Sendmail, 3rd Edition

The m4 Preprocessor

Creating a configuration file with m4(1) is simplicity itself. The m4(1) program is a macro preprocessor that produces a sendmail configuration file by processing a file of m4 commands. Files of m4 commands traditionally have names that end in the characters .m4 (the same as files used for building the sendmail binary). For building a configuration file, the convention is to name a file of m4 commands with an ending of .mc (for macro configuration). The m4 process reads that file and gathers definitions of macros, then replaces those macros with their values and outputs a sendmail configuration file.

With m4, macros are defined (given values) like this:

define(macro, value)

Here, the macro is a symbolic name that you will use later. Legal names must begin with an underscore or letter and can contain letters, digits, and underscores. The value can be any arbitrary text. A comma separates the two, and that comma can be followed by optional whitespace.

There must be no space between the define and the left parenthesis. The definition ends with the right parenthesis.

To illustrate, consider this one-line m4 source file named /tmp/x:

input text to be converted
            ↓
 
 define(A,B)A
  ↑ the m4 definition

When m4 is run to process this file, the output produced shows that A (the input) is redefined to become B:

% m4 /tmp/x
B

m4 Is Greedy

The m4 program is greedy. That is, if a macro is already defined, its value will replace its name in the second declaration. Consider this input ...