Unable to capture command exit code in makefile - bash

Question

I'm trying to setup my first makefile and am hitting a block at step 1. In my shell script, I did this:
which brew | grep 'brew not found' >/dev/null 2>&1
if [ $? == 0 ]; then
xcode-select --install
ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"
fi
This worked just fine as a bash script. After some googling, for a Makefile, I've so far come up with this one command:
BREW_INSTALLED = $(shell which brew | grep 'brew not found' >/dev/null 2>&1; echo $$?)
However, running it gets me
make: BREW_INSTALLED: No such file or directory
I'm equally unsure when I should be adding # to a command (seems like anything I don't want to output?).
I'm currently on GNU Make 3.81.

Answer 1

There are several odds in this line:
BREW_INSTALLED = $(shell which brew | grep 'brew not found' >/dev/null 2>&1; echo $$?)
In case of success, which writes its output to stdout, in case of failure to stderr. You are trying to capture the error message on stdout.
To feed the stderr of which to grep, you would need to write
which brew 2>&1 >/dev/null | grep 'brew not found'
(The order of 2>&1 and > also matters).
But you should not rely on the specific error message of which.
But you already get the return code you want from which, so you don't need grep at all.
Which returns the number of failed arguments, or -1 when no `programname' was given.
https://linux.die.net/man/1/which
Consider using grep -q 'expression' to supress output instead of redirecting stdout and stderr.
-q, --quiet, --silent
Quiet; do not write anything to standard output. Exit immediately with zero status if any match is found, even if an error was detected.
https://linux.die.net/man/1/grep
And the error message you get has nothing to do with what I'm writing above. This means the shell is trying to run BREW_INSTALLED as command, which probably means that make puts it at the beginning of a new shell.
Maybe you wrote it after a tabspace? see https://www.gnu.org/software/make/manual/html_node/Recipe-Syntax.html
To capture the return code (as string!):
BREW_INSTALLED := $(shell which brew >/dev/null 2>&1; echo $$?)

Answer 2

A typical makefile would check the presence of needed tools like this:
BREW := $(shell which brew)
# Check if variable brew is empty
ifeq ($(BREW),)
$(error brew not found)
else
$(info brew found: $(BREW))
endif
all:
#echo "Do something with brew"
$(BREW) --version
Note: There must be no tabspaces in the first two indented lines.
The two Recipe lines if the all Rule have to be indented with tabs.
The # at the beginning of a recipe supresses echoing: https://www.gnu.org/software/make/manual/html_node/Echoing.html