jgtr

README

jgtr - json go template renderer

jgtr is a very simple tool that uses JSON-encoded data and template files written in Go's text/template as input. It outputs rendered templates to stdout or a file. It can be used as a general purpose tool for rendering templates of whatever.

All the building blocks to make jgtr are already a part of Go, so it's mostly a matter of snapping blocks together. I just wanted a simple way to render templates with data, and I was already comfortable with JSON, and I was already somewhat comfortable with Go templates, and it looked like a good opportunity to learn more Go, so hey, why not.

Usage

jgtr normally reads all its input from stdin and writes all its output to stdout. Standard GNU-style flags allow you to set files for these purposes instead. Reading the template and the data from stdin at the same time is finicky, so at least one of those two has to be specified explicitly.

Consider these example files, test.json and test.template:

{
    "sweater": "blue",
    "pants": "red"
}

Sweater color is {{ .sweater }}
Pants color is {{ .pants }}

Then any of the following invocations will all result in the same output:

jgtr -j test.json < test.template > test.txt
jgtr -t test.template < test.json > test.txt
jgtr -j test.json -t test.template > test.txt
jgtr -j test.json -t test.template -o test.txt

Namely, test.txt, which will contain:

Sweater color is blue
Pants color is red

If you know Go's text/template language, then you can probably use jgtr. In brief, the top-level JSON value is exposed as {{ . }}. If that value is a JSON object, you can access its keys by their names, as shown in the above example. If that value is a JSON array, you can loop over its contents using the range action. Refer to the documentation for more details on how to use the templating language.

jgtr exposes some standard Go functions that you can use in your templates. Take a look at tmplFuncs in jgtr.go for the full list. As an example, jgtr exposes Go's time.Parse under the name timeParse. You can this to generate a Go time.Time object and invoke standard functions on it.

"2013-04-30"

{{ with timeParse "2006-01-02" . }}
The year is {{ .Year }}
The month is {{ .Month }}
The day is {{ .Day }}
Out of 365 days, we're at day number {{ .YearDay }}
All in all, the day is ISO8601 {{ .Format "2006-01-02T15:04:05Z07:00" }}
{{ end }}

will produce

The year is 2013
The month is April
The day is 30
Out of 365 days, we're at day number 120
All in all, the day is ISO8601 2013-04-30T00:00:00Z

Note the empty lines in the output. Those are from the with and end templates. Those templates evaluate to the empty string, so we only use them for their side effects. Since I put them on their own lines (for readability), they leave behind empty lines when rendered.

Todo

• add more functions. text/template supports introducing more functions into a template via Funcs. Feel free to submit issues or pull requests for more functions.
• add more data file types. While I personally prefer JSON, there are plenty of other encodings that are isomorphic to it and can represent the same data structures. As long as you can unmarshal it into an interface{}, it can probably be dropped into this code in place of JSON. TOML and YAML come to mind.
• add a flag to switch to html/template for security and proper escaping. I don't personally care much about this use case, but it should be a straightforward addition if I feel like it.

License

MIT/expat, see LICENSE.md.