Based on your description, I would say that the 'something inside' is either redirecting stderr to stdout, or it writes the prompt on stdout to begin with.

Standard shells write the prompt to stderr, which you can prove for yourself by running:

You will see a prompt, plus command line input, but no command generates any visible output, because it goes into the file.

You will need to determine what is generating the prompt, and see if you can fix it to write to stderr.

You may be able to avoid input prompts as suggested above, but in situations where you can't, one option is to print the last few lines after no output is detected for awhile. So output is suppressed unless the installer is sitting at a prompt for a bit. Usually the extra few seconds don't matter if the user is waiting for a long compile anyway.

The script below will act like a null sink until output quieces, then it will print the most recent 5 output lines. Pipe the output of tee in your current command line into:

my $timeout = 10;   # Seconds to wait, before printing
select *STDOUT; $| = 1; # Set standard output to auto flush
while (1) {
    my $buf = my $in = '';
    eval {
        local $SIG{ALRM} = sub { print $buf,$in; die };
        while (1) {
            alarm $timeout; sysread *STDIN, $in, 500 or exit; alarm 0;
            ($buf) = ($buf.$in) =~ m/((.*?\n){0,4}.*?\n?)$/; $in = '';
        }
    }
}
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Um, why?

Oh I see, Installing wxPerl 0.9923 with wxWidgets 3.0.1 on Unbuntu 14.04LTS 64bit

cpan will prompt, it usually doesn't prompt on STDERR (i think)

So, tell cpan and family not to prompt you

apt-get probably also has options not to prompt

Or better still, update wxPerl-Installer.sh so it duplicates/tees STDOUT/STDERR to a logfile so the user doesn't have to type extra stuff

My copy/paste from Re: One liner to config cpan to install all dependencies automatically along with temp modules?

?node_id=3989;HIT=cpan%20prompt;re=N

How to install module with CPANPLUS without prompting?

Getting CPAN.pm to accept Makefile.PL prompts

How to skip the prompt fot configuring CPAN.

export PERL_MM_USE_DEFAULT=1
export PERL5LIB=/home/user/stuff/lib/perl5
export PERL_MB_OPT=--install_base /home/user/stuff
export PERL_MM_OPT=INSTALL_BASE=/home/user/stuff
cpan -fi Module Module Module Module Module Module
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App::cpanminus/cpanm