4.2 Rule Syntax

In general, a rule looks like this:

     targets : prerequisites
             command
             ...

or like this:

     targets : prerequisites ; command
             command
             ...

The targets are file names, separated by spaces. Wildcard characters may be used (see Using Wildcard Characters in File Names) and a name of the form a(m) represents member m in archive file a (see Archive Members as Targets). Usually there is only one target per rule, but occasionally there is a reason to have more (see Multiple Targets in a Rule).

The command lines start with a tab character. The first command may appear on the line after the prerequisites, with a tab character, or may appear on the same line, with a semicolon. Either way, the effect is the same. There are other differences in the syntax of command lines. See Writing the Commands in Rules.

Because dollar signs are used to start make variable references, if you really want a dollar sign in a target or prerequisite you must write two of them, `$$' (see How to Use Variables). If you have enabled secondary expansion (see Secondary Expansion) and you want a literal dollar sign in the prerequisites lise, you must actually write four dollar signs (`$$$$').

You may split a long line by inserting a backslash followed by a newline, but this is not required, as make places no limit on the length of a line in a makefile.

A rule tells make two things: when the targets are out of date, and how to update them when necessary.

The criterion for being out of date is specified in terms of the prerequisites, which consist of file names separated by spaces. (Wildcards and archive members (see Archives) are allowed here too.) A target is out of date if it does not exist or if it is older than any of the prerequisites (by comparison of last-modification times). The idea is that the contents of the target file are computed based on information in the prerequisites, so if any of the prerequisites changes, the contents of the existing target file are no longer necessarily valid.

How to update is specified by commands. These are lines to be executed by the shell (normally `sh'), but with some extra features (see Writing the Commands in Rules).