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Ruby Programming
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Independent Children

Sometimes we don't need to be quite so hands-on: we'd like to give the subprocess its assignment and then go on about our business. Some time later, we'll check in with it to see if it has finished. For instance, we might want to kick off a long-running external sort.

exec("sort testfile > output.txt") if fork == nil
# The sort is now running in a child process
# carry on processing in the main program

# then wait for the sort to finish Process.wait

The call to Kernel::fork returns a process id in the parent, and nil in the child, so the child process will perform the Kernel::exec call and run sort. Sometime later, we issue a Process::wait call, which waits for the sort to complete (and returns its process id).

If you'd rather be notified when a child exits (instead of just waiting around), you can set up a signal handler using Kernel::trap (described on page 427). Here we set up a trap on SIGCLD, which is the signal sent on ``death of child process.''

trap("CLD") {
  pid = Process.wait
  puts "Child pid #{pid}: terminated"
  exit
}

exec("sort testfile > output.txt") if fork == nil

# do other stuff...

produces:
Child pid 31842: terminated
Ruby Programming
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