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m4

text processingLinux/Unix
The m4 command is one of the most frequently used commands in Linux/Unix-like operating systems. m4 Macro processor
See ExamplesView Syntax

Quick Reference

Command Name:

m4

Category:

text processing

Platform:

Linux/Unix

Basic Usage:

m4 [options] [arguments]

Common Use Cases

    Syntax

    m4 [options] [files]

    Options

    Option Description
    -D, --define=NAME[=VALUE] Define NAME as having VALUE or empty
    -I, --include=DIRECTORY Add DIRECTORY to include path
    -s, --synclines Generate line sync lines for debugging
    -P, --prefix-builtins Prefix all built-in macros with 'm4_'
    -Q, --quiet, --silent Suppress some warnings
    -E, --fatal-warnings Treat warnings as errors
    -G, --traditional Suppress all GNU extensions
    -H, --hashsize=PRIME Set symbol table hash size to PRIME
    -L, --nesting-limit=NUMBER Change nesting limit (default 1024)
    -F, --freeze-state=FILE Save frozen state to FILE at end
    -R, --reload-state=FILE Reload frozen state from FILE at start
    --error-output=FILE Redirect error messages to FILE
    -d, --debug[=FLAGS] Set debug level (no FLAGS implies 'aeq')
    -t, --trace=NAME Trace NAME when it is defined
    -l, --arglength=NUM Restrict macro tracing size
    -o, --debugfile=FILE Send debug and trace output to FILE

    Examples

    How to Use These Examples

    The examples below show common ways to use the m4 command. Try them in your terminal to see the results. You can copy any example by clicking on the code block.

    Basic Examples:

    m4 input.m4
    Process macros in input.m4 and output to stdout.
    echo "define(`NAME', `John') Hello NAME" | m4
    Define a macro NAME and use it, outputs "Hello John".

    Advanced Examples:

    m4 -D VERSION=1.0 input.m4
    Define VERSION as 1.0 from the command line.
    m4 --synclines input.m4 Include line number references in the output for debugging. m4 -P input.m4 Prefix all built-in macro names with 'm4_'. m4 -I directory input.m4 Add directory to the include path for searching included files. m4 --nesting-limit=100 input.m4 Change the macro nesting limit to 100 levels. m4 -s input.m4 Enable syncline output for better debugging. echo "define(`repeat', `ifelse(`$2', `1', `$1', `$1`'repeat(`$1', eval(`$2'-1))')')repeat(`Hello ', `3')" | m4 Create a recursive macro that repeats text, outputs "Hello Hello Hello ". echo "divert(1)Middle divert(2)Last divert(0)First undivert(1)undivert(2)" | m4 Demonstrate diversions to control output order.

    Try It Yourself

    Practice makes perfect! The best way to learn is by trying these examples on your own system with real files.

    Understanding Syntax

    Pay attention to the syntax coloring: commands, options, and file paths are highlighted differently.

    Notes

    The 'm4' command is a powerful macro processor that is used as a front-end to many software build systems and serves as a general-purpose text transformation tool. It copies its input to the output, expanding macros as it goes. The macro language includes facilities for macro definition, conditional operations, arithmetic expressions, file inclusion, and more. Key features of the m4 command: 1. Macro Processing: At its core, m4 allows you to define macros and have them expanded in text. These can be simple text substitutions or complex operations with arguments. 2. Built-in Functions: m4 comes with many built-in functions for string manipulation, arithmetic, file operations, and flow control, including ifelse, divert, include, eval, substr, and more. 3. Text Generation: m4 is often used to generate configuration files, code, documentation, or other text outputs based on templates and input parameters. 4. Recursive Expansion: Macros can call other macros, and m4 handles recursive expansion, making it possible to build complex text processing pipelines. 5. Diversions: The divert and undivert features allow redirecting output to temporary buffers and controlling the order in which content appears in the final output. 6. Quoting Mechanism: m4 provides a sophisticated quoting system to control when and how macro expansion occurs, allowing precise control over the processing. 7. Include Files: The include directive allows incorporating other m4 files, enabling modular organization of macros and templates. m4 is used in various contexts, including: - Autoconf/Automake: These GNU build tools use m4 extensively to generate configure scripts and makefiles that can adapt to different systems. - C/C++ Preprocessing: While C has its own preprocessor, m4 can be used for more advanced preprocessing needs. - Code Generation: Generating repetitive code or configurations from templates. - Document Generation: Creating documents with consistent formatting or content. - DNS Configuration: The BIND DNS server uses m4 for configuration file preprocessing. - SendMail Configuration: The sendmail email server traditionally used m4 for its configuration files. While powerful, m4 can be challenging to use due to its somewhat cryptic syntax and the complexity of controlling macro expansion. The quoting rules and recursive expansion behavior can be particularly tricky to master. However, for certain types of text processing tasks, especially those involving the generation of configuration files or code that needs to be customized for different environments, m4 remains a valuable tool in the Unix/Linux ecosystem.

    Related Commands

    These commands are frequently used alongside m4 or serve similar purposes:

    cpp
    sed
    awk
    autoconf
    make
    gpp
    envsubst
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    Use Cases

    Learn By Doing

    The best way to learn Linux commands is by practicing. Try out these examples in your terminal to build muscle memory and understand how the m4 command works in different scenarios.

    See ExamplesTry Lab Exercises
    $ m4
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