Exploring Expect

Example—su2

Suppose you accidentally type a command that requires root access, but are not root at the time. If it is just one command, you can type "su root !!“, but more commonly you need to enter several more commands that need root access. su provides no way to pass the old command in to the new root shell and then remain in the new root shell.

The usual solution is to invoke su, type the password, and then retype the command. If you have windows, you can start su and then copy and paste the command from one window to another. Either way, it is a bit of a hassle. The problem is that su provides no mechanism for going back in the history and bringing it to the new environment. If it did, you would not need to retype or copy and paste the old command—su would automatically know it.

Here is a script that address this problem. It starts su, gives it the command supplied as the script argument, and then passes control to the user. For simplicity, the password is taken from the command line. (In Chapter 8 (p. 197), I will show how to provide it in a much more secure way.) This version is called su2.

#!/usr/local/bin/expect --

set timeout −1
log_user 0
spawn su
expect "Password:"         ;# discard su's password prompt
send "[lindex $argv 0]\r"  ;# send password to su
expect "\r\n"
log_user 1
expect "Sorry" exit "# "
send "[lrange $argv 1 end]\r"
interact          ;# let user type more cmds to root shell

The script temporarily suppresses the output from the su command when the password is prompted for and ...