set -m +m -x and the element of chance or is it race conditions?

[jidanni]

Gentlemen, I am disturbed by these seemingly irregular results,
# perl -pwle 's/\d{4}/0000/' mm|sort|uniq -c
     88 ++ kill 0000
     44 ++ set +m
     44 ++ set -m
      1 ++ sleep 33
      2 ++ sleep 66
      1 ./rt: line 12:  0000 Terminated              sleep 33
      1 ./rt: line 12:  0000 Terminated              sleep 44
      1 ./rt: line 13:  0000 Terminated              sleep 33
      1 ./rt: line 13:  0000 Terminated              sleep 44
      1 ./rt: line 13:  0000 Terminated              sleep 55
      3 ./rt: line 14:  0000 Terminated              sleep 33
      3 ./rt: line 14:  0000 Terminated              sleep 44
      3 ./rt: line 14:  0000 Terminated              sleep 55
from running my program
# cat rt
set -x
set -m
sleep 33&
kill $!
set +m
sleep 44&
kill $!
sleep 55&
set -m
kill $!
sleep 66&
set +m
kill $!
# for i in `seq 22`; do ./rt; done 2> mm

We see some of the sleeps are reported some of the time, and on what
line something was terminated differs too. Also didn't I run it 22
times? Why do some things show up only a couple times instead of all or
none?

P.S., on the man page

              -m      Monitor mode. Job control is enabled. This option is on
                      by default for interactive shells on systems that
                      support it (see JOB CONTROL above). Background
                      processes run in a separate process group and a
                      line containing their exit status is printed upon
                      their completion.

Perhaps mention if what happens if this is turned on or off after
starting the job but before its completion.