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fmt

The fmt command is a simple text formatter that reflows and formats plain text paragraphs to a specified width. It’s especially useful for ensuring that text files, documentation, or email messages adhere to a consistent line length without breaking words inappropriately. In this comprehensive guide, we’ll explore the basic syntax, common options, practical examples, and advanced usage tips for the fmt command.

Table of Contents

  1. Introduction

  2. Basic Syntax and How fmt Works

  3. Common Options and Parameters

  4. Practical Examples

  5. Advanced Usage and Tips

  6. Conclusion and Further Reading

Introduction

The fmt command is a lightweight utility that is part of many Unix-like systems. Its primary purpose is to reformat paragraphs of text by adjusting line breaks so that each line is as close as possible to a specified maximum width without splitting words. This makes it ideal for cleaning up text files or reformatting output from other commands to improve readability.

Key benefits of fmt include:

  • Consistent Line Width: Automatically reflows paragraphs to a uniform width.

  • Word Preservation: Ensures words are not broken in the middle.

  • Ease of Use: Works directly with files or standard input, making it suitable for pipelines and scripts.

Basic Syntax and How fmt Works

The general syntax for fmt is:

fmt [OPTIONS] [FILE...]

  • FILE: One or more text files to be formatted. If no file is provided, fmt reads from standard input.

  • OPTIONS: Flags that allow you to control the formatting behavior, such as line width and paragraph handling.

By default, fmt processes the input text by grouping consecutive lines into paragraphs (separated by one or more blank lines) and then reflowing each paragraph to the default width (usually 75 characters).

Common Options and Parameters

Setting the Desired Width

  • -w NUM or --width=NUM:
    Specifies the maximum width (in columns) for the output lines. For example, to format text so that no line exceeds 50 characters:

    fmt -w 50 file.txt

    This option is the most frequently used, as it allows you to control the overall appearance of the text.

Controlling Paragraph Mode

  • -p:
    Enables "paragraph mode" where blank lines are preserved as paragraph separators. This is the default behavior, but you can use this flag explicitly to ensure paragraphs remain distinct.

  • -u:
    Tells fmt to treat each input line as a separate paragraph, which means no reflowing is done across lines. Use this option if you want to prevent merging of lines that might be logically separate.

Preserving Existing Line Breaks

  • -c:


    Prevents fmt from breaking long lines even if they exceed the specified width. This can be useful when you want to ensure that certain lines remain unchanged while still processing other paragraphs.

Other Useful Options

  • --help:
    Displays help information for fmt.

  • --version:
    Outputs version information about the fmt utility.

Practical Examples

Reformatting a Simple Text File

Suppose you have a file named example.txt that contains text with irregular line breaks. Running:

fmt example.txt

will reflow the paragraphs so that each line is roughly 75 characters long (the default width), resulting in a more uniform appearance.

Changing the Maximum Line Width

To format the text so that no line exceeds 60 characters, use:

fmt -w 60 example.txt

This ensures that the output lines are shorter, which might be preferable for narrow display windows or specific formatting requirements.

Preserving Indentation and Blank Lines

If your text file contains indented lines or you want to ensure that blank lines (paragraph breaks) are preserved, you can combine options like:

fmt -w 80 -p example.txt

This command preserves paragraph breaks while reflowing the text to a width of 80 characters.

Processing Standard Input in a Pipeline

fmt is often used in pipelines. For instance, if you want to reformat the output of another command, you can do:

cat example.txt | fmt -w 70

This reads the file through cat, pipes it into fmt, and formats the text to a maximum of 70 characters per line.

Advanced Usage and Tips

  • Handling Preformatted Text:
    If you have sections of text that should not be reflowed (such as code blocks), consider isolating them before processing with fmt or using options like -u to control how lines are treated.

  • Scripting and Automation:
    fmt can be easily integrated into scripts for automated text formatting, ensuring that documents and log files maintain a consistent appearance.

  • Combining with Other Text Utilities:
    Pair fmt with tools like sed, awk, or tr to perform more complex text transformations before or after formatting.

  • Locale and Character Widths:
    Be aware that fmt operates on character counts. In environments with multibyte or variable-width characters, the output may not perfectly match the specified width. Adjust your expectations or preprocess your text accordingly.

Conclusion and Further Reading

The fmt command is a straightforward yet powerful tool for reformatting plain text, making it easier to produce consistently formatted documents and readable output. Whether you’re cleaning up paragraphs for an email, preparing text for publication, or automating document processing, fmt can help ensure that your text adheres to the desired layout and width.

Further Reading and Resources

  • Manual Page:
    Access the detailed manual by typing:

    man fmt

  • Online Documentation:

  • Tutorials and Examples:
    Look for community examples on forums and Q&A sites like Stack Overflow to see creative uses of fmt in various scripting scenarios.

Experiment with fmt on your own text files to discover how it can streamline your document formatting workflows. Happy formatting!

Last modified: 08 February 2025
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