SSH via Perl

execute commands on remote machines using ssh Perl HowTo

You can execute commands on remote machines from a Perl script using the Net::SSH::Perl module.

This module allows you to execute a command remotely and receive the STDOUT, STDERR, and exit status of that remote command.

One big advantage of Net::SSH::Perl over other methods is that you can automate the login process, that way you can write fully automated perl scripts, no console interaction is required in order to authenticate in the remote machine.

Example:

#!/usr/bin/perl
 
use Net::SSH::Perl;
 
my $host = "perlhowto.com";
my $user = "user";
my $password = "password";
 
#-- set up a new connection
my $ssh = Net::SSH::Perl->new($host);
#-- authenticate
$ssh->login($user, $pass);
#-- execute the command
my($stdout, $stderr, $exit) = $ssh->cmd("ls -l /home/$user");
 

-- IMPORTANT NOTE!!

Execution of Net::SSH::Perl commands can be quite slow if you don't have the Math::BigInt::GMP module; so be sure that you have that module installed (or install it if you don't) in order to avoid the slowness problem. 

NEWS CONTENTS

• 200102 : NetOpenSSH - search.cpan.org ( NetOpenSSH - search.cpan.org, )
• 200102 : Thread: Perl (SSH to remote host, Issues the command and close the session without waiting it to finish…) ( )

Old News ;-)

NetOpenSSH - search.cpan.org

This package is implemented around the multiplexing feature found in later versions of OpenSSH. That feature allows reuse of a previous SSH connection for running new commands (I believe that OpenSSH 4.1 is the first one to provide all the required functionality).

When a new Net::OpenSSH object is created, the OpenSSH ssh client is run in master mode, establishing a permanent (actually, for the lifetime of the object) connection to the server.

Then, every time a new operation is requested a new ssh process is started in slave mode, effectively reusing the master SSH connection to send the request to the remote side.

Net::OpenSSH Vs Net::SSH::.* modules

Why should you use Net::OpenSSH instead of any of the other Perl SSH clients available?

Well, this is my (biased) opinion:

Net::SSH::Perl is not well maintained nowadays (update: a new maintainer has stepped in so this situation could change!!!), requires a bunch of modules (some of them very difficult to install) to be acceptably efficient and has an API that is limited in some ways.

Net::SSH2 is much better than Net::SSH::Perl, but not completely stable yet. It can be very difficult to install on some specific operative systems and its API is also limited, in the same way as Net::SSH::Perl.

Using Net::SSH::Expect, in general, is a bad idea. Handling interaction with a shell via Expect in a generic way just can not be reliably done.

Net::SSH is just a wrapper around any SSH binary commands available on the machine. It can be very slow as they establish a new SSH connection for every operation performed.

In comparison, Net::OpenSSH is a pure perl module that doesn't have any mandatory dependencies (obviously, besides requiring OpenSSH binaries).

Net::OpenSSH has a very perlish interface. Most operations are performed in a fashion very similar to that of the Perl builtins and common modules (i.e. IPC::Open2).

It is also very fast. The overhead introduced by launching a new ssh process for every operation is not appreciable (at least on my Linux box). The bottleneck is the latency intrinsic to the protocol, so Net::OpenSSH is probably as fast as an SSH client can be.

Being based on OpenSSH is also an advantage: a proved, stable, secure (to paranoic levels), interoperable and well maintained implementation of the SSH protocol is used.

On the other hand, Net::OpenSSH does not work on Windows, not even under Cygwin.

Net::OpenSSH specifically requires the OpenSSH SSH client (AFAIK, the multiplexing feature is not available from any other SSH client). However, note that it will interact with any server software, not just servers running OpenSSH sshd.

For password authentication, IO::Pty has to be installed. Other modules and binaries are also required to implement specific functionality (for instance Net::SFTP::Foreign, Expect or rsync(1)).

SSH to multiple servers and run commands PatchLog

You need to run a list of commands on a list of servers and record the output of each command.

Solution

Create a perl script using Net::SSH::Perl ( a ssh client written as a perl module ). This script will read a list of commands from a file, a list of servers form another file, will connect to each server, execute each command in in the commands file then go to the next server and do the same.

Installation

Download the script: SSH Batch-0.1 (1.5 KB)
install Net::SSH::Perl

sudo perl -MCPAN -e "install Net::SSH::Perl"

Now you can decompress the script and configure it

tar -xzpf ssh_batch-0.1.tar.gz
cd ssh_batch-0.1

put the commands in commands.txt ( one command per line )
put the servers in servers.txt (one per line ) in this format: user@hostname:port,password

Now you can test the script: run ./ssh_batch.pl and look at the output in log.txt

Thread: Perl (SSH to remote host, Issues the command and close the session without waiting it to finish…)

Hi, Script goes to the remote server and runs a shell script "snap.sh" using Net::SSH::Perl. This shell script takes almost 10mins to end, and my perl program waits until it gets output. I want to run the shell script on the remote sever and the program should close the SSH session without waiting for the script to finishes on the remote server. my $ssh = Net::SSH::Perl->new($host, protocol =>2...

Reply 1: Perl (SSH to remote host, Issues the command and close the session without waiting it to finish…)

Andrew White replied 11 months, 1 week ago

Lookup the nohup command. Here is quick post to get you started . For completeness here is what should work in your case... my $cmd="nohup ./bin/snap.sh &";

Reply 2: Perl (SSH to remote host, Issues the command and close the session without waiting it to finish…)

ephemient replied 11 months, 1 week ago

Untested, but can't you just do what ssh -f does? my $ssh = Net::SSH::Perl->new($host, protocol =>2); $ssh->login($username, $password); defined (my $pid = fork) or die "fork: $!"; if ($pid) { close $ssh->sock; undef $ssh; } else { my $cmd="./bin/snap.sh"; my($stdout, $stderr, $exit) = $ssh->cmd($cmd); POSIX::_exit($exit); }

Reply 3: Perl (SSH to remote host, Issues the command and close the session without waiting it to finish…)

Xomo replied 11 months, 1 week ago

Andrew, I tried this. It is running the cmd in the background on the remote server but the session is not closed..it is still waiting. I guess it had lot to do with NET::SSH::Perl module

Reply 4: Perl (SSH to remote host, Issues the command and close the session without waiting it to finish…)

Andrew White replied 11 months, 1 week ago

You'll still need to logout/exit to end the session. I assumed that was simply omitted from the question.

Reply 5: Perl (SSH to remote host, Issues the command and close the session without waiting it to finish…)

Xomo replied 11 months, 1 week ago

Not working same issue, program does not exits.., waiting to get some status from remote server

Reply 6: Perl (SSH to remote host, Issues the command and close the session without waiting it to finish…)

Xomo replied 11 months, 1 week ago

my($stdout, $stderr, $exit) = $ssh->cmd($cmd); At this command the script stops and wait for the remote command to end....

Reply 7: Perl (SSH to remote host, Issues the command and close the session without waiting it to finish…)

ephemient replied 11 months, 1 week ago

@Xomo: What is your platform? If it's Windows, fork emulation is problematic and could cause that.

Reply 8: Perl (SSH to remote host, Issues the command and close the session without waiting it to finish…)

mob replied 11 months, 1 week ago

To run a background job on a remote host, you also need to dissociate from any controlling ttys on the local machine. Try a command like: my $cmd = "./bin/snap.sh < /dev/null > /dev/null 2>&1 &"; I think using nohup is optional.

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Last modified: March 12, 2019