Trigger Jenkins build when pull request is merged in Github

This should be an easy, out-of-the-box configuration in Jenkins but I haven't found anything straightforward on the internet. All I want to do is a trigger a build ONLY when a pull request i merged in our Github repo.

To start with, Github aggregates almost all activity around the pull request into one webhook (versus bitbucket which allows you to differentiate between actions).

On the Jenkins side I've seen posts point towards the Generic Webhook Plugin which allows you to ingest the json of the webhook and create variables, however from here it looks like those need to be used in a script in order to trigger/not trigger a build.

Github Pull Request Build is another popular plugin, but again there is nothing explicit that states "only trigger this build when a PR is merged" or even seems to give the option of looking for a specific value in the webhook json.

Unless there are other plugins out there I haven't found the best option (i.e. least configuration to just get the build started) is to configure the GitHub hook trigger for GITSCM polling in Jenkins and on the Github side send the webhook only on push events... however this isn't the exact behavior we're looking for.

Right now this is all being done via the UI, and it's been awhile since I've used Jenkins so maybe the declarative pipeline infrastructure has passed the UI by, but it seems like this should be much more intuitive. Can someone explain the easiest implementation they've found, using Jenkins and Github, to trigger a build ONLY when a pull request is merged to a specific branch?

No need for webhooks anymore, since you now have GitHub Actions (assuming you are using github.com, although the Actions are coming with GHE, GitHub Enterprise, in Beta, starting Sept. 2020).

As explained on this thread, you can trigger, on GitHub side, a job when a pull request is merged on master:

on:
  pull_request:
    branches:
      - master
    types: [closed]

jobs:
  <job id>:
    if: github.event.pull_request.merged == true
    steps: // the rest of the code

And that job can then use a GitHub Action like Trigger Jenkins Job for GitHub Actions, which will call your Jenkins and trigger one or several Jenkins jobs.

jobs:

  build:
    name: Build
    runs-on: ubuntu-latest
    steps:
    - name: trigger single Job
      uses: appleboy/jenkins-action@master
      with:
        url: "http://example.com"
        user: "example"
        token: ${{ secrets.TOKEN }}
        job: "foobar"

After discussion with the OP and following the GitHub Actions tutorial, I confirm that:

• Jenkins does not have to be started with Docker, it only has to be addressable from GitHub through its public URL;
• The "trigger remote build" needs to be activated on Jenkins;
• a token needs to be generated for the Jenkins user with the right to launch the Jenkins job;
• A file triggerJenkinsBuild.yml (or any other name you want) must be created in the folder .github/workflows in your GitHub repository, with the two YAML sections mentioned above;
• The "url:" field is just the base URL of the Jenkins instance, no extra path.