Branch selection and branch specifier in Jenkins - Step-by-Step Execution

Process Flow - Branch selection and branch specifier

Start Jenkins Pipeline

↓

Read Branch Specifier

↓

Check Branch Exists?

No→Fail or Wait

Yes↓

Checkout Branch

↓

Run Pipeline Steps on Selected Branch

↓

End Pipeline

The Jenkins pipeline reads the branch specifier, checks if the branch exists, then checks out that branch to run the pipeline steps.

Execution Sample

Jenkins

pipeline {
  agent any
  stages {
    stage('Checkout') {
      steps {
        checkout([$class: 'GitSCM', branches: [[name: 'refs/heads/main']]])
      }
    }
  }
}

This pipeline checks out the 'main' branch from Git before running further steps.

Process Table

Step	Action	Branch Specifier	Branch Exists?	Result
1	Start Pipeline	refs/heads/main	N/A	Pipeline started
2	Read Branch Specifier	refs/heads/main	N/A	Branch specifier read
3	Check Branch Exists	refs/heads/main	Yes	Branch found
4	Checkout Branch	refs/heads/main	Yes	Branch 'main' checked out
5	Run Pipeline Steps	refs/heads/main	Yes	Pipeline runs on 'main' branch
6	End Pipeline	refs/heads/main	Yes	Pipeline completed

💡 Pipeline ends after running steps on the specified branch.

Status Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
branchSpecifier	undefined	refs/heads/main	refs/heads/main	refs/heads/main	refs/heads/main
branchExists	undefined	undefined	true	true	true
checkoutStatus	undefined	undefined	undefined	success	success

Key Moments - 2 Insights

Why does the pipeline fail if the branch specifier is incorrect?

Because in the execution_table at Step 3, if 'branchExists' is 'No', Jenkins cannot checkout the branch, causing failure.

Can the branch specifier be changed dynamically during the pipeline?

No, the branch specifier is read at the start (Step 2) and used for checkout; changing it later won't affect the already checked out code.

Visual Quiz - 3 Questions

Look at the execution table, what is the branchExists value at Step 3?

A No

B Yes

C Undefined

D Error

💡 Hint

Check the 'branchExists' column in row for Step 3 in execution_table.

At which step does Jenkins perform the actual branch checkout?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Look for 'Checkout Branch' action in execution_table.

If the branch specifier was 'refs/heads/dev', how would the variable 'branchSpecifier' change after Step 2?

A It would be 'refs/heads/dev'

B It would be 'refs/heads/main'

C It would be undefined

D It would be empty string

💡 Hint

Refer to variable_tracker 'branchSpecifier' value after Step 2.

Concept Snapshot

Jenkins branch specifier defines which Git branch to checkout.
It uses syntax like 'refs/heads/main' to specify branches.
Pipeline reads this specifier early, checks if branch exists.
If branch exists, Jenkins checks it out and runs pipeline steps.
Incorrect specifier causes checkout failure and pipeline error.

Full Transcript

In Jenkins pipelines, the branch specifier tells Jenkins which Git branch to use. The pipeline starts by reading this specifier, for example 'refs/heads/main'. Jenkins then checks if this branch exists in the repository. If it does, Jenkins checks out the branch so the pipeline can run on that code. If the branch does not exist, the pipeline fails because it cannot checkout the code. The branch specifier is fixed at the start of the pipeline and cannot be changed dynamically during execution. This process ensures the pipeline runs on the correct branch code.