The sendmail program supports three kinds of macros. Class macros ( Chapter 32, Class Macros ) are used to represent multiple values. Database macros ( Chapter 33, Database Macros ) represent values stored in external files or networked maps. In this chapter we discuss defined macros, which allow strings of text to be represented symbolically.
Defined macros can be declared (given names and assigned the strings of text that will become values) at four different times:
Macros can be used in any configuration-file command. Generally, they are expanded (their value is used) when mail is sent or received.
When sendmail first begins to run, it preassigns values to certain macros. The complete list of these macros is shown in Table 31.1 . Each is described in detail at the end of this chapter in Section 31.10, "Alphabetized Reference" .
All preassigned macros can be redefined in the configuration file
or in the command line.
Note that the m4 configuration technique uses many more macros than are shown here (see Table 31.5 ). But even with that technique this short list of macros is all that are internally defined by the sendmail program when it first starts up.