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Render jinja2 templates on the command line using shell environment variables

Project Description
envtpl
======

_Render jinja2 templates on the command line with shell environment variables_

Installation
------------

pip install envtpl

How-to
------

Say you have a configuration file called whatever.conf that looks like this

foo = 123
bar = "abc"

You can use envtpl to set `foo` and `bar` from the command line by creating a file called whatever.conf.tpl

foo = {{ FOO }}
bar = "{{ BAR }}"

If you run

FOO=123 BAR=abc envtpl < whatever.conf.tpl > whatever.conf

you'll get back the original whatever.conf.

You can also specify default values

foo = {{ FOO | default(123) }}
bar = "{{ BAR | default("abc") }}"

Running

FOO=456 envtpl < whatever.conf.tpl > whatever.conf

will generate

foo = 456
bar = "abc"

This is all standard [Jinja2 syntax](http://jinja.pocoo.org/docs/templates/), so you can do things like

{% if BAZ is defined %}
foo = 123
{% else %}
foo = 456
{% endif %}
bar = "abc"

If an environment variable is missing, envtpl will throw an error

$ echo '{{ FOO }} {{ BAR }}' | FOO=123 envtpl
Error: 'BAR' is undefined

You can change this behaviour to insert empty strings instead by passing the `--allow-missing` flag.

Instead of reading from stdin and writing to stdout, you can pass the input filename as an optional positional argument,
and set the output filename with the `--output-file` (`-o`) argument.

envtpl -o whatever.conf whatever.conf.tpl

As a convenience, if you don't specify an output filename and the input filename ends with `.tpl`, the output filename will be the input filename without the `.tpl` extension, i.e.

envtpl whatever.conf.tpl
# is equivalent to
envtpl -o whatever.conf whatever.conf.tpl

By default, envtpl will **delete** the input template file. You can keep it by passing the `--keep-template` flag.

There's a special `environment(prefix='')` function that you can use as a kind of wildcard variable. If you have `hello.tpl`

hello = {{ FOO }}
{% for key, value in environment('MY_') %}{{ key }} = {{ value }}
{% endfor %}

and compile it using

FOO=world MY_baz=qux MY_foo=bar envtpl hello.tpl

You end up with

hello = world
baz = qux
foo = bar

What's the point?
-----------------

I use this script quite a lot in Docker images. Usually I'll have the CMD execute some file, like /bin/start_container, that sets up the runtime configuration for the container by inserting environment variables into config files before starting the main process. A redis example could look like this

#!/bin/bash
# start_container

envtpl /etc/redis.conf.tpl

redis-server

This is the use case I've optimised for, so that's why envtpl by default will delete the original template file.
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