gomplate

README

gomplate

A Go template-based alternative to envsubst.

I really like envsubst for use as a super-minimalist template processor. But its simplicity is also its biggest flaw: it's all-or-nothing with shell-like variables.

Gomplate is an alternative that will let you process templates which also include shell-like variables. Also there are some useful built-in functions that can be used to make templates even more expressive.

Usage

The usual and most basic usage of gomplate is to just replace environment variables. All environment variables are available by referencing .Env (or getenv) in the template.

The template is read from standard in, and written to standard out.

Use it like this:

$ echo "Hello, {{.Env.USER}}" | gomplate
Hello, hairyhenderson

Commandline Arguments

--datasource/-d

Add a data source in name=URL form. Specify multiple times to add multiple sources. The data can then be used by the datasource function.

A few different forms are valid:

• mydata=file:///tmp/my/file.json
  • Create a data source named mydata which is read from /tmp/my/file.json. This form is valid for any file in any path.
• mydata=file.json
  • Create a data source named mydata which is read from file.json (in the current working directory). This form is only valid for files in the current directory.
• mydata.json
  • This form infers the name from the file name (without extension). Only valid for files in the current directory.

Syntax

About .Env

You can easily access environment variables with .Env, but there's a catch: if you try to reference an environment variable that doesn't exist, parsing will fail and gomplate will exit with an error condition.

Sometimes, this behaviour is desired; if the output is unusable without certain strings, this is a sure way to know that variables are missing!

If you want different behaviour, try getenv (below).

Built-in functions

In addition to all of the functions and operators that the Go template language provides (if, else, eq, and, or, range, etc...), there are some additional functions baked in to gomplate:

getenv

Exposes the os.Getenv function.

This is a more forgiving alternative to using .Env, since missing keys will return an empty string.

An optional default value can be given as well.

Example

$ echo 'Hello, {{getenv "USER"}}' | gomplate
Hello, hairyhenderson
$ echo 'Hey, {{getenv "FIRSTNAME" "you"}}!' | gomplate
Hey, you!

bool

Converts a true-ish string to a boolean. Can be used to simplify conditional statements based on environment variables or other text input.

Example

input.tmpl:

{{if bool (getenv "FOO")}}foo{{else}}bar{{end}}

$ gomplate < input.tmpl
bar
$ FOO=true gomplate < input.tmpl
foo

slice

Creates a slice. Useful when needing to range over a bunch of variables.

Example

input.tmpl:

{{range slice "Bart" "Lisa" "Maggie"}}
Hello, {{.}}
{{- end}}

$ gomplate < input.tmpl
Hello, Bart
Hello, Lisa
Hello, Maggie

json

Converts a JSON string into an object. Only works for JSON Objects (not Arrays or other valid JSON types). This can be used to access properties of JSON objects.

Example

input.tmpl:

Hello {{ (getenv "FOO" | json).hello }}

$ export FOO='{"hello":"world"}'
$ gomplate < input.tmpl
Hello world

jsonArray

Converts a JSON string into a slice. Only works for JSON Arrays.

Example

input.tmpl:

Hello {{ index (getenv "FOO" | jsonArray) 1 }}

$ export FOO='[ "you", "world" ]'
$ gomplate < input.tmpl
Hello world

datasource

Parses a given datasource (provided by the --datasource/-d argument).

Currently, only file:// URLs are supported, and only the JSON format can be parsed. More support is coming.

Example

person.json:

{
  "name": "Dave"
}

input.tmpl:

Hello {{ (datasource "person").name }}

$ gomplate -d person.json < input.tmpl
Hello Dave

ec2meta

Queries AWS EC2 Instance Metadata for information. This only retrieves data in the meta-data path -- for data in the dynamic path use ec2dynamic.

This only works when running gomplate on an EC2 instance. If the EC2 instance metadata API isn't available, the tool will timeout and fail.

Example

$ echo '{{ec2meta "instance-id"}}' | gomplate
i-12345678

ec2dynamic

Queries AWS EC2 Instance Dynamic Metadata for information. This only retrieves data in the dynamic path -- for data in the meta-data path use ec2meta.

This only works when running gomplate on an EC2 instance. If the EC2 instance metadata API isn't available, the tool will timeout and fail.

Example

$ echo '{{ (ec2dynamic "instance-identity/document" | json).region }}' | ./gomplate
us-east-1

ec2region

Queries AWS to get the region. Returns unknown if it can't be determined for some reason.

Example

$ echo '{{ ec2region }}' | ./gomplate
us-east-1

ec2tag

Queries the AWS EC2 API to find the value of the given user-defined tag. An optional default can be provided.

Example

$ echo 'This server is in the {{ ec2tag "Account" }} account.' | ./gomplate
foo
$ echo 'I am a {{ ec2tag "classification" "meat popsicle" }}.' | ./gomplate
I am a meat popsicle.

Some more complex examples

Variable assignment and if/else

input.tmpl:

{{ $u := getenv "USER" }}
{{ if eq $u "root" }}You are root!{{else}}You are not root :({{end}}

$ gomplate < input.tmpl
You are not root :(
$ sudo gomplate < input.tmpl
You are root!

Note: it's important for the if/else/end keywords to appear on the same line, or else gomplate will not be able to parse the pipeline properly

Releasing

Right now the release process is semi-automatic.

1. Create a release tag: git tag -a v0.0.9 -m "Releasing v0.9.9"
2. Build binaries: make build-x
3. (optional) compress the binaries with upx (may not work for all architectures)
4. Create a release in github!

License

The MIT License

Copyright (c) 2016 Dave Henderson