josh803316 has asked for the wisdom of the Perl Monks concerning the following question:

I'm attempting to use Net::OpenSSH to connect to a remote linux host and execute a tcpdump command in the background while then executing a ping, stopping the tcpdump and printing the result to the screen. I realize that each command runs in it's own session but I thought I could use nohup and & to create a background process that wouldn't exit. I know that Expect can be used for a more interactive process but I was wondering what the best/recommended way to do this would be (without using Expect).
#!/usr/bin/perl

use strict;
use warnings;
use Data::Dumper;
use Net::OpenSSH;

my $username = 'username';
my $password = 'password';
my $params;
my $session_obj;

$session_obj = Net::OpenSSH->new("10.2.200.12", user => $username, pas
+sword => $password, strict_mode => 0,master_opts => [ -o => "StrictHo
+stKeyChecking=no"]);
$session_obj->error and die "ssh failed: " . $session_obj->error;
print Dumper($session_obj);
# Each command runs in its own session
my ($out,$err) = $session_obj->capture2('nohup /usr/sbin/tcpdump -i et
+h1 -s 1500 -w /tmp/tmp.pcap &');
print "THE OUT IS $out\nTHE ERROR IS $err\n";
($out,$err) = $session_obj->capture2('ping -I eth1 -c 3 192.168.107.25
+4');
print "THE OUT IS $out\nTHE ERROR IS $err\n";
($out,$err) = $session_obj->capture2('killall tcpdump');
print "THE OUT IS $out\nTHE ERROR IS $err\n";
($out,$err) = $session_obj->capture2('tcpdump -r /tmp/tmp.pcap');
print "THE OUT IS $out\nTHE ERROR IS $err\n";

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Re: Net::OpenSSH and background process
by josh803316 (Beadle) on Aug 24, 2010 at 15:31 UTC
    If I change the command to this: 
    nohup /usr/sbin/tcpdump -i eth1 -s 1500 -w /tmp/tmp.pcap </dev/null >>
+myLogFile 2>>myErrorFile&

    [download]
    The background tcpdump process works and stays alive. I'm assuming this is more easily done with a combination of open_ex options just not sure what they should be or if my thought process is correct...
[reply]
[d/l]
      nohup only does its redirection magic when stdin/stdout/stderr are connected to a tty, that's why you need to explicitly redirect these streams.

      Besides that, you can use Net::OpenSSH spawn method to launch the command in the background: 

      # this code does not check for errors!

my $pid = $ssh->spawn('/usr/sbin/tcpdump -i eth1 -s 1500 -w /tmp/tmp.p
+cap');
($out,$err) = $ssh->capture2('ping -I eth1 -c 3 192.168.107.254');

$ssh->system('killall tcpdump');
waitpid($pid, 0);

($out,$err) = $ssh->capture2('tcpdump -r /tmp/tmp.pcap');
print "THE OUT IS $out\nTHE ERROR IS $err\n";

      [download]
[reply]
[d/l]
[select]
        Thank you for the info and helpful example. I was able to get it working both ways with both spawn and the nohup as you suggested. I sincerely appreciate the help!!!
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