variable assignment before vs after make command

Question

make can take variable assignments as part of his command line. Is DEBUG=1 make the same as make DEBUG=1? With this simple Makefile, both print echo 1.

$ cat Makefile
all:
        echo ${DEBUG}

It's clear in the latter case, DEBUG=1 is part of the argument to make, but the first one only seems like a variable assignment in shell. So I'd assume make gets the value differently. Some clarification about the difference will be helpful.

Answer 1

Is DEBUG=1 make the same as make DEBUG=1?

Not really.

• In DEBUG=1 make: make obtains the variable from the environment. That is, the shell sets the variable DEBUG, then make is executed and it (the process running make) inherits that environment variable.
• In make DEBUG=1: make obtains the variable from the command line (i.e.: make sets the variable itself).

---

Relevant differences

Variables obtained from the command line override the variables obtained from the environment. That is, in:

DEBUG=0 make DEBUG=1

DEBUG is 1, not 0.

Note as well that variables obtained from the command line override the variables set inside the makefile (unless the override directive is used), whereas environment variables don't (unless the option -e or --environment-override is passed to make).

Therefore, if the makefile sets a variable named DEBUG as in the following:

DEBUG = 0
all:
    @echo ${DEBUG}

Then:

make DEBUG=1

echoes 1, whereas:

DEBUG=1 make

echoes 0.

---

The origin built-in function

You can use the origin built-in function to find out where a variable comes from. Consider this modification of your makefile:

$ cat Makefile
all:
    @echo DEBUG is from $(origin DEBUG)

Then:

$ make DEBUG=1
DEBUG is from command line

but:

$ DEBUG=1 make
DEBUG is from environment