Mermaid CLI

Mermaid CLI. It is a command-line tool that renders diagrams and charts from text-based definitions, functioning as the standalone executable counterpart to the popular Mermaid JavaScript library. Developed primarily by Knut Sveidqvist, it allows developers and technical writers to automate the generation of complex visual documentation, such as flowcharts, sequence diagrams, and entity–relationship models, directly within CI/CD pipelines and build scripts. By parsing Markdown files or dedicated diagram definitions, it outputs static image files in formats like PNG or SVG, enabling seamless integration into technical documentation, software architecture wikis, and automated reporting systems.

● Overview

The tool emerged from the broader Mermaid project, which was created to simplify the creation and maintenance of technical diagrams through a text-based, version control-friendly syntax. It operates as a Node.js package, leveraging the same core rendering engine as the web-based library but packaged for headless execution in server environments and local development machines. This design philosophy aligns with principles of DevOps and Documentation as code, treating diagrams as artifacts that can be generated, tested, and versioned alongside application source code. Its ability to run without a graphical user interface makes it particularly valuable for use in automated documentation systems on platforms like GitHub Actions, Jenkins, and GitLab.

● Installation and setup

Installation is typically performed via the npm package manager, requiring a pre-installed version of Node.js on the host system. Users execute a global installation command, which downloads the package and its dependencies, including the Puppeteer library for controlling a headless Chromium instance to render the diagrams. Initial setup may involve configuring a PATH environment variable to ensure the command is accessible from any terminal session. For project-specific usage, it can also be installed as a development dependency within a project's `package.json` file, ensuring consistent versioning across different development environments and CI servers like Travis CI or CircleCI.

● Core features and usage

The primary command accepts input from stdin, a local file, or a specified text string, processing the Mermaid syntax to produce an output image. Supported diagram types include flowcharts, Gantt charts, pie charts, state diagrams, and class diagrams, covering a wide range of software engineering and project planning needs. Users can specify output parameters such as image format, scale, and CSS style overrides directly through command-line flags. A common workflow involves writing diagram definitions in separate `.mmd` files or embedding them within Markdown documents using code fences, then executing a build script that processes these files to generate a full set of visuals for a documentation site like those built with MkDocs or Jekyll.

● Integration with development workflows

Its strength lies in embedding diagram generation into automated documentation pipelines, often as a pre-commit hook or a build step in a static site generator. For instance, a GitHub Actions workflow can be configured to run the tool on every push to the `main` branch, ensuring all diagrams in a GitHub repository are up-to-date. It pairs effectively with documentation frameworks like VuePress, Docusaurus, and Sphinx, where the generated SVG or PNG files are directly referenced in the final published output. This integration supports the practice of Documentation as code, allowing teams to maintain a single source of truth for both application logic and its accompanying architectural diagrams.

● Configuration and customization

Behavior can be tailored using a JSON or YAML configuration file, which can define default output directories, theme settings, and Puppeteer launch arguments. Advanced customization includes injecting custom CSS to modify colors, fonts, and element styles to match corporate branding or documentation site themes. The configuration also supports setting parameters for the underlying Chromium instance, such as viewport size and device scale factor, which is crucial for generating pixel-perfect diagrams for high-resolution displays or print media. These settings ensure consistency across different execution environments, from a developer's macOS laptop to a Linux-based Docker container running on AWS.

● Comparison with related tools

Unlike Graphviz or PlantUML, which are also text-to-diagram tools, it uses a distinct, JavaScript-based syntax and is tightly coupled with the modern JavaScript web development ecosystem. While PlantUML has a broader historical adoption for UML diagrams, it is often favored for its simpler syntax and seamless integration with Markdown and contemporary static site generators. Compared to Microsoft Visio or Lucidchart, it is programmatic and command-driven, lacking a graphical user interface but excelling in automation and version control scenarios. Its reliance on a headless browser for rendering differentiates it from D2, which compiles to SVG natively, though both aim to serve the growing Infrastructure as code and Documentation as code movements.

Category:Command-line software Category:Diagramming software Category:Free software programmed in JavaScript