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 the system until you give the wait command, it is common to speak of waiting for or waiting on a process. Some people also like to use the term reap as in “reaping a process”.

Because wait follows close, it is very common to see people write "close;wait" on a single line. But if the connection is closed implicitly, the wait must appear by itself. Like close, the wait command can also occur implicitly. Unlike close, however, wait implicitly happens in only one case—when an Expect process (i.e., script) exits. On exit, all the spawned processes are waited for.

This means that Expect scripts that only spawn a single process and then exit, need not call wait since it will be done automatically. The example scripts so far have all taken advantage of this. Later on, I will show a script in which it is important to explicitly wait.

One last thing about wait: If you call it before a process has died, your Expect script will wait for the process to die—hence the name. It is possible to avoid the delay by using the -nowait flag.

wait -nowait