Hi Don, the creation of the missing file.list and the naming of the variable section_task3 was just for this sharing, as that file.list was created by the application, not by the bash completion.
Should have placed it outside the function, sorry.
Naming it 3 was a typo, should have been 2, which was corrected while you wrote.

The OPTS (-a -b) are just used as metapher for all the options there are, but its right, should have shorten it to '-c' only, for easier seperation.
Anyhow, the issue is, i want to morph the list of that file (abc def ghi ...) so that each entry of that list starts with letter 'b', as in babc, bdef bghi.

To be honest, i barley understood any of your question that were bash-completion specific, i just copied and modified the file, and played around trying to match/fit my needs.
As: Saying: I 'know' it works the way it is, but i dont know why or how exactly...
I thought prev="${COMP_WORDS[COMP_CWORD-1]}" is an integrated expression, similar to vars="${@[${#@}-1]}"

So i've adapted your suggestion, and, at least its not using 'b' as a single letter anymore.

Code:

+ files $ source ./reproducable.bash 
:) files $ XY -c a
a1024  a128   a192   a256   a384   a512   a768   a96    
:) files $ XY -c b

When i try to print the content of section_task2, i get this:

Code:

^C
130 files $ XY -c 


a  b  
130 files $ XY -c a


a1024  a128   a192   a256   a384   a512   a768   a96    
130 files $ XY -c b






bash: XY: Command not found.
127 files $

With this code on top:

Code:

	[ -f file.list ] || printf "%s\n" "section_task2=\"abc def ghi jkg lmn opq rst uvw xyz\"" > file.list
	section_task1="a96 a128 a192 a256 a384 a512 a768 a1024"
	#section_task2="b$(grep section_task2 file.list | sed s,section_task2=,, | sed s,\ ,\nb,)"
	section_task2=$(sed -e '/^section_task2=/!d' -e 's/^[^=]*="//' -e 's/"$//' -e 's/^/b/' -e 's/ /\b/g' file.list)
	set +x
	echo $section_task2 ##; return

Seems section_task2 is empty?

A final word about the use of array while its declared (set) as variable, have a look at the git bash completion, i copied from there.
I'm not saying its a temporary solution, but its working as is. -> never change a running system you dont fully understand
Yet i have/want to change this behaviour for better user support.

Thank you