Exploring Expect

Reconnecting

Unfortunately, there is no general way to reconnect a controlling terminal to a disconnected process. But with a little work, it is possible to emulate this behavior. This is useful for all sorts of reasons. For example, a script may discover that it needs (yet more) passwords. Or a backup program may need to get someone’s attention to change a tape. Or you may want to interactively debug a script.

To restate: You have two processes (your shell and a disconnected Expect processes) that need to communicate. However they are unrelated. The simplest way to have unrelated processes communicate is through fifos. It is necessary to use a pair since communication must flow in both directions. Both processes thus have to open the fifos.

The disconnected script executes the following code to open the fifos. infifo contains the name of the fifo that will contain the user’s input (i.e., keystrokes). outfifo contains the name of the fifo that will be used to send data back to the user. This form of open is described further in Chapter 13 (p. 287).

spawn -open [open "|cat $catflags < $infifo" "r"]
set userin $spawn_id

spawn -open [open $outfifo w]
set userout $spawn_id

The two opens hang until the other ends are opened (by the user process). Once both fifos are opened, an interact command passes all input to the spawned process and the output is returned.

interact -u $spawn_id -input $userin -output $userout

The -u flag declares the spawned process as one side of the interact and the ...