Automating tasks with pexpect

I started to use pexpect for some of the automation needs I have, especially for tasks that involve logging into a remote device and running commands there. I found the module extremely easy to use, and the documentation on the module's home page is very good. Basically, if you follow the recipe shown there for logging into an FTP server, you're set.

A couple of caveats I discovered so far:

• make sure you specify correctly the text you expect back; even an extra space can be costly, and make your script wait forever; you can add '.*' to the beginning or to the end of the text you're expecting to make sure you're catching unexpected characters
• if you want to print the output from the other side of the connection, use child.before (where child is the process spawned by pexpect)

Here's a complete script for logging into a load balancer and showing information about a load balanced server and its real servers:

#!/usr/bin/env python

import pexpect

def show_virtual(child, virtual):
    child.sendline ('show server virtual %s' % virtual)
    child.expect('SSH@MyLoadBalancer>')
    print child.before

def show_real(child, real):
    child.sendline ('show server real %s' % real)
    child.expect('SSH@MyLoadBalancer>')
    print child.before

virtuals = ['www.mysite.com']
reals = ['web01', 'web02']

child = pexpect.spawn ('ssh myadmin@myloadbalancer')
child.expect ('.* password:')
child.sendline ('mypassword')
child.expect ('SSH@MyLoadBalancer>')

for virtual in virtuals:
    show_virtual(child, virtual)

for real in reals:
    show_real(child, real)

child.sendline ('exit')