Perl execute command, capture and display output

Question

I need to execute a command from my Perl script, which is going to take a while (1-2 hours). I want to be able to see the output of the command, so that my script can check everything went OK, but as it takes such a long time, I'd like the user to see the commands output while it runs, too.

What I've tried:

backticks - Can only get output when command is finished
system - Can only get output when command is finished
open - Almost perfect - but the commands output is buffered, meaning users don't see an update for a long time. Internet is full of suggestions to set $| = 1 but apparently this only affects input buffering and doesn't work
Piping to tee - Similar results to open - 'tee' only seems to print later
Re-directing output and Using Proc::Background and File::Tail - Almost perfect again, but can't think of an elegant way to stop the print loop

Would love to have a suggestion!

Edit: I've accepted Barmars answer. I believe it works because Expect.pm uses a pseudo-terminal. Just to others looking at this question in future, this is how I've implemented it:

my $process = Expect->spawn($command, @params) or die "Cannot spawn $command: $!\n";

while ($process->expect(undef))
{
  print $process->before();
}

Setting autoflush to a true value disables buffering for the currently selected file handle. How is that different from what you are asking for? — TLP
– TLP, Commented Sep 3, 2012 at 9:19
TLP, as I said I tried that, and it didn't seem to work. Upon researching it, I read that autoflush only affects input buffering. — Frederik
– Frederik, Commented Sep 3, 2012 at 9:22
The phrase it doesn't work and all its variations is next to useless without elaboration. And no, it is output buffering. There is no input buffering. — TLP
– TLP, Commented Sep 3, 2012 at 9:32
@TLP, I would add more than one up to that comment if I could. — Axeman
– Axeman, Commented Sep 11, 2012 at 13:02

Barmar · Accepted Answer · 2012-09-03 09:14:42Z

2

Using Expect.pm should disable output buffering in the command.

answered Sep 3, 2012 at 9:14

Barmar

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1 Comment

Frederik Over a year ago

Seems to work beautifully and has an elegant implementation. Thanks!

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