Exploring Expect

More On The timed–read Script

In Chapter 3 (p. 77), I defined an Expect script called timed-read. Called from the shell with the maximum number of seconds to wait, the script waits for a string to be entered, unless the given time is exceeded. Here it is again:

set timeout $argv
expect "\n" {
    send [string trimright "$expect_out(buffer)" "\n"]
}

In that earlier example I used "string trimright" to trim off the newline. It is possible for the expect command to do this in the pattern matching process, but not with glob patterns. I will use a regular expression to rewrite the script above without using the string command.

# timed read using a regular expression
set timeout $argv
expect -re "(.*)\n" {
    send $expect_out(1,string)
}

In the expect command, the -re flag declares the following pattern to be a regular expression instead of a glob pattern. As before, the pattern is quoted with double quotes. The pattern itself is:

(.*)\n

This matches any string of characters followed by a "\n“. All the characters except for the \n get stored in expect_out(1,string). Since the pattern is successfully matched, the action is executed. The action is to send all the characters in $expect_out(1,string) to the standard output. Of course, these characters are exactly what was just typed without the terminating "\n“.