Exploring Expect

Partial Automation

Expect’s interact command turns control of a process over to you so that you can type directly to the process instead of through send commands.

Consider fsck, the UNIX program I mentioned earlier, which checks file system consistency. fsck provides almost no way of answering questions in advance. About all you can say is “answer everything yes” or “answer everything no”.

The following fragment shows how a script can automatically answer some questions differently than others. The script begins by spawning fsck and then in a loop answering yes to one type of question and no to another. The \\ prevents the next character from being interpreted as a wildcard. In this example, the asterisk is a wild card but the question mark is not and matches a literal question mark.

while 1 {
    expect {
        eof                     {break}
        "UNREF FILE*CLEAR\\?"   {send "y\r"}
        "BAD INODE*FIX\\?"      {send "n\r"}
        "\\? "                  {interact +}
    }
}

The last question mark is a catch-all. If the script sees a question it does not understand, it executes the interact command, which passes control back to you. Your keystrokes go directly to fsck. When done, you can exit or return control to the script, here triggered by pressing the plus key. If you return control to the script, automated processing continues where it left off.

Without Expect, fsck can be run non-interactively only with very reduced functionality. It is barely programmable and yet it is the most critical of system administration tools. Many other tools have similarly deficient user interfaces. In fact, the large number of these is precisely what inspired the original development of Expect.

The interact command can be used to partially automate any program. Another popular use is for writing scripts that telnet through a number of hosts or front-ends, automatically handling protocols as encountered. When such a script finally reaches a point that you would like to take over, you can do so. For example, you could browse through remote library catalogs this way. Using Expect, scripts can make a number of different library systems seem like they are all connected rather than different and disconnected.

The interact command also provides arbitrarily complex macros. If you find yourself repeatedly typing out long names, you can create a short character sequence to type instead. A simple example is the following interact command, which sends the (meaningless but long and hard to type) string "set def qwk/term=vt100 yhoriz=200" when (the short and easy to type) "y2" is entered. This ability to abbreviate is useful when dealing with interactive programs that require you to enter the same gobbledegook over and over again.

interact {
    "y2" {send "set def qwk/term=vt100 yhoriz=200"}
    "~~d" {send [exec date]}
}

This fragment also sends the current date if you press "~~d“. Arbitrary actions can be invoked, including other expect and interact commands. This example uses the exec command to run the UNIX date command.

Macros can be used in the other direction, too. If the program you are dealing with prints things out that you do not want to see or want to see but in a different way, you can change the appearance entirely.