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

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}"
      }
    }
  }
}