Unix in a Nutshell, 4th Edition

Conditional Input

Conditional statements allow you to include or exclude specific lines based on some condition. The condition can be that a macro is or is not defined, or that the value of a macro is or is not equal to a particular string. The equivalence/nonequivalence tests provide three different ways of quoting the values. Conditionals may have an optional “else” part; i.e., lines that are used when the condition is not true. The general form is as follows:

    ifXXX test
                  lines to include if true
    [ else
        lines to include if false ]
    endif

(The square brackets indicate optional parts of the construct; they are not to be entered literally.) Actual tests are as follows:

Condition	Meaning
ifdef macroname	True if macroname is a macro that has been given a value.
ifndef macroname	True if macroname is a macro that has not been given a value.
ifeq ( v1 , v2 )
ifeq ' v1 ' ' v2 '	True if values v1 and v2 are equal.
ifeq " v1 " " v2 "
ifneq ( v1 , v2 )
ifneq ' v1 ' ' v2 '	True if values v1 and v2 are not equal.
ifneq " v1 " " v2 "

For example:

    whizprog.o: whizprog.c
    ifeq($(ARCH),ENIAC)      # Serious retrocomputing in progess!
            $(CC) $(CFLAGS) $(ENIACFLAGS) -c $< -o $@
    else
            $(CC) $(CFLAGS) -c $< -o $@
    endif