
In order to find out Jenkins username, you need a plugin user build vars
Go to Jenkins settings

Section "Manage Plugins"

Go to the "Available" tab and specify "user build vars" in the search.

Install it.
An example of using a plugin from Pipeline:
pipeline {
agent any
stages {
stage('Output') {
steps {
script {
wrap([$class: 'BuildUser']) {
echo "Build was started by user: ${BUILD_USER}"
}
}
}
}
}
}
The module also has the following variables:
- BUILD_USER – Full name (first name + last name)
- BUILD_USER_FIRST_NAME – First name
- BUILD_USER_LAST_NAME – Last name
- BUILD_USER_ID – Jenkins user ID
- BUILD_USER_EMAIL – Email address