In a single Perl script, can I close and re-open STDIN?

user_1461307 0
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I want to write perl scripts that can read the STDIN that is given at invocation of the script, finish reading it, and then interactively prompt the user for a one-line STDIN. This one-line STDIN will tell the script how to proceed.

In a practical application, I would like the script to create a temporary file, report on the size of temporary file, and then ask the user if they really want to print the entire temporary file to STDOUT, or do they want to give a filename that will be clobbered by the temporary file's contents.

The following script behaves as desired if I give STDIN as a filename but does not work if I pipe STDIN to the script.

#! /usr/bin/perl
use strict; use warnings; 
my $count = 0;
while(<>)
{
    $count++;
}
print "you counted $count lines. Now do you want to proceed?";
my $answer = <STDIN>;
chomp $answer;
print STDERR "answer=$answer\n";
if ( $answer eq "yes" )
{
    print STDERR "you said $answer so we do something affirmative\n";
}
else
{
    print STDERR "you said $answer which is not \"yes\" so we do NOT proceed\n";
}

for instance

> wc junk
     193    1042   11312 junk
> junk.pl junk
you counted 193 lines. Now do you want to proceed?yes
answer=yes
you said yes so we do something affirmative
> junk.pl junk
you counted 193 lines. Now do you want to proceed?no
answer=no
you said no which is not "yes" so we do NOT proceed
> cat junk | junk.pl
Use of uninitialized value $answer in scalar chomp at /Users/BNW/u/kh/bin/junk.pl line 10.
Use of uninitialized value $answer in concatenation (.) or string at /Users/BNW/u/kh/bin/junk.pl line 11.
answer=
Use of uninitialized value $answer in string eq at /Users/BNW/u/kh/bin/junk.pl line 12.
Use of uninitialized value $answer in concatenation (.) or string at /Users/BNW/u/kh/bin/junk.pl line 18.
you said  which is not "yes" so we do NOT proceed
you counted 193 lines. Now do you want to proceed?>
stdin perl
2 Answers

Sort of. Maybe.

First off, in your first example, it's not true that you "gave STDIN as a filename". STDIN is the terminal throughout. <> is reading from the ARGV handle, not STDIN, so STDIN is available later when you need it.

The problem with the second example example is that the pipe from cat is STDIN. Closing it and reopening it to what it was initially doesn't do anything for you, because it will still be an exhausted pipe.

Many systems, though, have a special device /dev/tty which points to the controlling terminal of whichever process asks for it. On such a system, you could reopen STDIN from /dev/tty after it gives EOF, and you would get the console that the user invoked your program from, instead of whatever file or pipe they initially gave you as STDIN.

user_1529480

Thanks to @hobbs. Note that this works either way: piping the file junk into the script or passing junk as ARGV.

> printf "line 1 \nline 222 \n" > junk
> perl -e 'use strict; use warnings; while(<>) { print; } my $stuff = "/dev/tty"; my $h; open $h, "<", $stuff or die "waah $stuff"; print "give answer:"; my $answer=<$h>; print "answer=$answer\n";' junk
line 1 
line 222 
give answer:This is an answer!
answer=This is an answer!

> cat junk | perl -e 'use strict; use warnings; while(<>) { print; } my $stuff = "/dev/tty"; my $h; open $h, "<", $stuff or die "waah $stuff"; print "give answer:"; my $answer=<$h>; print "answer=$answer\n";' 
line 1 
line 222 
give answer:So, what was it she was saying?? ?? 
answer=So, what was it she was saying?? ?? 

>
user_14613070
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