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!

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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();
}

Answer 1

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