Exploring Expect

The expect_tty Command

If you have redirected the standard input, it is still possible to read from the terminal. This is done by using the expect_tty command. It is called just like expect or expect_user. The following example waits for the string foo to be entered followed by a return.

expect_tty "foo\n"

If this command is embedded in a script and the script is invoked with the input redirected, Expect will wait until the user physically types the string on the keyboard.

expect_tty has its advantages and its disadvantages. An advantage is that by overriding shell redirection, very powerful effects can be produced that are not possible any other way. For instance, the more program, which pages through files, reads from the standard input and the keyboard at the same time. This allows more to read files in a pipeline and still be controlled from the keyboard as it runs. To build a script that behaves like more, use expect_tty to read from the keyboard while using expect_user to read from the standard input.

The disadvantage to using expect_tty is that it cannot be redirected from the shell. This may seem circular. After all, that is the very point of the command. However, there is always a time when you want to force commands to come from a file—for example, if you are testing the more command. Because more behaves this way, it cannot be automated from the shell (although more can be automated from Expect). So expect_tty should not be used capriciously. expect_tty generally produces ...