Question

I’m trying to write a python script to access several Cisco network devices. Sadly the devices are not set up the same, some have banners up that appear at weird places. These banners might have the patterns that have some of the same stuff a prompt might have.

For example, once I log in I expect a prompt of 'hostname#', so I basically use a pexpect('#') I might have to handle certain other prompts such as 'hostname>' as well so pexpect('>') works. I’ll really have a list of possible prompts, plus a timeout and act accordingly.

This is fine unless a banner shows up in the middle somewhere. For example, after logging in a banner might show up that says ## Welcome ## and be there right before the prompt. I believe if I could tell pexpect to ignore the regular expression #.*\r I’d be fine, but I’m not sure this is possible. Is it possible to have pexpect ignore a string, or is there a better way to go about this?

Answer 1

A solution I used to use back in the BBS days was to look for a prompt, and then immediately after I got one, look for a line break or a space with a timeout of a second or two. If the second wait timed out, that meant I was really waiting at a prompt. Since pexpect uses regular expressions, you can actually check to see if you receive any character within a second after a prompt.

def wait_for_prompt(session, prompt, timeout=1):
    gotprompt = 0
    while not gotprompt:
        session.expect(prompt, timeout=None)
        gotprompt = session.expect([".", pexpect.TIMEOUT], timeout=timeout)

child = pexpect.spawn("ssh ...")
wait_for_prompt(child, "[#>] ?")

I’ve never actually used this library before so this may need some tweaking, but it’s an approach I used successfully in the ’80s. 🙂