Modern Jenkins Unit 3 / Part 3: Managing Secrets in GitHub

Encrypting files in the repo with Transcrypt


In production we will hopefully have a secrets management system, or at a very minimum private repos to store encrypted secrets in. For the purpose of this demo though I will be using a public repo and do not want to expose any sensitive data that we may need to add to this repo. WARNING: To be clear, storing any kind of secrets in a git repo, encrypted or not, may not be a good idea. Consult your local security team for advice. We however, don’t have any state secrets for this demo and very few options.


Transcrypt is a shell script that uses OpenSSL to encrypt and decrypt files in your git repo that are noted in the .gitattributes file. Let’s initialize our repo and confirm that it is working.

PWD: ~/code/modern-jenkins/

# On MacOS you can use brew
brew install transcrypt

# On Linux you have to install OpenSSL then place the script in your path
yum install openssl
# Download the script to our PATH
wget -O /usr/local/sbin/transcrypt
# Make it executable
sudo chmod +x /usr/local/sbin/transcrypt

# Confirm it works
trancrypt --help

NOTE: More information on the software can be found on the README here:

Initialize the repo

We will initialize Transcrypt within the repo on a clean branch. This will hopefully ensure that we can configure Transcrypt prior to adding any secrets to the repo.

PWD: ~/code/modern-jenkins/

# Check out a branch and initialize transcrypt
git checkout -b chore-install_transcrypt

# Encrypt using which cipher? [aes-256-cbc]
# Generate a random password? [Y/n]
# Repository metadata:
#   GIT_WORK_TREE:  /Users/matt.bajor/code/modern-jenkins
#   GIT_DIR:        /Users/matt.bajor/code/modern-jenkins/.git
#   GIT_ATTRIBUTES: /Users/matt.bajor/code/modern-jenkins/.gitattributes
# The following configuration will be saved:
#   CIPHER:   aes-256-cbc
#   PASSWORD: <a really good password, trust me>
# Does this look correct? [Y/n]
# The repository has been successfully configured by transcrypt.

Add the generated PASSWORD to your LastPass, keychain, text document on your desktop, or wherever you store secure passwords. It is very important to not lose that passphrase.

We now want to get the .gitattributes file added up and merged in so we can test if secrets encryption is working as expected so let’s branch, commit, push and PR.

PWD: ~/code/modern-jenkins/

git add .
git commit -m "Initialize Transcrypt in the repo"
git push origin chore-install_transcrypt

# PR, review, merge and then catchup master
git checkout master
git pull
git checkout -b test-secrets

Let’s create a secrets dir and initialize it with a README to confirm that transcrypt is working as expected without potentially exposing secrets on the internet. The way transcrypt works is that it looks at the .gitattributes in the repo root for files to encrypt. If it sees that there are paths to encrypt, it will encrypt them before adding to the tree. It does still allow you to view the changes as plaintext locally which sometimes can lead to confusion as to whether or not its working.

PWD: ~/code/modern-jenkins/

# Look at the .gitattributes
cat .gitattributes
# #pattern  filter=crypt diff=crypt
# Echo our new pattern in there
echo 'secure/* filter=crypt diff=crypt' >> .gitattributes
# Confirm that the file looks good:
cat .gitattributes
# #pattern  filter=crypt diff=crypt
# secure/* filter=crypt diff=crypt 

# Add a README with a bit of info
mkdir -p secure/
echo "# Transcrypt Secrets\n\n This repo is encrypted with transcrypt" > secure/

# Commit and push
git add .
git commit -m "Add test secrets"
git push origin test-secrets

When opening the PR, you should notice that there are two files changed:

  • .gitattributes
  • secure/

Transcrypt Commit

You should also notice that the contents of is encrypted. If this is the case, give yourself a thumbs up, merge the PR, and update your local branch.


NOTE: If you see that is not encrypted, do not open the PR. Instead, delete your branch and start over. Follow the instructions on the Transcrypt site if you need more help or information. It is important to confirm that the encryption mechanism is working the way we expect before we put secrets into the repo.

