Exploring Expect

Back To Eof

In the ping script on page 96, there was no specific handling of the eof. Here is that script again:

spawn ping $host
set timeout 2
expect "alive" {exit 0} timeout {exit 1}

If expect sees an eof, then ping terminates within the timeout but without producing output containing "alive“. How is this possible? After all, a host is either up or incommunicado. In fact, there is a third case. ping also reports if the host does not exist— that is, if there is no computer with such a name. In this case, ping says "unknown host“, closes the connection, and exits. expect sees an eof, but since there is no eof pattern and corresponding action, theexpect command returns. There are no more commands so the script ends.

When the script ends by running out of commands, an implied "exit 0" is executed. This is typical for interpreters, and UNIX commands conventionally return 0 to indicate that a command is successful. But in this case, the script returns 0 when given a non-existent host. This is clearly the wrong behavior. Unfortunately, the right behavior is not as clear. You could return 1 and revise the definition of what that means from “failure due to timeout” to simply “failure”. Or you could choose a different number, say, 2. Either can be justified depending on the use to which you want to put the script. ping returns 1 when the host is unknown so I will follow suit. Here is the revised script to handle the eof:

spawn ping $host
set timeout 2
expect "alive" {exit 0} timeout {exit 1} eof {exit 1}

In some ways this still does not handle the problem perfectly. For example, without looking directly at the source to ping, I do not know if there are other ways it could behave. For now, I am just lumping everything I do not know into an error.

But this may be sufficient. Indeed, one of the reasons for using Expect is that you may not be able to see the source in the first place. So taking a conservative approach of calling everything that is not expected an error, is a practical and common solution.

Timeout and eof are the only types of exception conditions possible. As in the ping example, both exceptions often deserve the same type of handling. For this reason, there is a special pattern called default that represents both conditions. The last line of the ping script could be rewritten to use default as:

expect "alive" {exit 0} default {exit 1}

Using default (or both timeout and eof) covers all possible conditions that an expect command can match. It is a good idea to account for all conditions in every expect command. This may seem like a lot of work, but it can pay off handsomely during debugging. In Chapter 11 (p. 255), I will describe how to use the expect_before and expect_after commands to catch all timeouts and eofs without specifying them on each expect. Those commands can greatly simplify your scripts.