m4 Command in Linux

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The m4 command in Linux is a powerful macro processor that allows you to define and manipulate macros in files. This command is invaluable for system administrators, developers, and anyone who needs to automate text processing tasks.

Table of Contents

Here is a comprehensive guide to the options available with the m4 command −

• Understanding m4 Command
• Syntax of m4 Command
• m4 Command Options
• Examples of m4 Command in Linux
• Advanced Features of m4 Command

Understanding m4 Command

m4 is a macro processor that reads input files, processes macros, and outputs the results. It is commonly used for generating configuration files, preprocessing source code, and automating repetitive text processing tasks. Unlike simple text substitution tools, m4 provides advanced features such as conditionals, loops, and recursion, making it a powerful tool for complex text processing.

Let's install it −

sudo apt install m4

Syntax of m4 Command

The basic syntax for the m4 command is −

m4 [options] [file...]

Without any options, the command reads from standard input and writes to standard output.

m4 Command Options

Here are some of the most commonly used options with the m4 command −

Examples of m4 Command in Linux

Let's explore some practical examples to understand how to use the m4 command effectively.

Processing Macros in a File

This command processes macros defined within a file. For example, if you have a file named example.m4 with the following content −

m4 path/to/file

Running m4 example.m4 will produce the following output −

m4
define(`HOSTNAME', `mymachine')
define(`NETWORK', `192.168.1.0')
define(`GATEWAY', `192.168.1.1')
Router: <<HOSTNAME>> with IP <<GATEWAY>> is the default gateway in the <<NETWORK>> network.

Router − mymachine with IP 192.168.1.1 is the default gateway in the 192.168.1.0 network.

Defining a Macro Before Processing Files

This command defines a macro and its value before processing files. For example, continuing with the previous example file example.m4, running m4 -DHOSTNAME=myrouter example.m4 will produce the following output −

m4 -DHOSTNAME=myrouter path/to/file

Router − myrouter with IP 192.168.1.1 is the default gateway in the 192.168.1.0 network.

Undefining a Macro Before Processing Files

This command undefines a macro before processing files. For example, if you want to remove the HOSTNAME macro from the previous example, you can use this command −

m4 -UHOSTNAME path/to/file

Including Files

This command adds a directory to the include path. For example, if you have a directory /path/to/include containing additional macro definitions, you can include them in your processing −

m4 -I /path/to/include path/to/file

Enabling Line Synchronization Output

This command enables line synchronization output, which is useful for debugging and tracking the source of errors −

m4 -s path/to/file

Prefixing Built-in Macros

This command prefixes all built-in macros with m4_. For example, the define macro becomes m4_define −

m4 -P path/to/file

Enabling Compatibility Mode

This command enables compatibility mode, which makes m4 behave more like traditional macro processors −

m4 -E path/to/file

Quoting the Output of Macros

This command quotes the output of macros, which can be useful for debugging and ensuring that macro expansions are correct −

m4 -Q path/to/file

Enabling Debugging Output

This command enables debugging output, which provides detailed information about macro expansions and processing −

m4 -d path/to/file

Tracing Macro Calls

This command traces macro calls, providing a detailed log of macro invocations and expansions −

m4 -t path/to/file

Setting the Maximum Number of Macro Call Nesting Levels

This command sets the maximum number of macro call nesting levels to 100. This is useful for preventing infinite recursion and excessive resource consumption −

m4 -l 100 path/to/file

Setting the Output File

This command sets the output file to output.txt, redirecting the processed output to a file instead of standard output −

m4 -o output.txt path/to/file

Advanced Features of m4 Command

In addition to the basic options and examples, m4 provides advanced features such as conditionals, loops, and recursion. Let's explore some of these features with practical examples.

Conditionals

This command evaluates the condition 1 == 1 and returns true. If the condition is false, it returns false −

ifelse(`1', `1', `true', `false')

Loops

This command defines a macro repeat that repeats a string a specified number of times. The output is −

define(`repeat', `ifelse(`$1', `0', , `repeat(decr(`$1'))$2')')
repeat(`5', `Hello ')

Recursion

This command defines a macro factorial that calculates the factorial of a number using recursion. The output is −

define(`factorial', `ifelse(`$1', `0', `1', `eval(`$1 * factorial(decr(`$1'))')')')
factorial(`5')

File Inclusion

This command includes the contents of file.m4 in the current processing −

include(`file.m4')

Diversions

This command diverts output to a different stream and then undiverts it back to the main output −

divert(`1')
This text goes to diversion 1.
divert(`0')
This text goes to the main output.
undivert(`1')

For more detailed information, you can refer to the official m4 documentation.

Conclusion

The m4 command is an essential tool for anyone working with Linux systems, providing powerful macro processing capabilities. By mastering the various options and examples provided in this tutorial, you'll be well-equipped to leverage the full power of the m4 command in your daily tasks.

Whether you're generating configuration files, preprocessing source code, or automating repetitive text processing tasks, m4 is a versatile and invaluable tool in your Linux toolkit.