How do I pass variables between stages in a declarative Jenkins pipeline?

Question

How do I pass variables between stages in a declarative pipeline?

In a scripted pipeline, I gather the procedure is to write to a temporary file, then read the file into a variable.

How do I do this in a declarative pipeline?

E.g. I want to trigger a build of a different job, based on a variable created by a shell action.

stage("stage 1") {
    steps {
        sh "do_something > var.txt"
        // I want to get var.txt into VAR
    }
}
stage("stage 2") {
    steps {
        build job: "job2", parameters[string(name: "var", value: "${VAR})]
    }
}

Answer 1

If you want to use a file (since a script is the thing generating the value you need), you could use readFile as seen below. If not, use sh with the script option as seen below:

// Define a groovy global variable, myVar.
// A local, def myVar = 'initial_value', didn't work for me.
// Your mileage may vary.
// Defining the variable here maybe adds a bit of clarity,
// showing that it is intended to be used across multiple stages.
myVar = 'initial_value'

pipeline {
  agent { label 'docker' }
  stages {
    stage('one') {
      steps {
        echo "${myVar}" // prints 'initial_value'
        sh 'echo hotness > myfile.txt'
        script {
          // OPTION 1: set variable by reading from file.
          // FYI, trim removes leading and trailing whitespace from the string
          myVar = readFile('myfile.txt').trim()

          // OPTION 2: set variable by grabbing output from script
          myVar = sh(script: 'echo hotness', returnStdout: true).trim()
        }
        echo "${myVar}" // prints 'hotness'
      }
    }
    stage('two') {
      steps {
        echo "${myVar}" // prints 'hotness'
      }
    }
    // this stage is skipped due to the when expression, so nothing is printed
    stage('three') {
      when {
        expression { myVar != 'hotness' }
      }
      steps {
        echo "three: ${myVar}"
      }
    }
  }
}

Answer 2

Simply:

  pipeline {
        parameters {
            string(name: 'custom_var', defaultValue: '')
        }

        stage("make param global") {
             steps {
               tmp_param =  sh (script: 'most amazing shell command', returnStdout: true).trim()
               env.custom_var = tmp_param
              }
        }
        stage("test if param was saved") {
            steps {
              echo "${env.custom_var}"
            }
        }
  }