I have a Jenkinsfile that looks roughly like this:

...
stage('Build') {
     steps {
         script {
             FOO = sh (script: "echo bar", returnStdout: true)
         }
     }
}
stage('Plan') {
     steps {
         sh "FOO=${FOO} ./tests/run.sh"
     }
}
...

Why am I not seeing the variable $FOO within run.sh? This works in a regular shell and bash. Furthermore it's not like Jenkins isn't picking up the value of $FOO. It correctly prints the value of $FOO as "bar" in other places that I've used it.