in reply to How to know whether user terminates system("$cmd|more") with q or not?

But how to know whether user terminates system("$cmd|more") with q or not in the first place?

It probably will get more complicated than you want. You want to intercept the q keypress.

Here is a simple example, (or you can move up to an eventloop system, or watch STDIN in the separate thread, and let it watch for q). I 'm not sure how to pass the q to killing your command , unless you use a piped open or some other IPC module that returns a pid. Maybe someone knows a clever trick for that using return values from system. 

#!/usr/bin/perl
use warnings;
use strict;
use Term::ReadKey;

#passing ReadKey() an argument of -1 to indicate not to block:
ReadMode('cbreak');

while(1){
my $char;
if (defined ($char = ReadKey(0)) ) {
        print "$char->", ord($char),"\n";    # input was waiting and i
+t was $char
        if(ord($char) == 113){ print "Got a q\n"; print "Do your thing
+ here\n"  }
        if(ord($char) == 27){ print "Got an Escape\n"; exit;  }
           
} else {
          # no input was waiting
        }
}

ReadMode('normal');                  # restore normal tty settings

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Old Perl Programmer Haiku ................... flash japh
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Re^2: How to know whether user terminates system("$cmd|more") with q or not?
by PerlOnTheWay (Monk) on Sep 13, 2011 at 12:06 UTC

    You mentioned

    watch STDIN in the separate thread, and let it watch for q

    Will it work theoretically?

    Say, will the thread be able to capture q if it's already consumed by more?

[reply]
[d/l]
      Will it work theoretically?Say, will the thread be able to capture q if it's already consumed by more?

      Theoretically yes,I think your only way to do it that way is to somehow dup stdin. See 

      Found in /usr/lib/perl5/5.14.1/pod/perlfaq5.pod
   How do I dup() a filehandle in Perl?
   If you check "open" in perlfunc, you'll see that several of the way
+s to
       call open() should do the trick. For example:

               open my $log, '>>', '/foo/logfile';
               open STDERR, '>&LOG';

       Or even with a literal numeric descriptor:

               my $fd = $ENV{MHCONTEXTFD};
               open $mhcontext, "<&=$fd";  # like fdopen(3S)

       Note that "<&STDIN" makes a copy, but "<&=STDIN" makes an alias
+. That
       means if you close an aliased handle, all aliases become inacce
+ssible.
       This is not true with a copied one.

       Error checking, as always, has been left as an exercise for the
+ reader.

      [download]
      Then your stdin watcher thread and main thread might be able to both get the q . As with error checking, seeing if it works is left as an exercise to the OP. :-). Googling for "perl duping stdin" should give you some already written code.

      But I think you are trying to do this the hard way, you probably would be better off putting your code into an eventloop of some sort, and using a piped open to run your $cmd.

      I'm not really a human, but I play one on earth.
      Old Perl Programmer Haiku ................... flash japh
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[d/l]

        What I want to run is not $cmd,but $cmd|more

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