Let’s take this time to add a line to the file reminding us to check for unencrypted secrets:


# #### Contribution checklist
# - [ ] The branch is named something meaningful
# - [ ] The branch is rebased off of current master}

Now that the repo has been initialized with Transcrypt, we can begin adding some configuration that depends on credentials. In the next segment, we will create a machine user for GitHub and configure the Git plugin to use those credentials when cloning.

The repo from this section can be found under the unit3-part3 tag here:

Next Post: Configuring the Jenkins GitHub plugin programmatically with Groovy

Modern Jenkins Unit 3 / Part 2: Configure Jenkins URL

Configuring the Jenkins URL

Configure Jenkins URL with Groovy

Currently it just so happens that the Jenkins URL in the management console does have the proper config as it is set to http://localhost:8080. This is merely a coincidence that the default value and the current address match though. Once we start moving this thing around, it will be very important that it is set to the right value else we’ll have all kinds of strange issues. Since it will definitely have to be configured, let’s start here. It doesn’t hurt that it is a fairly simple example of configuring Jenkins with an environment variable passed by Docker Compose.

Passing from Docker Compose

This is a setting that will change from environment to environment and so I think the best place to set it is in the compose file. Let’s create a variable in there that we can read it in during init and make the right configuration. Edit the compose file and create a JENKINS_UI_URL env variable as well as volumes for the configs themselves.


## deploy/master/docker-compose.yml
## Define the version of the compose file we're using
#version: '3.3'
## Define our services
#  # Jenkins master's configuration
#  master:
#    image: modernjenkins/jenkins-master
#    ports:
#      - "8080:8080"
        - JENKINS_UI_URL=http://localhost:8080
#    volumes:
#      - plugins:/usr/share/jenkins/ref/plugins
#      - warfile:/usr/share/jenkins/ref/warfile
       - groovy:/var/jenkins_home/init.groovy.d
#  # Jenkins plugins' configuration
#  plugins:
#    image: modernjenkins/jenkins-plugins
#    volumes:
#      - plugins:/usr/share/jenkins/ref/plugins
#      - warfile:/usr/share/jenkins/ref/warfile
       - groovy:/usr/share/jenkins/ref/init.groovy.d
## Define named volumes. These are what we use to share the data from one
## container to another, thereby making our jenkins.war and plugins available
#  plugins:
#  warfile:

Now that it is set, we should be able to write a groovy init script to read it. But first, we will have to restart Jenkins to pick up the new environment:

PWD: ~/code/modern-jenkins

cd deploy/master
docker-compose down -v
docker-compose up -d

# Confirm that the variable is there:
docker inspect master_master_1 | grep JENKINS_UI

# Should output
# "JENKINS_UI_URL=http://localhost:8080",

With the environment variable now being available to us, we can use the script console at localhost to develop our script that will set the URL of our Jenkins instance.

URL: http://localhost:8080/script


import jenkins.model.*

// Read the environment variable
url = System.env.JENKINS_UI_URL

// Get the config from our running instance
urlConfig = JenkinsLocationConfiguration.get()

// Set the config to be the value of the env var

// Save the configuration

// Print the results
println("Jenkins URL Set to " + url)

This should output:

URL Output

Deploying our Groovy Jenkins Configs

We now have a working script that will manage the URL of Jenkins in any environment that we would be deploying into simply by setting a variable in the compose file. Now we need to make this available to the master so that it can be executed on startup. To do that, we will add a director to the plugins image and then mount it into the master in a similar way to how the war and plugins work.

PWD: ~/code/modern-jenkins/

cd images/jenkins-plugins
mkdir -p files/init.groovy.d/

# Add the file above to this directory as
# files/init.groovy.d/01-cofigure-jenkins-url.groovy

Add the full directory (instead of the individual scripts) of Groovy configuration to the Docker image and then export it as a volume.


