Exploring Expect

Example—Preventing Bad Commands

The following fragment demonstrates how -reset could be used to prevent a user from entering certain commands. For example, suppose you want to provide an interface to another program that allows two commands that you would rather people not enter. You want to allow any others, just not these two.

Consider the following reasonable-looking but incorrect approach:

set badcmds "badcmd1|badcmd2"         ;# WRONG
interact -re "$badcmds.*\r" {         ;# WRONG
    put "command not allowed\r"       ;# WRONG
}                                     ;# WRONG

But as with the earlier modem monitor script, users can enter command-line editing characters to force the pattern to fail. For example, "ba<backspace>adcmd1" does not match the interact pattern and will be accepted by the spawned process.

One solution is to recognize the program prompt and shift into cooked mode while the user enters a command. Any command not in badcmds will be allowed. This algorithm could also be reversed to allow only a set of good commands.

set badcmds "badcmd1|badcmd2"
interact {
    -o -reset -nobuffer -re $prompt {
        expect_user {
            -re "^$badcmds.*\n" {
                puts "Command not allowed\n"
            }
            -re "(.*)\n" {
                send "$expect_out(1,string)\r"
            }
        }
    }
}

This script can also be written as an expect loop with an interact inside of it—similar to the one above but “inside out”. How you structure the script is not that important. What is important is that you use cooked mode to read the results of potentially edited input. In general, if the spawned process is using cooked ...