Declare and assign variable separately in Shell Script

What does this MR do?

This MR improves the consistency and maintainability, robustness of shell scripts in GitLab projects, make the scripts following the Shell scripting standards and style guidelines step by step.

This was originally something doing in gitlab-foss!31014 (comment 225759258), as @rpaik asked here(gitlab-foss!31014 (comment 225759258)), I'd like to continue shell script refactoring, but I can't comment there, so directly send another MR in the new repository, and separate different items into different MR, to minimize the effort on resolving conflicts.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content