how to read STDIN interactively

Allasso has asked for the wisdom of the Perl Monks concerning the following question:

Say I want to read multi-line user input, do something with it, then read some more. I try,


#!/usr/bin/perl
use strict;

my $input;

local $/;
$input = <STDIN>;
print $input;

$input = <STDIN>;
print $input;
[download]

It receives user input the first time, but prints and exits the script without letting me receive user input the second time. I tried undefining $/ in a separate block, and just seeing if I could read single line input the second time, but it does the same thing - just does the first read, then exits:


#!/usr/bin/perl
use strict;

my $input;
{
local $/;
$input = <STDIN>;
}
print $input;

$input = <STDIN>;
print $input;
[download]

this doesn't work either:


#!/usr/bin/perl
use strict;

my $holdRS = $/;
local $/;
my $input = <STDIN>;
$/ = $holdRS;

print $input;

my $holdRS = $/;
local $/;
my $input2 = <STDIN>;
$/ = $holdRS;

print $input2;
[download]

This works though:


#!/usr/bin/perl
use strict;

my @userinput = <STDIN>;
foreach (@userinput) { print; }

my @userinput = <STDIN>;
foreach (@userinput) { print; }
[download]

What is it about undefining $/ that allows me to read user input only one time? (even if I set it back to default first)

Comment on how to read STDIN interactively Select or Download Code

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Re: how to read STDIN interactively by ikegami (Patriarch) on Feb 24, 2011 at 15:54 UTC
If it's undef, you're saying there's no line terminator, so it reads until the end of the file.	[reply]
Re^2: how to read STDIN interactively by Allasso (Monk) on Feb 24, 2011 at 21:02 UTC
I don't understand, I thought I was sending it EOF when I hit ^D at the end of my input. That's how I get it to print the first time. Also, in the last example, I am returning $/ to a newline.	[reply]
Re^3: how to read STDIN interactively by ikegami (Patriarch) on Feb 24, 2011 at 21:39 UTC
I don't understand, My fault. I misunderstood the problem. Basically, I made the same mistake you did. Starting fresh: If you wanted to read to EOF, you'd write `while (<STDIN>) { print; }` [download] You want to read to EOF twice, so you'd write `for (1..2) { while (<STDIN>) { print; } }` [download] You don't want to read a line at a time, so `local $/; for (1..2) { while (<STDIN>) { print; } }` [download] Now count the number of times `<STDIN>` executed: 4. `local $/; $_ = <STDIN>; # Returns first chunk print; $_ = <STDIN>; # Returns EOF $_ = <STDIN>; # Returns second chunk print; $_ = <STDIN>; # Returns EOF` [download] In all but the last snippet, you are calling `readline` (`<>`) in scalar context. It returns undef on eof (and on error). `readline` (like `glob` and `each`) needs to signal when it has finished return results when used as an iterator. You're forgetting to absorb this signal. The simplest solution is to call `readline` in list context. `local $/; ($_) = <STDIN>; # Returns first chunk ($_) = <STDIN>; # Returns second chunk` [download]	[reply] [d/l] [select]
Re^4: how to read STDIN interactively by Allasso (Monk) on Feb 25, 2011 at 01:14 UTC
Re^5: how to read STDIN interactively by ikegami (Patriarch) on Feb 25, 2011 at 01:47 UTC
Some notes below your chosen depth have not been shown here
Re^3: how to read STDIN interactively by DrHyde (Prior) on Feb 25, 2011 at 10:48 UTC
By the time you reset `$/` the script has already read STDIN all the way to the end of the "file", so the second `<STDIN>` has nothing left to read.	[reply] [d/l] [select]
Re^3: how to read STDIN interactively by Allasso (Monk) on Feb 24, 2011 at 21:03 UTC
...excuse me, 2nd to the last example	[reply]
Re: how to read STDIN interactively by Corion (Patriarch) on Feb 24, 2011 at 15:55 UTC
Once you set `$/` to undef, `<...>` will keep reading until the filehandle is exhausted. After that, you will likely need to reopen STDIN to start reading from it again.	[reply] [d/l] [select]
Re^2: how to read STDIN interactively by ikegami (Patriarch) on Feb 24, 2011 at 16:11 UTC
No, not if STDIN is a terminal. Terminals automatically re-open, in effect. That's what you can rely on read functions to return EOF more than once, and why you sometimes have to signal EOF more than once to exit some applications.	[reply]
Re: how to read STDIN interactively by DrHyde (Prior) on Feb 25, 2011 at 10:29 UTC
Why doesn't what work? You're far more likely to get useful responses if you use a descriptive title, so that people can see what you're talking about from the index on the "newest nodes" page etc.	[reply]