Support running of an app.sh script.

[GrahamDumpleton]

First raised as comment against separate issue:

• Cannot deploy canonical WSGI hello world application. #92 (comment)

but breaking out as independent issue.

The ability to provide an app.sh file makes things easier when wishing to run an alternate WSGI server such as uWSGI or Waitress where there may not be a way to start it up by importing a Python module and executing some API.

Right now you would have to do the following.

1 - Create a wsgi.py file with your WSGI application.

2 - Create an app.py file containing:

import os

os.execl('/opt/app-root/src/app.sh', 'uwsgi')

3 - Create an app.sh file containing:

#!/bin/bash

exec uwsgi \
    --http-socket :8080 \
    --die-on-term \
    --master \
    --single-interpreter \
    --enable-threads \
    --threads=5 \
    --thunder-lock \
    --module wsgi

The reason for using the app.sh in this case is so that PATH searching for uwsgi program can be relied on. If you try and launch uwsgi from app.py, because os.exec calls require fill path, you have to either hardwire the path, the location of which would change if a switch is made to a Python virtual environment rather than per user site-packages, or you have to add a Python function to search PATH yourself.

Far simpler to add support for app.sh. So something like the following, which would go before existing check and execution of app.py.

APP_SCRIPT="${APP_SCRIPT:-app.sh}"
if [[ -x "$APP_SCRIPT" ]]; then
  echo "---> Running application from script ($APP_SCRIPT) ..."
  exec "$APP_SCRIPT"
fi

In general, easier to use shell script rather than Python code to do special stuff prior to executing command line based server.

[bparees]

@GrahamDumpleton you want to submit a PR updating the run script(s)?

[pkubatrh]

Implemented by #174