This is Part I in a series - 'Execute Commands on Multiple Linux or UNIX Servers Simultaneously'. The full series is Part I, Part II, and Part III. Some time it is necessary to execute commands on Multiple Linux or UNIX Servers, for example you would like to find out who is logged on and what they are doing on three Linux or UNIX boxes or better find out system utilization, disk space and much more. With the help of ssh you can easily setup such nice system.

Our Sample Setup

Admin Linux workstation -> Server # 1 with ssh
adm.my.com              -> server1.my.com

SSH client is a program for logging into a remote machine and for executing commands on a remote machine. If command is specified, command is executed on the remote host instead of a login shell.

$ ssh  user@server1.my.com  w

Above command will gather up all logged in users information. However if you put this command in script to gather information from three server as follows it will prompt for a password:

ssh user@server1.my.com w
ssh user@server2.my.com w
ssh user@server3.my.com w

To get rid of password you can setup ssh key based login. Once ssh keys are in place you can simply create a script as follows:

#!/bin/bash
# Linux/UNIX box with ssh key based login
SERVERS="192.168.1.1 192.168.1.2 192.168.1.3"
# SSH User name
USR="jadmin"
 
# Email
SUBJECT="Server user login report"
EMAIL="admin@somewhere.com"
EMAILMESSAGE="/tmp/emailmessage.txt"
 
# create new file
>$EMAILMESSAGE
 
# connect each host and pull up user listing
for host in $SERVERS
do
echo "--------------------------------" >>$EMAILMESSAGE
echo "* HOST: $host " >>$EMAILMESSAGE
echo "--------------------------------" >>$EMAILMESSAGE
ssh $USR@$host w >> $EMAILMESSAGE
done
 
# send an email using /bin/mail
/bin/mail -s "$SUBJECT" "$EMAIL" < $EMAILMESSAGE

You need to setup your hostname and email id and then execute shell script. This is very simple yet powerful method to execute commands simultaneously on multiple Linux/UNIX servers. If you are really interested to see application output then visit here, it was produced by this script (see our forum for more). This is just small script but you are only limited by your own imagination. Few more advanced tools do exist I will cover them some time later.

• Continue reading second part: Run distributed commands on Linux/BSD/UNIX systems using tentakel tool.