Gitlab CI variables and script section give different results - gitlab-ci

Question

I've searched a lot of examples but they did not work for me.
I'm trying to run linters for changed files when MR is opened.
My .gitlab-ci.yml
run_linters:
image: python:3
variables:
FILES: git diff --name-only $CI_MERGE_REQUEST_TARGET_BRANCH_NAME | grep *.py
before_script:
- python3 -m pip install black==21.5b1
- python3 -m pip install flake8==3.9.2
script:
- echo $FILES
- git diff --name-only $CI_MERGE_REQUEST_TARGET_BRANCH_NAME | grep *.py
- black --check $FILES
- flake8 $FILES
only:
- merge_requests
And I'm getting strange output.
echo $FILES says git diff --name-only main | grep incoming_file.py
incoming_file.py is the only file in that MR. Why is it around grep?
And git diff at script section says fatal: ambiguous argument 'main': unknown revision or path not in the working tree.
Why is filename present around grep?
Why are same git diff commands give different result?

Answer 1

Why is filename present around grep?
In bash when you refer to * this will expand and try to match the files/directories present in your current path, in your case since only the incoming_file.py is present, so it expands to this.
Why are same git diff commands give different result?
variables:
FILES: git diff --name-only $CI_MERGE_REQUEST_TARGET_BRANCH_NAME | grep *.py
When you define a variable in variables section, Gitlab doesnt execute the command, it simple populates the variable FILES with the string git diff --name-only $CI_MERGE_REQUEST_TARGET_BRANCH_NAME | grep *.py
Then in the script section, the runner expands *.py to incoming_file.py and $CI_MERGE_REQUEST_TARGET_BRANCH_NAME to main
that's why in echo you see git diff --name-only main | grep incoming_file.py
Here
- git diff --name-only $CI_MERGE_REQUEST_TARGET_BRANCH_NAME | grep *.py
You actually execute the command and you get the mentioned message