You need to install the lockable resources plugin to get this script to work https://wiki.jenkins.io/display/JENKINS/Lockable+Resources+Plugin PLUS ssh agent plugin https://wiki.jenkins.io/display/JENKINS/SSH+Agent+Plugin

Jenkinsfile.groovy

// Constants
// This is to ensure that a developer does not push to the wrong ready branch namespace.
TARGET_BRANCH_NAME = "master"
MERGE_BRANCH_PREFIX = "ready"
GIT_REPO= "GITREPO"
SSH_AGENT_ID="CREDENTIALS"
// State
inputSHA = "" // Why can't Jenkins tell me this?!
authorEmail = "none"
authorName = "none"
authorFull = "none <none>"

// Utils
//Does the branch namespace contain "ready", and should therefore be regarded as an integration branch.
def shouldMerge() {
    return "${BRANCH_NAME}" ==~ /^${MERGE_BRANCH_PREFIX}\/.*/
}


// Stages

println "Parameters: branch: ${BRANCH_NAME}, target: ${TARGET_BRANCH_NAME}, merge: ${shouldMerge()}"

lock("pip") {
    // The following stages are locked to ensure that only one pipeline runs at
    // at a time. The stages in here must contain everything between and
    // including the original git checkout and the final git push of the merged
    // branch.

  node{
    stage("Checkout") {
        deleteDir()
      buildNumber = currentBuild.number;
      //buildNumber = sh(script: "curl -sd '${TARGET_BRANCH_NAME}' ${MY_SQNZ_URL}", returnStdout: true).trim()
      currentBuild.displayName = "${currentBuild.displayName} (${buildNumber})"
      sshagent (credentials: ["${SSH_AGENT_ID}"]) {
          timeout(1) {
              sh "git clone --no-checkout ${GIT_REPO} ."
          }

          inputSHA = sh(script: "git rev-parse origin/${BRANCH_NAME}", returnStdout: true).trim()
          authorName = sh(script: "git log -1 --format='%an' ${inputSHA}", returnStdout: true).trim()
          authorEmail = sh(script: "git log -1 --format='%ae' ${inputSHA}", returnStdout: true).trim()
          authorFull = "${authorName} <${authorEmail}>"

          timeout(2) {

                  sh """
git config user.email "${authorEmail}"
git config user.name "${authorName}"
git checkout "${TARGET_BRANCH_NAME}"
if [ "\$(git branch --contains ${inputSHA} | wc -l)" -gt "0" ]
then
    echo "MERGE ERROR: origin/${BRANCH_NAME} already present in origin/${TARGET_BRANCH_NAME}"
    exit 1
fi
COMMITS="\$(git log --oneline ${TARGET_BRANCH_NAME}..${inputSHA} | wc -l)"
if [ "\${COMMITS}" -gt "1" ] || ! git merge --ff-only ${inputSHA}
then
    git reset --hard origin/${TARGET_BRANCH_NAME}
    git merge --no-ff --no-commit ${inputSHA}
    git commit --author "${authorFull}" --message "Merge branch '${BRANCH_NAME}' into '${TARGET_BRANCH_NAME}'"
fi
git submodule update --init --recursive
"""
    }

      sha = sh(script: "git rev-parse HEAD", returnStdout: true).trim()
      shaish = sh(script: "git rev-parse --short=5 HEAD", returnStdout: true).trim()


  }

}
stage("Build") {
        sh 'mvn clean package'
  }
  stage("Push") {
    sshagent (credentials: ["${SSH_AGENT_ID}"]) {
      timeout(1) {
        sh """
        git push origin ${TARGET_BRANCH_NAME}
        git fetch
        if [ "\$(git rev-parse origin/${BRANCH_NAME})" = "${inputSHA}" ]
        then
        git push origin :${BRANCH_NAME}
        fi
        """
      }
    }
      deleteDir()
  }
}
}