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 ...