Why is parameter not propagated to post-build parameterized build?

Question

I'm trying to apply the guidance I recently read at how to trigger parameterized builds from a different build in Jenkins and https://plugins.jenkins.io/parameterized-trigger/ - but this doesn't seem to work, and I'd like to ask the community for help understanding what I'm doing wrong.

My jobs are set up thus:

• foo2 is a non-parameterized build
  • foo2's Build is Execute shell with commands:

  string1="123abc"
  export string2="456def"
  

  • foo2's Post-build Action is Trigger parameterized build on other projects
    • It triggers foo3
    • It attempts to pass parameters to foo3 with Predefined parameters:

    a_string=${string1}
    b_string=${string2}
    

• foo3 is a parameterized build with two string paramters: a_string and b_string.
  • foo3's Build step is Execute shell with commands:

  echo ${a_string}
  echo ${b_string}
  

Question/Problem: when foo3 is triggered, its console output are the string literals ${string1} and ${string2}, instead of the desired 123abc and 456def -- what am I doing wrong? How can I pass variables set during foo2's Build stage to foo3?

Output of foo3 build:

Started by upstream project "foo2" build number 4
originally caused by:
 Started by user unknown or anonymous
Running as SYSTEM
Building remotely on builder-231 (docker-cleanup) in workspace /home/user/workspace/foo3
[foo3] $ /bin/sh -xe /tmp/jenkins2920927890739081404.sh
+ echo ${string1}
${string1}
+ echo ${string2}
${string2}
Finished: SUCCESS

---

Screenshots:

foo2's Build and Post-build action to trigger foo3:

---

foo3's parameters:

---

foo3's Build

Answer 1

Use a Pipeline.

pipeline {
  agent any
stages {
stage('run job foo3') { 
  steps {
    script {

      string1 = "123abc"
      string2 = "456def"

      build job: 'foo3', 
        parameters: [
          // both is possible
          string(name: 'a_string', value: "${string1}"),
          string(name: 'b_string', value: string2)
        ]
        wait: true

     }
   }
 }

}
 }