Exploring Expect

Signals In Spawned Processes

Most of this chapter covers signals occurring in the Expect process itself. But signals are also of concern to spawned processes. Unfortunately, there is little Expect can do to control the signal activity of a spawned process. In particular, there is no analog to the expect command for signals.

Signals can, however, be sent. As I mentioned on page 304, the UNIX kill command can be used to send arbitrary signals to a process.

Signals in spawned processes start out with the default behavior—corresponding to SIG_DFL. Processes override this for signals that they expect and care about. However, some unexpected signals may be delivered and the Expect programmer can control this to some extent.

As an example, recall that I mentioned in Chapter 4 (p. 101) that SIGHUP is delivered to a process when the Expect process closes its side of the connection. The default behavior of SIGHUP forces the spawned process to exit. Therefore, if you want the spawned process to continue after closing the connection, you must arrange for the signal to be ignored.

A signal is initially ignored in a spawned process by using spawn with the -ignore flag followed by a signal name. The -ignore flag understands the same style of signal names as the trap command; however, the signal names must be separated, one per flag. For example, the following command creates a sleep process immune to SIGHUP and SIGPIPE.

spawn -ignore SIGHUP -ignore SIGPIPE sleep 1000

Unless the spawned process overrides ...