Repeat Commands on a Dozen VMs with a Single Loop

04 Mar 2012

Category: utility. Tags: ssh.

When I was playing with a dozen of identical VirtualMachines, I always need to repeat some commands on each of them. This is painful: open VNC (or SSH) of the VM, type some long and boring commands and repeat the same thing on another VM. Then I figure out a way to avoid repeat. Everybody hates REPEAT, right?

First of all, all these VMs must be able to connected via SSH. If you have problem on SSH to VMs, you need to setup a network bridge (refer to my previous post for more detail).

Next, copy your public key to each VM (If you don’t have a public key yet, generate it with ssh-keygen). This step is necessary to login on VMs without typing a password. Assume there are ten VMs and their IP address are 192.168.1.10 to 192.168.1.19. Run the following commands on the host:

for i in `seq 0 9`
do
    ssh-copy-id user@192.168.1.1$i
done

You will be asked for password for each VM only once. And it’s time for a rest, let’s review SSH. I don’t know how many people know that ssh can has a optional command field. At least I didn’t know it just a moment ago.

ssh [user@]hostname [command]

When command field is specified, it is executed on the remote host instead of a login shell. Repeating commands on all VMs will be as simple as write a loop. A typical example is restart httpd on all VMs and display status of httpd:

for i in `seq 0 9`
do
    echo "Processing VM$i"
    ssh user@192.168.1.1$i "rc.d restart httpd; rc.d list httpd"
done
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