The fork Command
The fork
command creates a new process. The new process is an exact copy of the current Expect process except for one difference. In the new (child) process, the fork
command returns 0. In the original Expect process, fork
returns the process id of child process.
if [fork] { # code to be executed by parent } else { # code to be executed by child }
If you save the results of the fork
command in, say, the variable child_pid
, you will have two processes, identical except for the variable child_pid
. In the parent process, child_pid
will contain the process id of the child. In the child process, child_pid
will contain 0. If the child wants its own process id, it can use the pid
command. (If the child needs the parent’s process id, the child can call the pid
command before the fork
and save the result in a variable. The variable will be accessible after the fork
.)
Figure 17-1.
If your system is low on memory, swap space, etc., the fork
command can fail and no child process will be created. You can use catch
to prevent the failure from propagating. For example, the following fragment causes the fork
to be retried every minute until it succeeds:
while {1} if {[catch fork child_pid] == 0} break sleep 60 }
Forked processes exit via the exit
command, just like the original process. Forked processes are allowed to write to the log files. However, if you do not disable debugging ...
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