It is hard to comment when we don't know what the complete or compgen commands look like and we don't know how COMP_CWORD and the COMP_CWORDS array are set when you enter _XY_module(). But, a few things look strange:

You initialize OPTS to a string:

Code:

OPTS="-a -b"

but you refer to it as an array:

Code:

COMPREPLY=( $(compgen -W "${OPTS[*]}" -- $cur) )

Decide whether it is an array or a string and use it consistently in your script.

You call: compgen -W "${OPTS[*]}" -- $cur
which will expand to: compgen -W "-a -b" -- $cur
You also call: compgen -W "a b" -- $cur
Does the compgen -W option expect an option-argument both with and without hyphens?

If the file file.list does not exist, you create that file and put a definition for the variable section_task2 in it. In the trace output you showed us, the file does exist and only contains a definition for the variable section_task3.

Two lines later in your script you set the variable section_task2 to the string to which section_task2 had been defined in that file with a leading b added and the 1st (and only the 1st) space changed to a <newline> character or to the letter n (depending on what operating system and version of sed you're using) followed by the letter b. I would have guessed that if you were going to change any spaces in that string, you would want to change them all. But, since there is no section_task2 defined in that file, section_task2 in your script is defined to just be the string b.

In the trace log and in the code you use if the file doesn't exist, you double quote the string that is placed in the file as the value of section_task2 or section_task3. Your sed script doesn't remove those quotes!

And, why are you using two sed commands, when you could use a single sed command with two substitute commands? (Of course, you could also skip the grep command, if your sed script would delete lines that don't start with the variable you're interested in before processing the substitute commands.) Assuming that the variable you're looking for in the file is section_task2 and not section_task3, maybe you would want something like:

Code:

section_task2=$(sed -e '/^section_task2=/!d' -e 's/^[^=]*="//' -e 's/"$//' -e 's/^/b/' -e 's/ /\
b/g' file.list)