The “eqn” command is an equation preprocessor that is used in conjunction with the “groff” (GNU Troff) document formatting system. It allows you to include mathematical equations and formulas in your documents, which are then rendered beautifully and correctly formatted by “groff”.

Here’s a more detailed explanation of the “eqn” command and its key features:

**Equation Formatting**: The primary purpose of the “eqn” command is to format mathematical equations and expressions in a document. It provides a rich set of syntax and features to represent various mathematical concepts, including algebraic equations, fractions, exponents, square roots, integrals, matrices, and more.**Preprocessing**: “eqn” acts as a preprocessor for the “groff” document formatting system. It takes input files that contain equations written in a specific syntax and processes them to generate the appropriate output that “groff” can understand and render correctly.**Mathematical Expressions**: With “eqn”, you can express complex mathematical expressions using a combination of mathematical notation and special syntax. It supports a wide range of mathematical symbols, operators, functions, and formatting options to accurately represent your equations.**Equation Numbering**: “eqn” allows you to automatically number equations in your documents. You can specify which equations should be numbered and which should be left unnumbered. This is particularly useful when referencing equations in the text or when you want to provide a clear reference for your readers.**Integration with “groff”**: Once the equations are processed by “eqn”, they can be seamlessly integrated into your “groff” documents. “groff” understands the output generated by “eqn” and takes care of rendering the equations correctly in the final formatted document, ensuring that they appear as intended.**Flexible Output Options**: “eqn” provides options to control the output format and style of equations. You can specify the font, size, spacing, alignment, and other formatting attributes to customize the appearance of your equations to suit your document’s needs.**Cross-Platform Compatibility**: “eqn” and “groff” are available on multiple platforms, including Unix-like systems such as Linux, macOS, and BSD variants. This ensures cross-platform compatibility and allows you to create and format documents with equations on different operating systems.**Integration with Document Typesetting**: “eqn” seamlessly integrates with the broader “groff” typesetting system. You can combine equations with text, images, tables, and other elements supported by “groff” to create professional-looking documents with consistent formatting and layout.**Extensibility**: “eqn” is extensible, allowing you to define custom macros and functions to enhance its capabilities. This enables you to create reusable templates or simplify the representation of complex equations by defining your own shortcuts and notations.

The “eqn” command, together with the “groff” formatting system, provides a powerful solution for including mathematical equations and expressions in your documents. By using the rich syntax and features of “eqn”, you can accurately represent mathematical concepts and ensure that they are correctly rendered in the final formatted document.

## eqn Command Examples

1. Process input with equations, saving the output for future typesetting with groff to PostScript:

# eqn /path/to/input.eqn > /path/to/output.roff

2. Typeset an input file with equations to PDF using the [me] macro package:

# eqn -T pdf /path/to/input.eqn | groff -me -T pdf > /path/to/output.pdf