Jenkins.instance.getAllItems(AbstractProject.class).each {it ->
  println it.fullName;
}

The above script, did not give the complete list of jobs, I have tried the below command and it listed all jobs:

import jenkins.model.*
import hudson.model.*
Jenkins.instance.getAllItems(AbstractItem.class).each { println(it.fullName) };

Because of it doesn't show jobs in folders

There are 2 methods:

1) Recursion

import com.cloudbees.hudson.plugins.folder.*
void processFolder(Item folder) {
folder.getItems().each{
if(it instanceof Folder){
processFolder(it)
}else{
processJob(it)
}
}
}
void processJob(Item job){
println job.name
}

Jenkins.instance.getItems().each{
if(it instanceof Folder){
processFolder(it)
}else{
processJob(it)
}
}

2) Using getAllItems which is recursive by itself

import hudson.matrix.*
import jenkins.model.*;
import com.cloudbees.hudson.plugins.folder.*
Jenkins.getInstance().getAllItems().each {
// MavenModule is superfluous project returned by getAllItems()
if (!(it instanceof MavenModule || it instanceof MatrixConfiguration)) {
println it
}
}