/envtpl

Render jinja2 templates on the command line with shell environment variables

Primary LanguagePython

envtpl

Render jinja2 templates on the command line with shell environment variables

Build Status PyPI version

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, 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

If you need more complex data structures you can pass in JSON as a string and use the from_json filter to turn it into an object you can use in your template:

FOO='[{"v": "hello"}, {"v": "world"}]' envtpl <<< '{% for x in FOO | from_json %}{{ x.v }}{% endfor %}'

gives

helloworld

and

FOO='{"bar": "baz"}' envtpl <<< '{{ (FOO | from_json).bar }}'

renders

baz

What's the point?

I use this script quite a lot in Docker images. Here is an example.

In the CMD script I set up the runtime configuration using environment variables, e.g.

#!/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.