Makefile does not call shell script when setting variable

Question

I am trying to follow the meaty skeleton tutorial on osdev. The Makefile is not running one of the shell scripts. I have set all of the permissions on each of the files to be executable.

In lib/Makefile, I have the below few lines set:

$(info DEFAULT_HOST!=../default-host.sh)
$(info HOST?=DEFAULT_HOST)
$(info HOSTARCH!=../target-triplet-to-arch.sh $(HOST))

after these lines have executed, neither DEFAULT_HOST nor HOSTARCH get set.

default-host.sh:

#!/bin/sh
echo i686-elf

arget-triplet-to-arch.sh:

#!/bin/sh
if echo "$1" | grep -Eq 'i[[:digit:]]86-'; then
  touch here.txt
  echo i386
else
  touch there.txt
  echo "$1" | grep -Eo '^[[:alnum:]_]*'
fi

Note, I added the touch statements in arget-triplet-to-arch.sh. When run from the shell, one or other of those files is created, but not when the Makefile is run. This means that make seems to not be running the shell commands. How can I get make to run the shell commands?

The info command doesn't assign values to variables, as you seem to think it does. — Beta
– Beta, Commented Apr 29, 2018 at 23:30
I don't think it does that, sir; I think it allows me to see the value of the that line being evaluated. I placed it on the line that does the assignments to see that the line is at least being read by makefile. I realize that it is not going to show me the value. — K. Shores
– K. Shores, Commented Apr 29, 2018 at 23:32
(We may have a language problem.) Please post the part of the makefile which you expect to execute the scripts. — Beta
– Beta, Commented Apr 29, 2018 at 23:37
Those are the lines, the ones with the info statements. Even without the info statements, nothing happens. — K. Shores
– K. Shores, Commented Apr 29, 2018 at 23:49
If I change the lines that use !=<script> to use :=$(shell <script>), the scripts run. However, later expansion of the variables does not happen, for instance $HOSTARCH would evaluate to the empty string. — K. Shores
– K. Shores, Commented Apr 30, 2018 at 0:25

MadScientist · Accepted Answer · 2018-04-30 14:04:41Z

As Beta says, info doesn't "allow you to see the value of that line being evaluated". info expands its argument then prints it to stdout. "Expands" means it resolves any variable references, it doesn't mean interpreting it as a makefile command. So if you run $(info hi) it prints "hi". If you run $(info foo = bar) if prints foo = bar but does not set the value of the variable foo to bar.

For using !=, note that this feature was added to GNU make 4.0. If your version is older than that then this assignment doesn't do what you expect. In particular, a line like FOO!=echo bar will be interpreted as if it were FOO! = echo bar... in other words it sets the make variable named FOO!.

Personally I always put whitespace around the assignment statements in my makefiles... this makes it clear that they are make assignments, not shell variable assignments (not that it shouldn't be clear anyway for anyone who knows makefile syntax, but...). In newer versions of GNU make, variable names cannot contain whitespace.

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