You need to use who command, to print who is logged on. It also displays the time of last system boot. Use last command to display system reboot and shutdown date and time.
$ who –b
Output:

system boot Apr 30 15:08

Use last command to display listing of last logged in users and system last reboot time and date:
$ last reboot | less
Or better try:
$ last reboot | head -1
Output:

reboot system boot 2.6.15.4 Sun Apr 30 15:08 – 16:22 (01:13)

last command searches back through the file /var/log/wtmp and displays a list of all users logged in (and out) since that file was created. The pseudo user reboot logs in each time the system is rebooted. Thus last reboot command will show a log of all reboots since the log file was created.

To display last shutdown date and time use following command:
$ last -x|grep shutdown | head -1
Output:

shutdown system down 2.6.15.4 Sun Apr 30 13:31 – 15:08 (01:37)

Where,

* -x: Display the system shutdown entries and run level changes.

For more information read last command man page.

[user1@machine1 ~]$ cat ssh_loop.sh
#!/bin/sh

$HOSTS="machine1, machine2, machine3";

for i in $HOSTS;
do ssh $i uname -a;
done;
[user1@machine1 ~]$

This one-liner iterates through each machine I’ve
listed in the HOSTS environment variable and runs
uname -a. You can, of course, replace uname -a
with any command-line command that you would
want to run on the hosts. For instance, one need I
had was to keep all of my Debian servers up to
date. I created a small shell script on each Debian
host called /usr/local/bin/apt-automate:

[user1@machine ~]$ cat automate.sh
#!/bin/sh
apt-get update && apt-get -u upgrade
[user1@machine ~]$