Shell Tricks
============

How to Execute Two Processes in Parallel
----------------------------------------

We want to :command:`ping` two machines parallel.  *Kapow!* can get IP addresses
from query params:

.. code-block:: console
   :linenos:

   $ cat parallel.pow
   kapow route add '/parallel/{ip1}/{ip2}' - <<-'EOF'
   	ping -c 1 -- "$(kapow get /request/matches/ip1)" | kapow set /response/body &
   	ping -c 1 -- "$(kapow get /request/matches/ip2)" | kapow set /response/body &
   	wait
   EOF

Calling with :program:`curl`:

.. code-block:: console
   :linenos:

    $ curl -v http://localhost:8080/parallel/10.0.0.1/10.10.10.1

Script debugging
----------------

Bash provides the ``set -x`` builtin command that "After expanding each simple command,
for command, case command, select command, or arithmetic for command, display the
expanded value of PS4, followed by the command and its expanded arguments or associated
word list". This feature can be used to help debugging the `.pow` scripts and, together
the ``--debug`` option in the server sub-command, the scripts executed in user requests.