Exploring Expect

The disconnect Command

The disconnect command disconnects the Expect process from its controlling terminal. To prevent the terminal from ending up not talking to anything, you must fork before calling disconnect. After forking, the terminal is shared by both the original Expect process and the new child process. After disconnecting, the child process can go on its merry way in the background. Meanwhile, the original Expect process can exit, gracefully returning the terminal to the invoking shell.

This seemingly artificial and arcane dance is the UNIX way of doing things. Thankfully, it is much easier to write using Expect than to describe using English. Here is how it looks when rendered in an Expect script:

if {[fork] != 0} exit
disconnect
# remainder of script is executed in background

A few technical notes are in order. The disconnected process is given its own process group (if possible). Any unredirected standard I/O descriptors (e.g., standard input, standard output, standard error) are redirected to /dev/null. The variable tty_spawn_id is unset.

The ability to disconnect is extremely useful. If a script will need a password later, the script can prompt for the password immediately and then wait in the background. This avoids tying up the terminal, and also avoids storing the password in a script or passing it as an argument. Here is how this idea is implemented:

stty -echo send_user "password? " expect_user -re "(.*)\n" send_user "\n" set password $expect_out(1,string) # got ...