Redirect STDOUT to a file while process is waiting for user input - perl

Question

I am trying to redirect the STDOUT and STDERR into a log file, but I also want to print those streams to the console. I am using Perl, and my code looks like this:
use Capture::Tiny ':all';
my $stderr, $stdout;
($stdout, $stderr) = capture {
system($command);
};
print $stdout;
print $stderr;
It works, but if the command waits for a user input, the program doesn't print $stdout to STDOUT until a key is pressed. Is there any way to print $stdout to STDOUT before it needs user input? Line by line approach would be fine.
Thank you in advance!

Answer 1

Well, I'm not familiar with Capture::Tiny so this may not be entirely relevant - generally though, if I'm looking to handle STDIN, STDOUT and/or STDERR then I look towards either open (if it's just one), or IPC::Open2 and [IPC::Open3][1] which open multiple file descriptors attached to a process.
use IPC::Open3;
$pid = open3(\*CHLD_IN, \*CHLD_OUT, \*CHLD_ERR,
'some cmd and args', 'optarg', ...);
use IPC::Open2;
$pid = open2(\*CHLD_OUT, \*CHLD_IN, 'some', 'cmd', 'and', 'args');
Although I would suggest rather than the examples - you can use lexical filehandles:
my($chld_out, $chld_in);
$pid = open2($chld_out, $chld_in, 'some cmd and args');
You can then read and write from your filehandles (bear in mind though - by default a read will be blocking).
You do need to close and then (ideally) waitpid to clear up the process when you're done though.

Answer 2

You need to use Capture::Tiny's tee instead of capture.
The tee function works just like capture, except that output is captured as well as passed on to the original STDOUT and STDERR.
Just replace the function call and your output will end up both in the variables and on the screen.
use Capture::Tiny ':all';
my $stderr, $stdout;
($stdout, $stderr) = tee {
system($command);
};

Answer 3

Simple approach that I could think of:
#! /usr/bin/perl -w
# Using perl one liner as a command here
# which prints a string to STDOUT and STDERR
my $cmd = "perl -e 'print STDOUT \"stdout\n\"; print STDERR \"stderr\n\";'";
my $log = "./test.log";
# using "2>&1" we are redirecting stderr also to stdout
system("$cmd 2>&1 | tee $log");
# Sample run results in both the strings getting printed to console as well as to log file
> perl test.pl
stderr
stdout
> cat test.log
stderr
stdout