Managing file interactions with make

Executable commands

If a target must be created, the sequence of commands is executed. Normally, each command line is printed and then passed to a separate invocation of the shell after substituting for macros. The printing is suppressed in the silent mode (-s option of the make command) or if the command line in the description file begins with an ``@'' sign. make normally stops if any command signals an error by returning a nonzero error code. Errors are ignored if the -i flag has been specified on the make command line, if the fake target name .IGNORE appears in the description file, or if the command string in the description file begins with a hyphen (-). If a program is known to return a meaningless status, a hyphen in front of the command that invokes it is appropriate. Because each command line is passed to a separate invocation of the shell, care must be taken with certain commands (cd and shell control commands, for instance) that have meaning only within a single shell process. These results are forgotten before the next line is executed.

Before issuing any command, certain internally maintained macros are set. The $@ macro is set to the full target name of the current target. The $@ macro is evaluated only for explicitly named dependencies. The $? macro is set to the string of names that were found to be younger than the target. The $? macro is evaluated when explicit rules from the makefile are evaluated. If the command was generated by an implicit rule, the $< macro is the name of the related file that caused the action; and the $* macro is the prefix shared by the current and the dependent file names. If a file must be made but there are no explicit commands or relevant built-in rules, the commands associated with the name .DEFAULT are used. If there is no such name, make prints a message and stops.

In addition, a description file may also use the following related macros: $(@D), $(@F), $(*D), $(*F), $(<D), and $(<F) (see below).

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