Remote script execution using Net::SSH::Expect in perl

Question

I am trying to execute remote script using following command by connecting to one of the server using Net::SSH::Expect perl module.

$ssh->exec("python test.py");

Script is shown below:

#!/usr/bin/perl 

use Net::SSH::Expect;

my $ssh = Net::SSH::Expect->new (
    host => 'hostip', #deb-2-scripting ip
    user => 'user',
    password => 'password',
    raw_pty => 1
);

..
..
$result = $ssh->exec("python test.py");
..
..
$ssh->close();

Actually this remote script (test.py) is taking longer than expected to execute (Because its having so many checks inside).

Is there any way that I can just fire this script (test.py) and continue with my perl script execution (I dont want to wait till test.py execution completion). OR
How can I wait till the test.py execution completion without giving timeout. Since I could see by giving $ssh->exec("python test.py", 10); this waits for 10 seconds so that test.py execution gets succeeded.

Why not just run the command in the background with the shell: $ssh->exec("python test.py &"); ? See Run a Unix process in the background — Håkon Hægland
– Håkon Hægland, Commented Sep 3, 2019 at 18:38
Good idea @hakon. So in this case test.py will execute from background and the Perl script will not bother about the test.py executed successfully or not and continues it's flow. Correct me if I'm wrong. — vkk05
– vkk05, Commented Sep 3, 2019 at 18:52

Håkon Hægland · Accepted Answer · 2019-09-05 06:38:52Z

0

Since the command handed to $ssh->exec("command") is run by the shell on the server, you can run the Python command in the background like you would do for a normal shell command, by including an & (an ampersand) at the end of the command:

$ssh->exec("python test.py &");

This will cause exec() to return immediately, while the python command is running as a background process on the server.

Edit:

It seems like $ssh->exec($cmd, $timeout) is not able to tell exactly when the $cmd finished. I wonder if this is a bug? It always waits $timeout seconds, even if the command finishes before that. A workaround is use $ssh->send() combined with $ssh->waitfor() like this:

$str = '__DONE__';
$ssh->send("python test.py; echo $str");
my $result = $ssh->waitfor($str, undef);  #<-- undef means wait for ever or 
                                          # until $str is matched.

edited Sep 5, 2019 at 6:38

answered Sep 3, 2019 at 19:03

Håkon Hægland

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11 Comments

vkk05 Over a year ago

So in order to see if test.py is executed successfully or not what should I do (In my question (2)) ? As of now I am giving timeout inside exec command. Is there any other solution for this?

Håkon Hægland Over a year ago

I think if you set timeout to undef it will wait forever (i.e. until the command finishes). You could use this to fork a new process that waits for the python script, while the parent process continues working

vkk05 Over a year ago

In this case yes, its waiting forever and never comes back to perl script.

Håkon Hægland Over a year ago

I thought it could be because the python script had turned on output buffering, but I ran small test now with python -u, and it still does not work.. strange. I will have to look closer into this issue

vkk05 Over a year ago

Thanks a lot @Hakon. But I don't wish to give timeout inside exec command.

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