Using variables in Makefile

问题:

Here is a simple Makefile.

FILENAME=test.`date +"%Y.%m.%d %H:%M:%S"`.txt
test:
    @echo ${FILENAME}
    @sleep 2s
    @echo ${FILENAME}

The output of make test is

test.2013.02.18 15:30:23.txt
test.2013.02.18 15:30:25.txt

The problem is that FILENAME is being calculated each time it is used. I want it to be calculated only once and be the same while script is running. Am I doing it wrong?

回答1:

GNU Make has two flavours of variables. You have used a recursively-expanded variable, but you want a simply-expanded variable. See http://www.gnu.org/software/make/manual/html_node/Flavors.html#Flavors

With your current Makefile, the variable contains the exact text test.date +"%Y.%m.%d %H:%M:%S".txt and every time you reference it that text gets substituted verbatim, and so your Makefile is equivalent to:

test:
    @echo test.`date +"%Y.%m.%d %H:%M:%S"`.txt
    @sleep 2s
    @echo test.`date +"%Y.%m.%d %H:%M:%S"`.txt

Seen like this it should be obvious that the shell runs the date command twice.

To get the behaviour you want, you need to set a simply-expanded variable and you need to run the shell command at the point where you define it, which can't use backticks because they are shell metacharacters but Make ignores them, so you use Make's shell function instead:

FILENAME := test.$(shell date +"%Y.%m.%d %H:%M:%S").txt