use of variables in gnu makefile

Question

This is a GNU make question. Maybe a simple one, but I did search some textbooks before and didn't find the answer.

Short description of what I want to do:

copy a range of bytes from a file in a temporary file
calculate the checksum of this file with crc32 utility
just print the checksum in the build process for now
delete the temporary file

The problem I see is that every command is done in a separate shell, but I need a way to get input from the previous command to execute the next one.

eg:

/opt/rtems-4.11/bin/arm-rtems4.11-nm -a px_pmc.elf | grep bsp_section_start_begin | awk '{print $$1}'
/opt/rtems-4.11/bin/arm-rtems4.11-nm -a px_pmc.elf | grep _Check_Sum | awk '{print $$1}'

These commands will print in shell the limits of the range of bytes I want, but how do I store them in two variables, say low_limit/high_limit so I can copy that range in the temp file in the next make command ?

dd if=px_pmc.bin skip=low_limit bs=(high_limit-low_limit) count=1 of=temp.bin

(in C you can do this with a simple variable, I'm looking for the equivalent here)

regards, Catalin

It would help us understand more clearly what you're trying to do if you included the makefile you've tried to create. Renaud's answer is perfect, if the commands you're trying to run are all in a single recipe to build one target in your Makefile. — MadScientist
– MadScientist, Commented Dec 3, 2020 at 18:43

Renaud Pacalet · Accepted Answer · 2020-12-03 16:39:35Z

You can chain all your shell commands such that they are all executed in the same shell:

cmd1; cmd2; cmd3

If you prefer one command per line you can also use the line continuation (\):

cmd1; \
cmd2; \
cmd3

(be careful, no spaces after the \). And you can assign the output of a shell command to a shell variable:

a="`cmd1`"

So, the only subtlety here is that make will expand the recipe before passing it to the shell and this will eat all $ signs. If you want to preserve them such that they are passed to the shell, you must double them ($$):

a="`cmd1`"; \
b="`cmd2`"; \
cmd3 "$$a" "$$b"

In your case you can try this (untested):

target:
    low_limit="`/opt/rtems-4.11/bin/arm-rtems4.11-nm -a px_pmc.elf | grep bsp_section_start_begin | awk '{print $$1}'`"; \
    high_limit="`/opt/rtems-4.11/bin/arm-rtems4.11-nm -a px_pmc.elf | grep _Check_Sum | awk '{print $$1}'`"; \
    bs="`expr "$$high_limit" - "$$low_limit"`"; \
    dd if=px_pmc.bin skip="$$low_limit" bs="$$bs" count=1 of=temp.bin

I added the computation of high_limit-low_limit using expr. This should be more or less compatible with the bourne shell which is the default shell make uses.

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