#!/usr/bin/expect
log_file switches.log

set command "show unit"
set prompt "#"
set password "password"
set switches [list 8.8.8.8 8.8.4.4]

package require Expect

foreach ip $switches {
  puts $ip

  spawn telnet $ip
  set timeout 3

  expect Username:
  send admin\r

  expect Password:
  send $password\r

  expect -exact $prompt
  send "$command\r"

  expect -exact $prompt
  send exit\r
}

# then we can extract all SN with: grep -o '\bRZ\w*' switches.log