Question

I’ve never used expect before but need a script to do the following for a large list of hosts

• ssh into a machine
• su to root and enter the root password
• sed a file in /etc/passwd to replace some text with some other text, for this example lets just say the original text is TEXT and the text to replace it with is NEWTEXT

Can anyone help?

Answer 1

Understandably, executing a script to process commands as root is a huge security issue, and is generally not allowed.

However, Expect has the ability to run a pseudo terminal, so it can act just like a human typing away at a keyboard.

This is more or less what you asked, but is untested.

#!/bin/sh
# if run from shell, restart with wish \
exec wish "$0" "$@"

package require Expect

proc fix_a_host {host usrname password root_password} {
        # open session to host, wait for a username prompt
        spawn ssh $host
        expect "username:"

        # send username, wait for a password prompt
        send "$usrname\r"
        expect "password:"

        # send password, wait for shell prompt
        send "$password\r"
        expect "%"

        # become root, wait for prompt
        send "su\r"
        expect "#"

        # change TEXT to NEWTEXT in password file
        send "sed 's/TEXT/NEWTEXT'" /etc/passwd
        expect "#"

        # exit root, exit host connection
        send "exit\r"
        expect "%"

        send "exit\r"
        expect eof
}       

fix_a_host {"host1" "someuser" "user_password"  "root_password"}
fix_a_host {"host2" "someuser" "user_password"  "root_password"}

If it were me, I’d change the sed command to something far less destructive, like grep TEXT /etc/passwd until confident it works well. For a remote command which display output you want to see, use

set results $expect_out(buffer)
send "some_command\r"
expect "#"    # use the command prompt to indicate output is complete
puts "command output is got `$buffer`"