LTF

Status: alpha

LTF is a minimal, transparent Terraform wrapper. It makes Terraform projects easier to work with.

In standard Terraform projects, the *.tf files are typically duplicated in each environment, with minor differences for the backend configuration. Every environment directory contains nearly identical *.tf files and *.tfvars files. It requires some effort to maintain all of these environments and keep them consistent. Changes take longer because they involve more files.

terraform
├── dev
│   ├── dev.auto.tfvars
│   ├── main.tf
│   ├── outputs.tf
│   └── variables.tf
├── qa
│   ├── qa.auto.tfvars
│   ├── main.tf
│   ├── outputs.tf
│   └── variables.tf
└── live
    ├── blue
    │   ├── live.blue.auto.tfvars
    │   ├── main.tf
    │   ├── outputs.tf
    │   └── variables.tf
    └── green
        ├── live.green.auto.tfvars
        ├── main.tf
        ├── outputs.tf
        └── variables.tf

Using LTF, the *.tf files are shared between all environments. A single *.tfbackend file is used to configure the backend for all environments, making use of variables. Environment directories only contain what is unique about the environment: the *.tfvars file. Maintenance is easier and environments are consistent by default. Changes take less time because they involve fewer files.

ltf
├── auto.tfbackend
├── main.tf
├── outputs.tf
├── variables.tf
├── dev
│   └── dev.auto.tfvars
├── qa
│   └── qa.auto.tfvars
└── live
    ├── blue
    │   └── live.blue.auto.tfvars
    └── green
        └── live.green.auto.tfvars

Using LTF is very easy. It avoids tedious command line arguments. Change to an environment directory and use ltf just like you would normally use terraform in a simple, single-directory Terraform project.

$ cd dev
$ ltf init
$ ltf plan
$ ltf apply
$ cd ../live/green
$ ltf init
$ ltf plan
$ ltf apply -target=random_id.this

Why use LTF?

LTF only does a few things:

• It finds and uses a parent directory as the configuration directory.
• It finds and uses tfvars and tfbackend files from the current and parent directories.
• It supports variables in tfbackend files.
• It supports hooks to run custom scripts before and after Terraform.

LTF is good because:

• It avoids tedious command line arguments, so it's quick and easy to use.
• It is a transparent wrapper, so all standard Terraform commands and arguments can be used.
• It is released as a single binary, so installation is easy.
• It keeps your Terraform configuration DRY using only a simple directory structure.
• It only does a few things, so there's not much to learn.
• It runs Terraform in the configuration directory, so there are no temporary files or build/cache directories to complicate things.

LTF is purposefully simple and feature-light, so it doesn't do everything:

• It does not create backend resources for you.
• It does not generate Terraform configuration using another language.
• It does not support remote configurations.
• It does not support run-all or similar commands.

Installation

Install manually

Download the appropriate binary for your system from the releases page, move it to your PATH, and make it executable.

Install using bin

bin manages binary files downloaded from different sources. Run the following to install the latest version of LTF:

bin install github.com/raymondbutcher/ltf

Usage

Run ltf instead of terraform. All standard Terraform commands and arguments can be used.

Example:

$ ltf init
$ ltf plan
$ ltf apply -target=random_id.this

How LTF works

LTF searches the directory tree for a Terraform configuration directory, tfvars files, and tfbackend files, and passes their details to Terraform.

When LTF finds no *.tf or *.tf.json files in the current directory, it does the following:

• Finds the closest parent directory containing *.tf or *.tf.json files, then adds -chdir=$dir to the Terraform command line arguments, to make Terraform use it as the configuration directory.
• Sets the TF_DATA_DIR environment variable to make Terraform use the .terraform directory inside the current directory instead of the configuration directory.

When running ltf init, it does the following:

• Finds *.tfbackend files in the current directory and parent directories, stopping at the configuration directory, then updates the TF_CLI_ARGS_init environment variable to contain -backend-config=$attribute for each attribute.
  • The use of Terraform variables in *.tfbackend files is supported.

It always does the following:

• Finds *.tfvars and *.tfvars.json files in the current directory and parent directories, stopping at the configuration directory, then sets the TF_VAR_name environment variable for each variable.
  • Terraform's precedence rules are followed when finding variables, with the additional rule that variables in subdirectories take precendence over variables in parent directories.
  • If any tfvars files exist in the configuration directory, variables from those files take precedence over any environment variables. LTF raises an error if it tries to set an environment variable that would be ignored by Terraform. This can be avoided by using variable defaults instead of tfvars files, or by moving the tfvars files into a subdirectory.
• Runs hook scripts before and after Terraform.

Hooks

LTF also supports hook scripts defined in ltf.yaml. It looks for this file in the current directory or any parent directory. Hook scripts are just Bash scripts; they can contain multiple lines, and they can even export environment variables. Environment variables will persist to subsequent hooks and to the Terraform command.

Hooks can be configured to run before specific Terraform commands, and/or after they have completed successfully, and/or after they have failed.

Schema

hooks:
  $name: # the name of the hook
    before: # (optional) run the script before these commands
      - terraform # the hook will always run
      - terraform $subcommand # the hook will only run before this subcommand
    after: [] # (optional) run the script after these commands finish successfully
    failed: [] # (optional) run the script after these commands have failed
    script: $script # bash script to run

Example: running commands

hooks:
  greetings:
    before:
      - terraform
    script: |
      echo "Hello, this is a hook!"
      echo "The date is $(date -I)"

Example: Setting environment variables

hooks:
  TF_VAR_hook:
    before:
      - terraform apply
      - terraform plan
    script: export TF_VAR_hook=hello