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TPL

A simple template-to-stdout renderer with support for various data sources.

MIT Software License Say Thanks!

The original motivation behind tpl was that we had docker-compose files that needed to reference things like secrets from an external Vault instance. In order to do that we needed to know the external IP address inside the configuration file. While things like container-juggler solve that for docker-compose files, we wanted a slighly more generic tool for use in other environments. Being able to access secrets directly from a Vault instance was also very high on the requirements-list 😉

Let's look at a small example where we have a service that should be able to talk to a Vault server running on the host system:

version: "3"
services:
    core-service:
        external_hosts:
            - "vault:{{ .Network.ExternalIP }}"
        ...

tpl now parses that docker-compose file, attempts to set all the variables it find in it (see the reference down below for details on what variables and functions are available), and writes its output to standard-output:

$ tpl docker-compose.yml
version: "3"
services:
    core-service:
        external_hosts:
            - "vault:123.123.123.123"
        ...

Depending on what data points you want to use inside your template you might also have to set specific environment variables (like VAULT_ADDR and VAULT_TOKEN for interacting with a Vault server). You can find details for what datapoints are available and what settings they might require in the next chapter.

Installation

You have a couple of options to install tpl:

  • If you're on macOS and are using brew:

    $ brew tap zerok/main https://github.com/zerok/homebrew-tap
$ brew install zerok/main/tpl

  • If you want to install tpl manually, you can find binaries for all releases on Github.

  • If you have Go installed, you can also install directly from the master branch:

    $ go get -u github.com/zerok/tpl/cmd/tpl

Supported data points

System information

Platform name and architecture

{{ .System.OS }} and {{ .System.Arch }} can be used to inspect the operating system and architecture tpl is executed on.

Shell output

Using {{ .System.ShellOutput "..." }} you can open a bash shell, run a command inside of it, and work with the output of that command. Note that, for now, /bin/bash is hardcoded as the path to the shell to be executed.

Network information

tpl exposes various data points about your local network.

Host IP address

You can access the host's external IP address using {{ .Network.ExternalIP }}. This can be useful to, for instance, configure host services inside a docker-compose file.

File-system

File existance

Especially when working with docker-compose, being able to check if a certain file exists before building volume mounts for it:

{{ .FS.Exists "path/to/file" }}

File content as string

Sometimes it can be useful to include the content of another file as is:

{{ .FS.ReadFile "path/to/file" }}

HashiCorp Vault secrets

If you have the environment variables:

  • VAULT_ADDR
  • VAULT_TOKEN

then you can access secrets from that Vault using the following syntax:

{{ vault "secrets/path" "fieldname" }}

Azure keyvault secrets

If you have the environment variables:

  • AZURE_TENANT_ID
  • AZURE_CLIENT_ID
  • AZURE_CLIENT_SECRET
  • AZURE_KEY_VAULT_URL
  • AZURE_API_VERSION

then you can also access secrets from that Azure keyvault using the following syntax:

{{ .Azure.Secret "secrets--path" }}

Alternatively you can set:

  • AZURE_TOKEN

to your personal token via az account get-access-token --resource https://vault.azure.net, then you only additionally need AZURE_KEY_VAULT_URL to access your secrets.

The path in keyvault can only contain alphanumeric characters and dashes.

Secrets as JSON

If you have secrets saved in JSON format you can read their values this way:

{{ .Azure.Secret "secrets--path" | jsonToMap | jmsepathValue "database.password" }}

This would assume that the secret saved in the path secrets--path in an Azure keyvault is in this format:

{
    "database": {
        "password": "123456"
    }
}

The returned value would in this case be "123456".

Path overrides

To allow for generic templates to be overridden with local path overrides, you can specify a custom path prefix flag for all secrets with the --vault-prefix PREFIX for vault and --azure-prefix PREFIX for azure.

For more fine-grained mappings, you can also create a mappings file which maps a path as it is written inside your template to a path as it should be looked up in the Vault:

$ cat vault.tpl
{{ vault "secret/old-path" "field" }}

$ cat vault-mapping.csv
secret/old-path;secret/new-path

$ vault write secret/new-path "field=test-value"
Success! Data written to: secret/new-path

$ tpl vault.tpl --vault-mapping vault-mapping.csv
test-value

If you are using Azure keyvault the --azure-mapping flag does the same for azure.

Note: If you also specify a --vault-prefix or --azure-prefix, this will be applied before the path is mapped.

Data files

You can load data also from previously generated data-files into the template using the --data flag:

$ cat test.tpl
{{ range .Data.items }}> .
{{ end }}

$ cat test.yaml
- 1
- 2
- 3

$ tpl --data=items=test.yaml test.tpl
> 1
> 2
> 3

Data can be loaded from files using one of these extensions:

  • .json
  • .yaml
  • .yml

Different template delimiters

The Go template language used {{ and }} as delimiters for working with variables or actions by default. This can become quite tedious when working within systems that also use these characters (or at least make the template hard to read). For this reason, you can override these delimiters using the --left-delimiter and --right-delimiter command-line flags.

And more...

tpl also bundles sprig which offers lots of general-purpose template functions. Please see its website for details.

Third-party libraries

This tool wouldn't be possible (or at least would have been a lot harder to write) without the great work of the Go community. The following libraries are used:

Big thanks to everyone who has contributed to any of these projects!