#  # Install our base set of plugins and their dependencies that are listed in
#  # plugins.txt
#  ADD files/plugins.txt /tmp/plugins-main.txt
#  RUN `cat /tmp/plugins-main.txt`
   # Add our groovy init files
   ADD files/init.groovy.d /usr/share/jenkins/ref/init.groovy.d

#  # Export our war and plugin set as volumes
#  VOLUME /usr/share/jenkins/ref/plugins
#  VOLUME /usr/share/jenkins/ref/warfile
   VOLUME /usr/share/jenkins/ref/init.groovy.d
#  ...

We can now rebuild the image and pick up our new config that should (hopefully) configure the URL of our Jenkins on boot!

PWD: ~/code/modern-jenkins


cd deploy/master/
docker-compose down -v
docker system prune -f
docker-compose up -d
docker-compose logs -f

Ok you should now see that the URL in the Jenkins management console is set to http://localhost:8080! Ahhhh, it was like that before? Hmm. Ok well then let’s break it to confirm it is working. Modify the value in the compose file to something different and restart Jenkins:

PWD: ~/code/modern-jenkins/deploy/master

# Change the JENKINS_UI_URL to something different in docker-compose.yml
perl -pi -e 's/JENKINS_UI_URL=.*/JENKINS_UI_URL=http:\/\/derpyhooves/g' docker-compose.yml

# Restart the stack
docker-compose down -v
docker-compose up -d
docker-compos logs

Homer Fatfinger

Did that work? A typo you say? I can’t imagine it. I’ve typed docker-compose over 1000 times today, it’s not possible for me to misspell it. In addition, if you notice the difference between this set of commands and the one earlier, we seem to be drifting to chaos. Let’s take note of that, but address it after we confirm that this current change is working as expected.

Gooood, it does work :) The URL in the management console has been updated to the new, wrong, custom value so we can confirm it works. Let’s commit our code now, but attend to that tiny little pile of tech debt we found (starting and stopping the system differently every time is definitely tech debt). NOTE: Don’t forget to set the JENKINS_UI_URL back to http://localhost:8080

PWD: ~/code/modern-jenkins/

git checkout -b feat-configure_jenkins_url
git add .
git commit -m "Configure the Jenkins URL with Groovy

This change adds an environment variable to the compose file
that sets the URL of the Jenkins instance upon boot. This is
done via the script added to init.groovy.d"



So we’ve noticed something a bit stinky in our code as we’ve been going about our business. This happens very often in our work lives and attending to little tech debts like this is critical to having quality software. I certainly encourage everyone to leave the code better than they found it and to refactor things when they see something turning into a turd like object.

I also encourage you to make note of these things and take care of them after you are in a place to save game. Switching context between one problem and another can be very expensive mind and time wise so feel free to take a note, create a ticket or something, then finish what you are doing (unless there is an actual issue that needs to be addressed before your code will work). When you submit your PR for review, then jump on that Jira and refactor your heart away.

We need to do everything, but we can only do one thing at a time. Try to be aware of time management.

Let’s get that squirrel

What we noticed was that we were beginning to type the command differently every time we did it. That seems like it should be replaced by a shell script. Let’s create a start script so that this thing starts consistently every time:


#!/bin/bash -el

echo "INFO: (re)Starting Jenkins"
docker-compose down -v
docker-compose up -d

echo "INFO: Use the following command to watch the logs: "
echo "docker-compose logs -f"

Write that guy out to deploy/master/ and make it executable with chmod +x deploy/master/ and we’ve now got ourselves a script that will consistently restart our app.

Add that onto the branch with a good message and push, PR, merge, etc.. See, you’re getting the hang of it!

Next let’s get ready to handle some secrets. Shhhhhh!

The repo from this section can be found under the unit3-part2 tag here:

Next Post: Managing Jenkins secrets in GitHub with Transcrypt

Modern Jenkins Unit 3 / Part 1: Intro to the Jenkins Groovy Init System

Professor Frink Configuring Bender Image from Simpsons Mathematics

Configuring the Jenkins Master on Boot with Groovy

One of the greatest strengths of Jenkins is it’s ability to do almost anything. This comes from it’s extremely customizable nature. I think of it as a scheduler that can do any given task on any given trigger with enough configuration. What we are building specifically though is a system to build, test, and deploy our piece of custom software that most likely is at least a little bit different than anything else out there requiring the same tasks. We will need to use Jenkins’ ultra powerful customization toolkit, but do it in a way that strives to be:

  1. Deterministic: Given a set of inputs, there should be only one output. If that output is not as we expect, there is a problem that we should spend time fixing. ie: removing “flaky” tests that fail for “no reason”.

