Jenkins pipeline if else not working

I am creating a sample jenkins pipeline, here is the code.

pipeline {
    agent any 

    stages {    
        stage('test') { 
            steps { 
                sh 'echo hello'
            }            
        }
        stage('test1') { 
            steps { 
                sh 'echo $TEST'
            }            
        }
        stage('test3') {
            if (env.BRANCH_NAME == 'master') {
                echo 'I only execute on the master branch'
            } else {
                echo 'I execute elsewhere'
            }                        
        }        
    }
}

this pipeline fails with following error logs

Started by user admin
org.codehaus.groovy.control.MultipleCompilationErrorsException: startup failed:
WorkflowScript: 15: Not a valid stage section definition: "if (env.BRANCH_NAME == 'master') {
                echo 'I only execute on the master branch'
            } else {
                echo 'I execute elsewhere'
            }". Some extra configuration is required. @ line 15, column 9.
           stage('test3') {
           ^

WorkflowScript: 15: Nothing to execute within stage "test3" @ line 15, column 9.
           stage('test3') {
           ^

But when i execute the following example from this url, it executes successfully and print the else part.

node {
    stage('Example') {
        if (env.BRANCH_NAME == 'master') {
            echo 'I only execute on the master branch'
        } else {
            echo 'I execute elsewhere'
        }
    }
}

The only difference i can see is that in the working example there is no stages but in my case it has.

What is wrong here, can anyone please suggest?

your first try is using declarative pipelines, and the second working one is using scripted pipelines. you need to enclose steps in a steps declaration, and you can't use if as a top-level step in declarative, so you need to wrap it in a script step. here's a working declarative version:

pipeline {
    agent any

    stages {
        stage('test') {
            steps {
                sh 'echo hello'
            }
        }
        stage('test1') {
            steps {
                sh 'echo $TEST'
            }
        }
        stage('test3') {
            steps {
                script {
                    if (env.BRANCH_NAME == 'master') {
                        echo 'I only execute on the master branch'
                    } else {
                        echo 'I execute elsewhere'
                    }
                }
            }
        }
    }
}

you can simplify this and potentially avoid the if statement (as long as you don't need the else) by using "when". See "when directive" at https://jenkins.io/doc/book/pipeline/syntax/. you can also validate jenkinsfiles using the jenkins rest api. it's super sweet. have fun with declarative pipelines in jenkins!

It requires a bit of rearranging, but when does a good job to replace conditionals above. Here's the example from above written using the declarative syntax. Note that test3 stage is now two different stages. One that runs on the master branch and one that runs on anything else.

stage ('Test 3: Master') {
    when { branch 'master' }
    steps { 
        echo 'I only execute on the master branch.' 
    }
}

stage ('Test 3: Dev') {
    when { not { branch 'master' } }
    steps {
        echo 'I execute on non-master branches.'
    }
}

If you wanted to create a condition to execute only a stage based on expression you can use keyword when

stage ('test3'){
    when { expression { return env.BRANCH_NAME == 'master'} }
    steps {
           echo 'I only execute on the master branch.'
          }
    }
}

With the expression key word you can add any condition.

e.g. if stage is dependent on generated file in workspace.

stage ('File Dependent stage'){
 when { expression { return fileExists ('myfile') } }
    steps {
           echo "file exists"
          }
    }
}

        if ( params.build_deploy == '1' ) {
            println "build_deploy 是 ${params.build_deploy}"
              jobB = build job: 'k8s-core-user_deploy', propagate: false, wait: true, parameters: [
                         string(name:'environment', value: "${params.environment}"),
                         string(name:'branch_name', value: "${params.branch_name}"),
                         string(name:'service_name', value: "${params.service_name}"),                      
                     ]
            println jobB.getResult()
        }