Makefile run an action before everything - gnu-make

Question

For building my target I have a list of prerequisites contained in a file list.txt and a script for generating this file generate-list.sh.
I need the script to be executed as first thing every time I invoke the make in order to have the list.txt updated and to give ti make the right list of prerequisites.
prebuild:
touch list.txt
.SECONDEXPANSION:
exe: prebuild $$(shell cat list.txt)
touch exe
<files in list.txt>:
<rules for generating these files>
In this way when I run make I first get an error from cat saying that list.txt does not exist, then list.txt is generated but since the cat failed the prerequisites contained in list.txt are not generated.

Answer 1

One method you could use, given that generate_list.sh must be executed at the very start every time, would be to explicitly execute it using the shell function. This would mean altering your makefile to something like
$(shell ./generate_list.sh > /dev/null)
.SECONDEXPANSION:
exe: $(shell cat list.txt)
touch exe
#echo $?
<files in list.txt>:
<rules for generating these files>
Executing this makefile produces
$ make
touch exe
deps.c test.c
where my generate_list.sh file contains
#!/bin/bash
touch test.c deps.c
echo deps.c test.c > list.txt
echo 'Created prerequisites list.'
Notes
/dev/null is included in $(shell ./generate_list.sh > /dev/null) incase your generate_list.sh produces an output as this would cause an error in make of
$ make
GNUmakefile:1: *** missing separator. Stop.
otherwise.
#echo $? shows that all of the prerequisites in list.txt are now included as prerequisites of exe.
Alternate Method Based on Auto Dependency Generation
What you are attempting to do is very similar to automatic dependency generation which can be accomplished using the -include directive in make. For future usage you may want to consider going down this route and altering your generate_list.sh script to create a makefile that can be included in your main makefile.