Exploring Expect

Opening Ttys

Normally, Expect calls upon tty-aware programs such as tip or cu to interact with ttys. However, using the technique described in the previous section, it is possible to open ttys or other devices directly. For example, if you have a tty port named /dev/ttya, you can open it as follows:

spawn -open [open /dev/ttya w+]

Unfortunately, that is not the whole story. Tty devices must be initialized. This is something that a program such as tip would do for you. Without such a program, you must do it yourself. In the interest of generality, Expect performs no special handling depending upon the type of file it has been handed. That is up to you.

You can initialize a tty using stty with the appropriate arguments. The only hard part is figuring out the arguments. There is no standard, and the parameters can vary depending on your task.

For most tasks, you want to disable special character processing. You also want to disable echo. These are accomplished as follows:

stty raw -echo < /dev/ttya

Flags such as -echo apply to the tty input. Unlike a traditional tty to which you log in, this tty is being used to “go out”. So the tty’s input (in the sense that stty cares) is provided by the remote side. For example, if you use the tty to connect to another serial port, then the output of that serial port is the input of this one.

If the serial port generates parity, you may need to handle that either by disabling it or telling stty to ignore it. Another stty command suffices:

stty istrip ...