Exploring Expect

The wait Command

After closing the connection, a spawned process can finish up and exit. Processes exit similarly to the way Expect scripts do, with a number (for example, "exit 0“). The operating system conveniently saves this number and some other information about how the process died. This information is very useful for non-interactive commands but useless for interactive commands. Consequently, it is of little value to Expect. Nonetheless, Expect must deal with it.

Expect must retrieve this information—even if only to discard it. The act of retrieving the information frees various valuable resources (process slots) within the computer. Until the information is retrieved, the operating system maintains the information indefinitely. This can be seen from the output of ps. Assuming a spawned process has died and the connection has been closed, ps shows something like this:

PID   TT  STAT  TIME  COMMAND
4425  ?   Z     0:00  <defunct>

The Z stands for zombie—someone’s attempt to humorously describe a process that is dead but still haunts the system in an almost useless way. Even the process name and arguments have been discarded—no matter what they were originally, they show up here as <defunct>.

To get rid of this zombie, use the wait command. It is called simply as:

wait

The wait command returns a list of elements including the spawn id and process id. These elements are further described in Chapter 14 (p. 309). For now, ignore the return value of wait.

Because a process will not disappear from ...