In .gitlab-ci.yml it is possible to add a default before_script that runs before all script in defined jobs.
My question is why define before_script in a job instead of just using the script in the job? Is using before_script inside a job is only to override the default before_script, or are there other circumstances or reasons for it?
It's mostly useful for jobs to add or override steps when using include: (templates) and/or extends: and/or globals/default for before_script as you mentioned.
For example, you may create a hidden key job that defines script: intended to be extended by other jobs. Those jobs which extend it can provide a before_script: key without overriding script: or vice versa.
.install_dependencies:
before_script:
- pip install --upgrade pip
- pip install -r requirements.txt
my_test_job:
extends: .install_dependencies
script:
- pytest
So, it's just for composition of jobs. Otherwise, there is no difference. before_script: and script: are simply concatenated together when the job runs.
It's worth mentioning also that after_script: is significantly different than script:/before_script:. after_script runs in a separate shell instance and will run under different circumstances. See the docs for more info.