$(filter pattern…,text)

Returns all whitespace-separated words in text that do match any of the pattern words, removing any words that do not match. The patterns are written using ‘%’, just like the patterns used in the patsubst function above.

The filter function can be used to separate out different types of strings (such as file names) in a variable. For example:

sources := foo.c bar.c baz.s ugh.h

foo: $(sources)

        cc $(filter %.c %.s,$(sources)) -o foo

says that foo depends of foo.c、bar.c、baz.s and ugh.h but only foo.c、bar.c and baz.s should be specified in the command to the compiler.

$(filter-out pattern…,text)

Returns all whitespace-separated words in text that do not match any of the pattern words, removing the words that do match one or more. This is the exact opposite of the filter function.

For example, given:

objects := main1.o foo.o main2.o bar.o

mains   := main1.o main2.o

the following generates a list which contains all the object files not in ‘mains’:

$(filter-out $(mains),$(objects))