How to use if-else condition on gitlabci

Hereunder three syntax options for that kind of statement. From gitlab-ci documentation :

Using shell variable

deploy-dev:
image: testimage
environment: dev
tags:
 - kubectl
script:
 - if [ "$flag" == "true" ]; then MODULE="demo1"; else MODULE="demo2"; fi
 - kubectl apply -f ${MODULE} --record=true

Using shell variable with yaml multiline block

deploy-dev:
image: testimage
environment: dev
tags:
  - kubectl
script:
  - >
    if [ "$flag" == "true" ]; then
      kubectl apply -f demo1 --record=true
    else
      kubectl apply -f demo2 --record=true
    fi

Using gitlab rules

workflow:
  rules:
    - if: '$CI_PIPELINE_SOURCE == "schedule"'
      when: never
    - if: '$CI_PIPELINE_SOURCE == "push"'
      when: never
    - when: always

Using gitlab templates and variables

demo1-deploy-dev:
  extends: .deploy-dev
  only:
    variables: [ $flag == "true" ]
  variables:
    MODULE: demo1
        
demo2-deploy-dev:
  extends: .deploy-dev
  only:
    variables: [ $flag == "false" ]
  variables:
    MODULE: demo2
        
.deploy-dev:
image: testimage
environment: dev
tags:
  - kubectl
script:
  - kubectl apply -f ${MODULE} --record=true

Note that with GitLab 13.3 (August 2020), there is an improvement to the if-else rule syntax:

CI/CD rules:if support logical expressions with parentheses

If you use the rules keyword with if clauses, it’s now even more powerful, with support for bracketed expressions evaluated by the pipeline processor.

You can use more complex and efficient AND (&&) / OR (||) expressions, making your pipelines rules more logical, powerful, and easier to manage.

See Documentation and Issue.

And, with GitLab 13.8 (January 2021)

Support variables for pipeline rules

Previously, the rules keyword was limited in scope and only determined if a job should be included or excluded from pipelines. In this release, you can now decide if certain conditions are met and subsequently override variables in jobs, providing you with more flexibility when configuring your pipelines.

See Documentation and Issue.

With GitLab 13.12 (May 2021):

Support variables in CI/CD pipeline 'workflow:rules'

Previously, the rules keyword was limited in scope and only determined if a job should be included or excluded from pipelines. In 13.8, we added the ability to use the variables keyword with rules to set variable values in a job based on which rule matched.

In this release we’ve extended this ability to workflow: rules, so you can set variable values for the whole pipeline if certain conditions match.
This helps you make your pipelines even more flexible.

See Documentation and Issue.