  2. Reliable: The system should have high availability to the users who depend on it. Having a system that is sometimes down and sometimes up does not encourage teams to inject automated builds into their workflows.

  3. Repeatable: This system should be able to be recreated without persistent data from the repo.

  4. Agile: The system should evolve to meet the needs of it’s consumers in a sustainable way. If one team’s jobs or configs are breaking another team’s pipelines, it is a good indication that it is time to split the monolith into two independent systems.

  5. Scalable: As the system becomes more popular, more people are going to utilize it. When this happens, it’s critical to be able to support the increased capacity in not only job runners, but also collaboration from more teams.

Luckily we can treat the code that configures the system in the same way we treat the code the builds and runs the system :)

Intro to the Jenkins init system

Jenkins has a not-much-talked about feature that I have yet to see much information on. It is the Jenkins Groovy Init system and really the only documentation I have been able to find are two articles on the Jenkins wiki:

  Post-initialization script
  Created by Kohsuke Kawaguchi, last modified by Daniel Beck on Dec 10, 2015
  You can create a Groovy script file $JENKINS_HOME/init.groovy, or any .groovy
  file in the directory $JENKINS_HOME/init.groovy.d/, (See Configuring Jenkins
  upon start up for more info) to run some additional things right after Jenkins
  starts up. This script can access classes in Jenkins and all the plugins. So for
  example, you can write something like:
  import jenkins.model.*;
  // start in the state that doesn't do any build.
  Output is logged to the Jenkins log file. For Debian based users, this is

which points to this:

  Jenkins can execute initialization scripts written in Groovy if they are present
  during start up. See Groovy Hook Script for details. The hook name for this
  event is "init". Those executions happen at the very end of the initialization,
  and therefore this can be used to pre-configure Jenkins for a particular OEM
  While one can always write a plugin to participate in the initialization of
  Jenkins, this script-based approach can be useful as it doesn't require any
  compilation and packaging.

Not super impressive documentation considering how powerful this mechanism is. Using this init system you are able to configure any aspect of the master that you are able to using “Manage Jenkins”. This includes (but is not limited to):

  • The URL and name of this instance
  • Authentication and security settings
  • Secrets in the credential store
  • Global plugin configuration
  • Global tool configuration
  • Adding and removing nodes
  • Creation of jobs (though we’ll only use it to create one special job)

Groovy Configuration

Jenkins Groovy Script Console

The Groovy Script Console

Not only does it support configuring so much of the system, it has a direct REPL like environment to code in. This is called the “Script Console” (available at http://localhost:8080/script on our instance) and can be considered a shell into the Jenkins system. This shell has the same abilities of the init system’s shell so it makes for super quick and easy development of the scripts that we will use.

Jenkins Groovy Hello World

Let’s kill two stones with one bird. We will do a quick little Hello World that will introduce you to bot the syntax of groovy as well as how to use the script console.

  • Stand up your development Jenkins (cd deploy/master && docker-compose up -d)
  • Browse to the script console at http://localhost:8080/script
  • Enter the following into the box in front of you:

URL: http://localhost:8080/script

  import jenkins.model.*

  def jenkins = Jenkins.getInstance()
  jenkins.setSystemMessage("I'm Bender, baby! Oh god, please insert liquor!")
  // You can change the message if you please. I'm not at the office :)

  • Browse back to the main Jenkins interface
  • Check out the cool message for all the users of your system to see. I bet your boss will love it!

I'm Bender Baby!

Nothing too crazy, but this should give you a good idea of how we are going to configure our master to build our particular brand of software. Inside Old Mr. Jenkins is just a series of objects (I think of them as his organs) that we can grab and squeeze and modify to fit our needs. I hope Janky is ready to play “Operation”!

Next Post: Configure Jenkins URL with Groovy on boot