Is a Jenkinsfile valid standalone groovy? - jenkins

Question

I'm trying to wrap my head around how this declarative Jenkinsfile is Groovy. I want to write supporting code to execute this outside the Jenkins environment, in pure Groovy, if that's possible. I've been writing example groovy code but still am unsure what "pipeline", "agent", and "stages" are.
Any tips to understand this structure is appreciated
EDIT: I edited this question with simplified code below. I'm just wondering if there is a way that this can be turned into valid groovy code without the preprocessor/groovyshell environment that is utilized by Jenkins
pipeline {
stages {
// extra code here
}
}

Answer 1

No, you can't run Jenkinsfile as a standalone Groovy script. In short, Jenkins executes the pipeline code inside a pre-configured GroovyShell that knows how to evaluate things like pipeline, agent, stages, and so forth. However, there is a way to execute Jenkinsfie without the Jenkins server - you can use JenkinsPipelineUnit test library to write JUnit/Spock unit tests that will evaluate your Jenkinsfile and display the call stack tree. It uses mocks, so you can treat it as interaction-based testing, to see if a specific part of your pipeline gets executed. Plus, you can catch some code errors prior to running the pipeline on the server.
A simple unit test for the declarative pipeline can look like this:
import com.lesfurets.jenkins.unit.declarative.*
class TestExampleDeclarativeJob extends DeclarativePipelineTest {
#Test
void should_execute_without_errors() throws Exception {
def script = runScript("Jenkinsfile")
assertJobStatusSuccess()
printCallStack()
}
}
You can find more examples in the official README.md - https://github.com/jenkinsci/JenkinsPipelineUnit
Alternatively, you can try Jenkinsfile Runner command-line tool that can execute your Jenkinsfile outside of the Jenkins server - https://github.com/jenkinsci/jenkinsfile-runner
UPDATE
I edited this question with simplified code below. I'm just wondering if there is a way that this can be turned into valid groovy code without the preprocessor/groovyshell environment that is utilized by Jenkins.
Your pipeline code example looks like a valid Jenkinsfile, but you can't turned it into a Groovy code that can be run e.g. from the command-line as a regular Groovy script:
$ groovy Jenkinsfile
This won't work, because Groovy is not aware of the Jenkins Pipeline syntax. The syntax is added as a DSL via the Jenkins plugin, and it uses a dedicated GroovyShell that is pre-configured to interpret the pipeline syntax correctly.
If you are interested in checking if the syntax of the Jenkins Pipeline is correct, there are a few different options:
npm-groovy-lint (https://github.com/nvuillam/npm-groovy-lint) can validate (and even auto-fix) the syntax of your Jenkinsfile without connecting to the Jenkins server,
Command-Line Pipeline Linter (https://www.jenkins.io/doc/book/pipeline/development/#linter) can send your pipeline code to the Jenkins server and validate its syntax.
These are a few tools that can help you with catching up the syntax errors before you run the pipeline. But that's just a nice addon to your toolbox. The first step, as always, is to understand what the syntax means, and the official documentation (https://www.jenkins.io/doc/book/pipeline/syntax) is the best place to start.