Render jinja2 templates on the command line using shell environment variables

template, environment, variables, parameter, substitution, shell, jinja2, docker
pip install envtpl==0.6.0



Render jinja2 templates on the command line with shell environment variables

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pip install envtpl


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") }}"


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 %}'




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



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.

# start_container

envtpl /etc/redis.conf.tpl


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