How to assign the output of a command to a Makefile variable

makefileshell

I need to execute some make rules conditionally, only if the Python installed is greater than a certain version (say 2.5).

I thought I could do something like executing:

python -c 'import sys; print int(sys.version_info >= (2,5))'

and then using the output ('1' if ok, '0' otherwise) in a ifeq make statement.

In a simple bash shell script it's just:

MY_VAR=`python -c 'import sys; print int(sys.version_info >= (2,5))'`

but that doesn't work in a Makefile.

Any suggestions? I could use any other sensible workaround to achieve this.

Best Answer

Use the Make shell builtin like in MY_VAR=$(shell echo whatever)

me@Zack:~$make
MY_VAR IS whatever

me@Zack:~$ cat Makefile 
MY_VAR := $(shell echo whatever)

all:
    @echo MY_VAR IS $(MY_VAR)

Related Solutions

Python – How to execute a program or call a system command

Use the subprocess module in the standard library:

import subprocess
subprocess.run(["ls", "-l"])

The advantage of subprocess.run over os.system is that it is more flexible (you can get the stdout, stderr, the "real" status code, better error handling, etc...).

Even the documentation for os.system recommends using subprocess instead:

The subprocess module provides more powerful facilities for spawning new processes and retrieving their results; using that module is preferable to using this function. See the Replacing Older Functions with the subprocess Module section in the subprocess documentation for some helpful recipes.

On Python 3.4 and earlier, use subprocess.call instead of .run:

subprocess.call(["ls", "-l"])

The difference between the GNU Makefile variable assignments =, ?=, := and +=

Lazy Set

VARIABLE = value

Normal setting of a variable, but any other variables mentioned with the value field are recursively expanded with their value at the point at which the variable is used, not the one it had when it was declared

Immediate Set

VARIABLE := value

Setting of a variable with simple expansion of the values inside - values within it are expanded at declaration time.

Lazy Set If Absent

VARIABLE ?= value

Setting of a variable only if it doesn't have a value. value is always evaluated when VARIABLE is accessed. It is equivalent to

ifeq ($(origin VARIABLE), undefined)
  VARIABLE = value
endif

See the documentation for more details.

Append

VARIABLE += value

Appending the supplied value to the existing value (or setting to that value if the variable didn't exist)