envtpl

envtpl renders Go templates on the command line using environment variables.

It is directly inspired by the original envtpl, a Python tool for rendering Jinja2 templates.

This port was motivated by the desire to add templating support for template-driven configuration files that needed to be part of a base Docker image without also requiring the installation of Python. For the same reason, I decided not to add variable support to my previous template utility njx, which depends on Node.js.

Despite the difference between Jinja and Go templates, an attempt was made to match the command line syntax of the original envtpl.

The biggest obvious difference is that Go template variables represent a path within a data context, so envtpl variables will need to be prepended with a leading . to match the keys of the internal environment variable map object (see example).

Get it

$ go get github.com/subfuzion/envtpl/...

Usage

envtpl [-o|--output outfile] [-m|--missingkey option] [template]

If template is not provided, envtpl reads from stdin
If outfile is not provided, envtpl writes to stdout
If missingkey is unset or set to either default or invalid, envtpl follows the default behavior of the golang template library and missing keys in the template will be filled in with the string <no value>. If missingkey is set to zero, missing keys will be filled in with the zero value for their data type (ie: an empty string). If missingkey is set to error, envtpl will fail and an error returned to the caller.

Example

greeting.tpl

Hello {{.USER}}

Render the template (assume the value of $USER is 'mary')

envtpl greeting.tpl  # writes "Hello mary" to stdout

USER=bob envtpl greeting.tpl  # overrides "mary" and writes "Hello bob" to stdout

echo "greetings {{.USER}}" | envtpl  # writes "greetings mary" to stdout

envtpl < greeting.tpl > out.txt  # writes "Hello mary" to out.txt
envtpl > out.txt < greeting.tpl  # same thing
cat greeting.tpl | envtpl > out.txt  # same thing

unset USER; envtpl greeting.tpl            # => "Hello <no value>"
unset USER; envtpl -m zero greeting.tpl    # => "Hello "
unset USER; envtpl -m error greeting.tpl   # => "map has no entry for key "USER"", aborts

# Use a GitHub gist with curl
curl -s https://gist.githubusercontent.com/subfuzion/d5a6e3b7c2577902408069deb1d8e4d7/raw/2e4c0b894e2983411a20ffa5ee84aeafa5c6ebfb/greeting.tpl | envtpl
# => Hello, Tony

See test/test.tpl for more examples, including conditional functions and looping over environment variables.

Template Functions

sprig

In addition to the standard set of template actions and functions that come with Go, envtpl also incorporates sprig for additional, commonly used functions.

For example:

echo "Greetings, {{.USER | title}}" | envtpl  # writes "Greetings, Mary" to stdout

In the example, the environment name of the user mary is converted to Mary by the title template function.

For reference, see sprig functions.

environment

To mimic the environment function for the original envtpl, an environment function allows to filter the environment with a prefix string

{{ range $key, $value := environment "TAG_"  }}{{ $key }}="{{ $value }}"{{ end }}

filters all environment variables starting with TAG_.

For example:

$ echo '{{ range $key, $value := environment "GO"  }}{{ $key }} => {{ $value }} {{ "\n" }}{{ end }}' | envtpl
GOPATH => /Users/tony/go
GOROOT => /usr/local/go

Building an envtpl Docker image

An image is available on Docker Hub subfuzion/envtpl

You can use run a container like this:

$ echo 'Hello {{ .NAME | title | printf "%s\n" }}' | docker run -i --rm -e NAME=world subfuzion/envtpl
Hello World

# using a gist
$ curl -s https://gist.githubusercontent.com/subfuzion/d5a6e3b7c2577902408069deb1d8e4d7/raw/2e4c0b894e2983411a20ffa5ee84aeafa5c6ebfb/greeting.tpl | docker run -i -e USER=$USER subfuzion/envtpl
Hello, Tony

To build your own local container:

$ make image

The final image is based on scratch and weighs in at less than 7MB:

$ docker images --format "{{ .Repository }}:{{ .Tag }} => {{ .Size }}" subfuzion/envtpl
subfuzion/envtpl:latest => 6.65MB

Test

The Dockerfile for the image explicitly runs tests, so if the image builds successfully, then the tests passed.

There is an automated build for subfuzion/envtpl on Docker Hub. Docker Hub runs tests based on the presence of docker-compose.test.yml, so there is a docker-compose.test.file that simply builds the image and runs envtpl --version. You can test this yourself:

$ make test

Similar Tools

As mentioned above, this tool was inspired by the original envtpl project and motivated to provide something similar without adding a Python dependency to Docker base images.

A search for similar Go-based tools turns up the following:

I haven't spent any time evaluating either yet. However, mattrobenolt/envtpl looks elegantly simple and arschles/envtpl offers tests, glide package management support and more template functionality using sprig.

Neither of these two packages appear to conform to the original envtpl command line syntax, which was one of my goals, although I don't think this is a big deal since all of these spin-off versions use an entirely different template syntax anyway. However, at first glance at least, this variant does offer more input/output options modeled after the original.

~~I'm inspired by arschles/envtpl to add sprig support for extended template functions, potentially glide support, and definitely tests.~~ This version now has sprig template support, tests, and uses Go 1.1. modules instead of glide.

License

MIT

peregrinator/envtpl