Run a command once on each Jenkins slave using the CloudBees Workflow / Jenkins Pipeline plugin

once-per-jenkins-slave.groovy

// The ArrayList of slaves is not serializable, so fetching them should be marked as @NonCPS so that
// no attempt is made to serialize and save the local state of the function. See here for details:
// https://github.com/jenkinsci/pipeline-plugin/blob/master/TUTORIAL.md#serializing-local-variables
@NonCPS
def getSlaves() {
    def slaves = []
    hudson.model.Hudson.instance.slaves.each {
        slaves << it.name
    }
    return slaves
}

// Run a command on each slave in series
getSlaves().each {
    node(it) {
        sh "hostname"   
    }
}

// Run a command on each slave in parallel
def jobs = [:]
getSlaves().each {
    
    // Use a local variable to avoid closing over a reference to 'it',
    // the value of which changes on each iteration
    def slave = it
    
    // Create a closure for each slave and put it in the map of jobs
    jobs[slave] = {
        node(slave) {
            sh "hostname"
        }
    }
}

// Run the closures in parallel
parallel jobs

I've found (at least with Jenkins 2, Workflow plugin 2.4, Groovy sandboxed) this second each iteration doesn't work. Using a for loop, however, does. e.g.

for (String slave : getSlaves()) {
    node(slave) {
        sh "hostname"   
    }
}