; Perl waits for your system. Though the details depend upon your system and its configuration, you can generally correct your mistyping with a backspace key before you press return since your system handles that, ...