[Solved] Question regarding Perl's "defined" function...

[cgmd]

Hi, all...

I hope to teach myself Perl, using "Learning Perl" by Randal L. Schwartz, et al. I have no background writing scripts, so this is all pretty much new to me.

Within the early part of the book, I ran across a script meant to help with the understanding of the defined function:

Code: Select all

#!/usr/bin/perl

$madonna = <STDIN>;
if ( defined($madonna) ) {
  print "The input was $madonna";
} else {
  print "No input available!\n";
}

I'm unable to make this script print out "No input available!"... If I run the script, and not enter anything, nothing happens. If I run it and only hit return, I get:

Code: Select all

 cgmd  ~/bin $ defined 

The input was 
cgmd  ~/bin $ 

I'm guessing the return key sends an "end of line signal" to <STDIN> , causing defined to return true, and, therefore avoid the "else" alternative. If that's the case, how can I make this script print out the line "No input available!"??

What am I missing here??

Thanks!

[grimm26]

You're right. The newline that happens when you hit enter is input into the script.
If your goal is to find if someone entered something meaningful (non whitespace). you should go this way:

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while(<STDIN>) {
  if(/\S+/) {
    print "You typed $_";
  }
}

If you just want to make that script tell you it didn't get any STDIN, try running like so:

Code: Select all

 ./defined < /dev/null

[vandien]

If that script is straight from the book, I'm not sure I would trust it..

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$madonna = <STDIN>;
chomp $madonna;
if ($madonna) {
  print "The input was $madonna\n";
} else {
  print "No input available!\n";
}

Note, the three things that changed. I added chomp. This safely removes the newline from the user input. This is important to use on input from stdin, files, or system commands.

I took out the defined bit. I'm not sure how it's supposed to work (I never use it), but it obviously doesn't in this example. Testing the variable itself tells you whether it has data in it or not.

The last change was a minor cosmetic fix, adding a newline to make the output look better.

[cgmd]

grimm26 wrote:

If you just want to make that script tell you it didn't get any STDIN, try running like so:
Code:
./defined < /dev/null

That's exactly the information I needed...

Not entering keystrokes into <STDIN>, but rather sending "/dev/null" to the script, as you have shown, produces the output I was seeking!

I also appreciate the alternative script suggestions from both posts...

Thanks!