Checks the status of a list of machines using SSH blank firing.

check_machine_status.sh

#!/bin/bash

function check_user_and_host {
	_ssh_result=$(ssh -o ConnectTimeout=5 -o BatchMode=yes -o StrictHostKeyChecking=no $1@$2 "echo \"\`who | grep -c . \`\"; true" 2>/dev/null)
}

function check_and_output_host {
	check_user_and_host $1 $2 && echo "$2: up, $_ssh_result users logged in" || echo "$2: down"
}

function store_worker_pid {
	_workers[${#_workers[@]}]=$1
}

function kill_all_workers () {
	for pid in ${_workers[*]}; do
		kill $pid >/dev/null 2>&1
	done
}

function usage {
	cat <<EOF
Usage: $0 <user> <host> [<host> ...]

Tests SSH logins by <user> to all <hosts> in parallel, printing
reports as information becomes available and waiting until all
commands exit.  Keeps a list of PID's and kills all of them if SIGINT,
SIGQUIT, SIGTSTP are signalled.

If <host> is up and allows passwordless authentication for <user>,
(highly recommended as otherwise password prompts will be given) runs
\`who | grep -c .\` and prints output before passing.  Useful for
gathering user counts on all servers and checking status at a high
level.

You can (and are encouraged) to use brace expansion in your argument
list, to simplify generating a list of hosts.  There is theoretically
no maximum imposed by this script, instead just by the maximum number
of file descriptors open on your kernel.
EOF
}

case $1 in
	--help|-h)
		usage
		exit
		;;
	*)
		trap kill_all_workers SIGINT SIGQUIT SIGTSTP

		for i in ${@:2}; do
			check_and_output_host $1 $i &
			store_worker_pid $!
		done

		for pid in ${_workers[*]}; do
			wait $pid >/dev/null 2>&1
		done
		;;
esac