How to write loop in a Makefile?


If you're using GNU make, you could try

NUMBERS = 1 2 3 4
        $(foreach var,$(NUMBERS),./a.out $(var);)

which will generate and execute

./a.out 1; ./a.out 2; ./a.out 3; ./a.out 4;

I want to execute the following commands:

./a.out 1
./a.out 2
./a.out 3
./a.out 4
. and so on

How to write this thing as a loop in a Makefile?

You can use set -e as a prefix for the for-loop. Example:

    set -e; for a in 1 2 3; do /bin/false; echo $$a; done

make will exit immediately with an exit code <> 0.

I realize the question is several years old, but this post may still be of use to someone as it demonstrates an approach which differs from the above, and isn't reliant upon either shell operations nor a need for the developer to schpeel out a hardcoded string of numeric values.

the $(eval ....) builtin macro is your friend. Or can be at least.

define ITERATE
$(eval ITERATE_COUNT :=)\
$(if $(filter ${1},0),,\
  $(call ITERATE_DO,${1},${2})\

$(if $(word ${1}, ${ITERATE_COUNT}),,\
  $(eval ITERATE_COUNT+=.)\
  $(info ${2} $(words ${ITERATE_COUNT}))\
  $(call ITERATE_DO,${1},${2})\

  $(call ITERATE,5,somecmd)
  $(call ITERATE,0,nocmd)
  $(info $(call ITERATE,8,someothercmd)

That's a simplistic example. It won't scale pretty for large values -- it works, but as the ITERATE_COUNT string will increase by 2 characters (space and dot) for each iteration, as you get up into the thousands, it takes progressively longer to count the words. As written, it doesn't handle nested iteration (you'd need a separate iteration function and counter to do so). This is purely gnu make, no shell requirement (though obviously the OP was looking to run a program each time -- here, I'm merely displaying a message). The if within ITERATE is intended to catch the value 0, because $(word...) will error out otherwise.

Note that the growing string to serve as a counter is employed because the $(words...) builtin can provide an arabic count, but that make does not otherwise support math operations (You cannot assign 1+1 to something and get 2, unless you're invoking something from the shell to accomplish it for you, or using an equally convoluted macro operation). This works great for an INCREMENTAL counter, not so well for a DECREMENT one however.

I don't use this myself, but recently, I had need to write a recursive function to evaluate library dependencies across a multi-binary, multi-library build environment where you need to know to bring in OTHER libraries when you include some library which itself has other dependencies (some of which vary depending on build parameters), and I use an $(eval) and counter method similar to the above (in my case, the counter is used to ensure we don't somehow go into an endless loop, and also as a diagnostic to report how much iteration was necessary).

Something else worth nothing, though not significant to the OP's Q: $(eval...) provides a method to circumvent make's internal abhorrence to circular references, which is all good and fine to enforce when a variable is a macro type (intialized with =), versus an immediate assignment (initialized with :=). There are times you want to be able to use a variable within its own assignment, and $(eval...) will enable you to do that. The important thing to consider here is that at the time you run the eval, the variable gets resolved, and that part which is resolved is no longer treated as a macro. If you know what you're doing and you're trying to use a variable on the RHS of an assignment to itself, this is generally what you want to happen anyway.


  # will error.  Comment out and re-run

  # works

  @echo ${SOMESTRING}

Happy make'ing.

This is not really a pure answer to the question, but an intelligent way to work around such problems:

instead of writing a complex file, simply delegate control to for instance a bash script like: makefile

foo : bar.cpp baz.h

and looks like:

for number in 1 2 3 4
    ./a.out $number