Expect script explainer

The dialogue

Command by command

1. 1#!/usr/bin/expect

   Runs under /usr/bin/expect: the first line names the interpreter; to Tcl it is a comment.

2. 2spawn telnet 10.0.0.1

   Starts telnet with 10.0.0.1 as the current process. From here, expect reads its output and send types at it.

3. 3expect "login:"

   Waits for the program's output to match: one pattern.

   • login:
4. 4send "admin\r"

   Types admin\r at the program, ending with a return.

5. 5expect "Password:"

   Waits for the program's output to match: one pattern.

   • Password:
6. 6send "Cisco123\r"

   Types Cisco123\r at the program, ending with a return.

7. 7expect ">"

   Waits for the program's output to match: one pattern.

   • >
8. 8send "terminal length 0\r"

   Types terminal length 0\r at the program, ending with a return.

9. 9expect ">"

   Waits for the program's output to match: one pattern.

   • >
10. 10send "show running-config\r"

    Types show running-config\r at the program, ending with a return.

The script, lines marked

Findings

1. noteE2line 3Timeout
   expect "login:"

   No timeout was set before this expect, so the default of 10 seconds applies (expect(1)). Slow logins and long commands need more; say what you mean with set timeout.

2. noteE7line 3No timeout handler
   expect "login:"

   4 expects wait on a pattern with no timeout handler. When the pattern does not arrive in time, expect(1) executes an implicit null action: the expect returns silently and the script carries on as if it had matched, typing commands at a program that is not where the script thinks it is. Add a timeout pattern that reports and exits.

3. errorE1line 6Secret in clear text
   send "Cisco123\r"

   A secret typed in clear text: right after a password prompt the script sends ••••••••, written in the file. Anyone who can read the script can read the password. Take it from the command line (lindex $argv), from the environment ($env(NAME)), or ask for it with stty -echo and expect_user.

4. noteE3line 7Pattern that matches too soon
   expect ">"

   A one-character pattern, >. Patterns are unanchored and match anywhere in the output, so a banner or an earlier line that contains this character ends the wait early. Match more of the prompt, and anchor a regexp with $ (expect(1) encourages it when you can describe the end of the string).

5. noteE3line 9Pattern that matches too soon
   expect ">"

   A one-character pattern, >. Patterns are unanchored and match anywhere in the output, so a banner or an earlier line that contains this character ends the wait early. Match more of the prompt, and anchor a regexp with $ (expect(1) encourages it when you can describe the end of the string).

6. warningE4line 10Ends on a send
   send "show running-config\r"

   The script's last act is a send. When the script ends, Expect closes the connections to its spawned processes (expect(1)), so whatever the program prints in answer is never read, and the program may not have finished. End with expect eof, an expect for the final prompt, or interact.