Jobs and Concurrency
====================

commands

* ``jobs``
* ``ps``
* ``fg`` bring background process to the foreground
* ``bg`` resume a stopped background process if it is stopped
* ``disown``
* ``kill``


Running Tasks in the background
-------------------------------

Tasks can be run concurrently in the background with the ``&`` operator.

.. code-block:: bash

    # run 3 commands in a background jobs
    tail -f /var/log/dmesg &
    tail -f /var/log/syslog &
    nano /tmp/file &

    # Three jobs in the background

    # list background jobs and their statuses and their PIDs
    jobs -l

    # list the PIDs only
    jobs -p

    # list jobs in the current process and its children
    ps

UNIX Signals
------------

Resources

* man 7 signal
* kill -L

.. csv-table::
    :file: signals.csv

Stopping and Resuming
---------------------

The main signals we are concerned about are:

* ``kill -19`` stop process
* ``kill -18`` continue if stopped
* ``kill -15`` soft termination
* ``kill -9`` hard termination

jobs can be selected by their order in which they were started with ``%1`` being the first job,
``%2`` being the second job and onwards.

.. code-block:: bash

    tail -f /var/log/dmesg &
    tail -f /var/log/syslog &
    nano /tmp/file &

    # stop the second job
    kill -19 %2

    # stop the first job
    kill -19 %1

    # resume the first job
    kill -18 %1

    # when we terminate the second job, jobs 3 onward maintain their order in the job list
    kill -15 %2


Current and Child PID
---------------------

.. code-block:: bash

    # prints the current PID VS a child subshell PID
    echo $BASHPID; ( echo $BASHPID )