jobber examples: stop-start-more

This example shows the interplay of

jobber task_list stop
jobber task_list start
jobber task_list more

which can be used to interrupt processing of the task queue. more can be employed in a batch job chain to decide whether a subsequent job should be submitted.

The stop action can be used to make all jobber processes that are actually running stop after finishing their current tasks. This works for jobber processes that are running in interactive and batch environments. In both cases stop will be issued from a separate terminal session. In the example stop is issued in the same session (due to the sequential nature of the presentation format). However, the example demonstrates the effects of stop: a subsequent work action does nothing although there are 7 more unprocessed tasks, and the .cont file disappears (which is practically irrelevant but is mentioned for the sake of completeness).
After the start action work actions will continue to process the task queue. (And the .cont file exists again.)
The more action was implemented in order to simplify scripting, in particular for batch job chains where the typical usage is expected to be:
```
   jobber task_list more && submit same_jobber_job_again
```
From the command line above it is clear that the exit status is the essential outcome. In the example the exit status is translated into clear text with an if-else statement. Note that the exit status of more is non-zero if all tasks are completed or processing was interrupted by the stop action (and in either case no subsequent job would be submitted).

shell$ cat task.list
sleep 1; echo '### this is task 1 ###'
sleep 1; echo '### this is task 2 ###'
sleep 1; echo '### this is task 3 ###'
sleep 1; echo '### this is task 4 ###'
sleep 1; echo '### this is task 5 ###'
sleep 1; echo '### this is task 6 ###'
sleep 1; echo '### this is task 7 ###'
sleep 1; echo '### this is task 8 ###'
sleep 1; echo '### this is task 9 ###'
sleep 1; echo '### this is task 10 ###'
shell$ jobber task.list init
[2022-04-01T13:53:47] appended 10 tasks
shell$ jobber task.list 3
[2022-04-01T13:53:47] 1.1 task.worker: sleep 1; echo '### this is task 1 ###'
### this is task 1 ###
[2022-04-01T13:53:48] 1.1 exit status: 0
[2022-04-01T13:53:48] 2.2 task.worker: sleep 1; echo '### this is task 2 ###'
### this is task 2 ###
[2022-04-01T13:53:49] 2.2 exit status: 0
[2022-04-01T13:53:49] 3.3 task.worker: sleep 1; echo '### this is task 3 ###'
### this is task 3 ###
[2022-04-01T13:53:50] 3.3 exit status: 0
shell$ ls -l task.list*
-rw------- 1 user group 391 Apr  1 13:53 task.list
-rw------- 1 user group   0 Apr  1 13:53 task.list.cont
-rw------- 1 user group 596 Apr  1 13:53 task.list.queue
shell$ jobber task.list stop
shell$ ls -l task.list*
-rw------- 1 user group 391 Apr  1 13:53 task.list
-rw------- 1 user group 596 Apr  1 13:53 task.list.queue
if jobber task.list more; then echo 'there is more work'; else echo 'there is nothing to do at the moment'; fi
there is nothing to do at the moment
shell$ jobber task.list 3
shell$ jobber task.list start
shell$ ls -l task.list*
-rw------- 1 user group 391 Apr  1 13:53 task.list
-rw------- 1 user group   0 Apr  1 13:53 task.list.cont
-rw------- 1 user group 596 Apr  1 13:53 task.list.queue
if jobber task.list more; then echo 'there is more work'; else echo 'there is nothing to do at the moment'; fi
there is more work
shell$ jobber task.list 3
[2022-04-01T13:53:50] 4.4 task.worker: sleep 1; echo '### this is task 4 ###'
### this is task 4 ###
[2022-04-01T13:53:51] 4.4 exit status: 0
[2022-04-01T13:53:51] 5.5 task.worker: sleep 1; echo '### this is task 5 ###'
### this is task 5 ###
[2022-04-01T13:53:52] 5.5 exit status: 0
[2022-04-01T13:53:52] 6.6 task.worker: sleep 1; echo '### this is task 6 ###'
### this is task 6 ###
[2022-04-01T13:53:53] 6.6 exit status: 0
shell$