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I've create a jenkins pipeline and it is pulling the pipeline script from scm.
I set the branch specifier to 'all', so it builds on any change to any branch.

How do I access the branch name causing this build from the Jenkinsfile?

Everything I have tried echos out null except

sh(returnStdout: true, script: 'git rev-parse --abbrev-ref HEAD').trim()

which is always master.

Answers

If you have a jenkinsfile for your pipeline, check if you see at execution time your branch name in your environment variables.

You can print them with:

pipeline {
    agent any

    environment {
        DISABLE_AUTH = 'true'
        DB_ENGINE    = 'sqlite'
    }

    stages {
        stage('Build') {
            steps {
                sh 'printenv'
            }
        }
    }
}

However, PR 91 shows that the branch name is only set in certain pipeline configurations:

• Branch Conditional (see this groovy script)
• parallel branches pipeline (as seen by the OP)