Gitlab CI variables and script section give different results

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.

1. Why is filename present around grep?
2. Why are same git diff commands give different result?

>Solution :

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