Exploring Expect

Sending Breaks

A break is a special condition of a serial communications line. It does not correspond to a particular ASCII character sequence, so it requires special handling in order to produce it using send. A break condition is generated by calling send with the -break flag.

send -break

The spawned program to which you are sending must understand and expect the break. Most programs do not understand a break as input even if they can generate it on output. For example, the tip program translates the input character sequence \r~# into an output break condition. tip itself does not accept the break condition from the user.

Historically, keyboards contained special keys to generate break conditions. However, such keyboards are rare today. Hence, most modern programs are like tip, accepting some combination of keystrokes to signify a break.

The only situation in which you are likely to generate a break directly by using "send -break" is when the spawned process is not an actual process but is instead a serial device. I have not yet described how to make such a connection, but I will in Chapter 13 (p. 286).

A break condition cannot be detected using the expect command. However, breaks can be turned into signals. This is similar to how a ^C in cooked mode is interpreted as an interrupt. I will discuss signals and how to detect them in Chapter 14 (p. 303). See your local stty documentation for more information on the break condition.