Perl script to check if possible to ssh to remote server

Question

I have a perl script in which i want to first check if it possible to ssh into the server before i actually do ssh. So if it possible to ssh, then go ahead and ssh into that server, otherwise handle this exception appropriately.

So i have this script that goes through a textfile with a list of databases, and tries to ssh into each server to perform a bash command that checks if the server is performing MySQL SST or not:

    #!/usr/bin/perl -w

use strict;
use warnings;

use 5.010;
use DBI;
use Encode;
use IO::File;
use JSON;
use utf8;

BEGIN {
    binmode STDERR, ':utf8';
    binmode STDIN, ':utf8';
    binmode STDOUT, ':utf8';
}

my $time = localtime();
my $file = '/opt/db-servers/db.txt';
open my $info, $file or die "Could not open $file: $!";

while( my $hostname = <$info>)  {
    chomp( $hostname );
    my $xtrabk_check = `ssh $hostname ps -ef |grep mysql | grep wsrep_sst_xtrabackup-v2`;
    my $role_check = `ssh $hostname ps -ef |grep mysql | grep donor`;
    my $error_log = `ssh $hostname ps -ef |grep mysql`;
        if ( $xtrabk_check ne "" ){
            if ( $role_check ne "" ){
                my $cmd ="curl -vs -o /dev/null -X POST --data-urlencode 'payload={\"channel\": \"#db-wsrep-status-log\", \"username\": \"db-wsrep-status-log\", \"text\": \"$time: $hostname: --role Donor \n```$error_log```\", \"icon_emoji\": \":scorpion:\"}' 2>&1 /dev/null https://hooks.slack.com/services/GVadasdd/B6LSMF5GV/BIApnzoIldfdsrw343wf";
                system($cmd);
            }
            else {
                my $cmd ="curl -vs -o /dev/null -X POST --data-urlencode 'payload={\"channel\": \"#db-wsrep-status-log\", \"username\": \"db-wsrep-status-log\", \"text\": \"$time: $hostname: State transfer in progress, setting sleep higher mysqld \n```$error_log```\", \"icon_emoji\": \":scorpion:\"}' 2>&1 https://hooks.slack.com/services/GVadasdd/B6LSMF5GV/BIApnzoIldfdsrw343wf";
                system($cmd);
            }
       }
}
close $info;

So probably before performing the three lines below, i want to be able to know if that server is down (hence cannot ssh into it) or not (then ssh into it with the commands below):

my $xtrabk_check = `ssh $hostname ps -ef |grep mysql | grep wsrep_sst_xtrabackup-v2`;
    my $role_check = `ssh $hostname ps -ef |grep mysql | grep donor`;
    my $error_log = `ssh $hostname ps -ef |grep mysql`;

How can i go about achieving this? By the way, the slack URL have been altered so security worries there.

I have added suggested code below, but still not seem to be working:

    ...
    while( my $hostname = <$info>)  {
    chomp( $hostname );
    my $ssh_ping = `$ssh_cmd\'@\'$hostname true`;
    if ( `$ssh_ping true`){
    my $xtrabk_check = `ssh $hostname ps -ef |grep mysql | grep wsrep_sst_xtrabackup-v2`;
    my $role_check = `ssh $hostname ps -ef |grep mysql | grep donor`;
    my $error_log = `ssh $hostname ps -ef |grep mysql`;
        if ( $xtrabk_check ne "" ){
            if ( $role_check ne "" ){
                my $cmd ="curl -vs -o /dev/null -X POST --data-urlencode 'payload={\"channel\": \"#db-wsrep-status-log\", \"username\": \"db-wsrep-status-log\", \"text\": \"$time: $hostname: --role Donor \n```$error_log```\", \"icon_emoji\": \":scorpion:\"}' 2>&1 /dev/null https://hooks.slack.com/services/GVadasdd/B6LSMF5GV/BIApnzoIldfdsrw343wf";
                system($cmd);
            }
            else {
                my $cmd ="curl -vs -o /dev/null -X POST --data-urlencode 'payload={\"channel\": \"#db-wsrep-status-log\", \"username\": \"db-wsrep-status-log\", \"text\": \"$time: $hostname: State transfer in progress, setting sleep higher mysqld \n```$error_log```\", \"icon_emoji\": \":scorpion:\"}' 2>&1 https://hooks.slack.com/services/GVadasdd/B6LSMF5GV/BIApnzoIldfdsrw343wf";
                system($cmd);
            } 
       }
    } #end $ssh_ping if
} #end while

Answer 1

You can do something like this :

`ssh $hostname true`;

if ($? == 0) {
    ...;
} else {
    die "ssh failed";
}