Jenkins declarative when condition to check if a variable is NULL - jenkins-pipeline

Question

I want to skip Build stage if AMI already exists using declarative syntax.
stage('Build') {
environment {
AMI = sh(returnStdout: true, script: 'aws ec2 describe-images').trim()
}
when {
expression { AMI = null }
}
steps {
sh 'packer build base.json -machine-readable'
}
}
But when I'm running this pipeline I get groovy.lang.MissingPropertyException: No such property: AMI for class: groovy.lang.Binding
At the same time scripted pipeline works perfectly fine
stage('Build') {
steps {
script {
env.AMI = sh(returnStdout: true, script: 'aws ec2 describe-images').trim()
if (env.AMI == '') {
sh 'packer build base.json -machine-readable'
}
}
}
}
}
I'd really love to switch to the declarative pipelines just stuck with this condition. Any help is really appreciated. Thanks
I tried a lot things without any luck
when {
expression {
return AMI.isEmpty()
}
}
when {
not {
expression {
AMI == ''
}
}
when {
not {
expression { env.AMI }
}
}
Nothing works. I suspect it is somehow related to env variable association through sh

Answer 1

You can do something like this.
pipeline {
agent any
stages {
stage('Build') {
when {
expression {
return isAMIAvailable()
}
}
steps {
sh 'packer build base.json -machine-readable'
}
}
}
}
def isAMIAvailable() {
AMI = sh(returnStdout: true, script: 'aws ec2 describe-images').trim()
return AMI == null
}

Answer 2

With #ycr help I was able to build it!
Just in case here is the whole thing
pipeline {
agent any
environment {
ENV = 'dev'
}
stages {
stage('Build') {
when { not { expression { return isAMIAvailable() } } }
steps {
sh 'packer build base.json -machine-readable'
}
}
}
}
def isAMIAvailable() {
AMI = sh(returnStdout: true, script: "aws ec2 describe-images --owners self --filters 'Name=name,Values=base-${ENV}-1' --query 'Images[*].[Name]' --output text").trim()
if (AMI == '') {
return AMI == null
}
return AMI
}