Mastering the Packer Command Line Interface

Packer Fundamentals

This briefing document reviews key concepts and insights of HashiCorp Packer, a tool for creating machine images.

1. Interacting with Packer

The primary and sole method of interacting with Packer is through its command-line interface (CLI). There is currently no UI or API available.

Key Commands:

• packer: Lists available commands.

• packer --help: Displays detailed help information.

• packer -autocomplete-install: Enables tab completion for enhanced CLI usability.

Command Structure:

Packer commands follow a consistent structure similar to other HashiCorp tools:

packer [subcommand] [arguments/options] [template]

For example, packer build -var-file variables.json template.pkr.hcl would execute a build using the specified variable file and template.

2. Template Formats

Packer supports multiple template formats:

JSON:

• Legacy format (.json extension).

• Newer format aligning with HCL2 structure.

HCL (HashiCorp Configuration Language):

• Preferred format for new templates (.pkr.hcl extension).

• Offers improved readability and features compared to JSON.

3. Key Template Components

Variables:

• Allow for reusable templates by defining customizable values.

• Support default values, types, and validation rules in HCL2.

Builders:

• Define the environment for image creation (e.g., AWS, VMware, Docker).

• Act as sources in HCL2.

Provisioners:

• Customize the image by running scripts, installing packages, etc.

Post-Processors:

• Manipulate the built image, such as creating a manifest.

4. Building Images with Packer

The packer build command initiates the image creation process:

• It provisions resources on the target platform as defined by the builder.

• It executes provisioners to customize the image.

• It runs post-processors on the generated image.

5. Useful Commands

• packer console: Allows experimenting with variable interpolations and expressions.

• packer fmt: Formats HCL2 configuration files for consistent style.

• packer hcl2_upgrade: Translates JSON templates to HCL2 format.

• packer init: Initializes a template and downloads necessary plugins.

• packer inspect: Displays the components of a template.

• packer validate: Checks the syntax and configuration of a template.

6. Troubleshooting and Debugging

Packer offers a -debug flag to enable debug mode during builds. The -machine-readable flag outputs logs in a machine-parsable format for easier analysis in automated environments.

7. Plugin Installation

Packer uses plugins to extend its functionality. Plugins can be installed using the packer plugins install command. The packer init command will also install plugins specified in a template.

Conclusion:

This document provides a high-level overview of Packer's core concepts and functionalities. For further information, consult the official Packer documentation on the HashiCorp website.