The before_script keyword in GitLab CI/CD allows you to define a list of shell commands that should be executed before the main script commands of a job. This is commonly used for setup tasks like:

• Logging information.
• Setting up environment variables.
• Installing dependencies.
• Authenticating to services.

You can define before_script at two main levels:

1. Globally (as a default): Using default:before_script:. These commands will run before every job unless a job overrides it.
2. Job-level: Defined within a specific job. This overrides any global before_script for that particular job.

Example .gitlab-ci.yml with before_script:

YAML

# .gitlab-ci.yml

# 1. Default before_script: Applied to all jobs unless overridden
default:
  image: alpine:latest # Using a common default image
  before_script:
    - echo "--- Default before_script (Global) ---"
    - echo "Current date: $(date)"
    - echo "Setting up common environment..."
    - export COMMON_SETUP_VAR="Set by default before_script"

stages:
  - setup
  - build
  - test

# Job 1: Uses the default before_script
job_with_default_before_script:
  stage: setup
  script:
    - echo "--- Main script for job_with_default_before_script ---"
    - echo "COMMON_SETUP_VAR is: $COMMON_SETUP_VAR"
    - echo "This job used the global before_script."

# Job 2: Overrides the default before_script with its own
job_with_custom_before_script:
  stage: build
  before_script: # Job-level before_script overrides the default one
    - echo "--- Custom before_script for job_with_custom_before_script ---"
    - echo "Performing specific setup for this job..."
    - apk add --no-cache curl # Example: installing a tool
    - curl --version
  script:
    - echo "--- Main script for job_with_custom_before_script ---"
    - echo "This job used its own custom before_script."
    - echo "COMMON_SETUP_VAR is: $COMMON_SETUP_VAR (This will be empty as default before_script didn't run)"

# Job 3: Disables before_script (even the default one)
job_without_any_before_script:
  stage: test
  before_script: [] # Explicitly set to an empty array to disable
  # You could also use `before_script: null` in some GitLab versions, but `[]` is safer.
  script:
    - echo "--- Main script for job_without_any_before_script ---"
    - echo "No before_script (neither default nor custom) was executed for this job."
    - echo "COMMON_SETUP_VAR is: $COMMON_SETUP_VAR (This will be empty)"

Code language: PHP (php)

Explanation:

1. default:before_script:
   • The default: block allows you to set default configurations for all jobs.
   • before_script: within default: defines a list of commands that will be executed before the main script of every job in the pipeline, provided the job doesn’t have its own before_script or explicitly disables it.
   • In this example:
     • It prints some introductory lines.
     • It prints the current date.
     • It simulates setting up a common environment variable COMMON_SETUP_VAR.
2. job_with_default_before_script:
   • This job does not have its own before_script keyword.
   • Therefore, it inherits and executes the commands from the default:before_script: section before its own script block runs.
   • You’ll see the output from the default before_script, and the COMMON_SETUP_VAR will be available in its main script.
3. job_with_custom_before_script:
   • This job defines its own before_script: block.
   • This job-level before_script completely overrides (replaces) the default:before_script:. The commands from default:before_script: will not run for this job.
   • Only the commands listed in this job’s before_script (e.g., installing curl) will execute before its main script.
   • Consequently, COMMON_SETUP_VAR (set in the default before_script) will not be defined or will be empty when this job’s main script runs.
4. job_without_any_before_script:
   • This job explicitly sets before_script: [] (an empty array).
   • This tells GitLab CI not to run any before_script commands for this job, not even the ones defined in the default:before_script: section.
   • The job will proceed directly to its script block.
   • COMMON_SETUP_VAR will be undefined or empty.

Key Concepts and Behavior:

• Purpose: before_script is intended for setup tasks that prepare the environment for the main script. It’s not for the core logic of your job.
• Execution Order: For any given job, if a before_script is active (either default or job-specific), its commands run first. If all commands in before_script succeed, then the commands in the job’s script section are executed.
• Failure Handling: If any command in before_script fails (exits with a non-zero status), the job is typically marked as failed immediately. The main script section for that job will not be executed.
• Array of Commands: before_script takes an array of strings, where each string is a shell command to be executed sequentially.
• Inheritance and Overriding:
  • Job-level before_script overrides default:before_script.
  • To prevent any before_script (including a global one) from running for a specific job, set before_script: [] or before_script: null (though [] is generally preferred for clarity and consistency).
• Common Use Cases:
  • Printing debug information (versions of tools, environment variables).
  • Logging into private repositories or services.
  • Installing required software packages or dependencies that are not part of the Docker image.
  • Setting dynamic environment variables needed by the main script.
  • Fetching configuration files.

Using before_script helps keep your main script section clean and focused on the primary task of the job, while common or job-specific setup steps are neatly organized.