Dynamic Environment Variables in Jenkins

Jenkins is a powerful automation server, and a key aspect of automation is configuring the environment in which jobs execute. Often, you’ll need to set environment variables dynamically – meaning their values aren’t fixed but are determined during the build process itself, perhaps by running a shell command or retrieving information from an external source. This tutorial explores various techniques for accomplishing this in Jenkins.

Understanding Environment Variables

Environment variables are named values that influence the behavior of processes. They provide a way to configure applications without modifying the application code itself. In Jenkins, environment variables can be used to:

Pass information between build steps
Configure tools and scripts
Control the build process

Setting Static Environment Variables

Before diving into dynamic variables, let’s quickly review setting static ones. Jenkins offers several ways to define static environment variables:

Global Properties: Navigate to Manage Jenkins > Configure System > Global properties > Environment variables. Adding variables here makes them available to all jobs on the Jenkins instance.
Job Configuration: Within a specific job’s configuration, you can define environment variables under the “Build Environment” section. These are specific to that job.

However, these methods are limited when you need variables whose values change with each build.

Dynamically Setting Environment Variables

The true power comes from dynamically setting environment variables during the build process. Here are a few effective techniques:

1. Using Shell Scripting within Build Steps

The simplest approach is to use a shell script build step to compute the variable’s value and then export it.

#!/bin/bash
# Compute the value
AOEU=$(echo aoeu)

# Export the variable (important!)
export AOEU

# You can then use $AOEU in subsequent build steps
echo "The value of AOEU is: $AOEU"

This script first computes the desired value for AOEU and then uses export to make it available as an environment variable for all subsequent build steps in that job.

2. Utilizing the EnvInject Plugin

The EnvInject plugin provides a flexible way to inject environment variables into the build process. This is particularly useful for more complex scenarios.

Create a Properties File: In a "Execute shell" build step, write the variable assignments to a properties file:
```
echo AOEU=$(echo aoeu) > propsfile
```
Inject the Variables: Add an "Inject environment variables" build step, specifying the "Properties File Path" as propsfile. The plugin will read the file and set the variables accordingly.

3. Jenkins Pipeline Scripting

For Jenkins Pipeline users, setting environment variables is even more streamlined. Within a script block, you can directly define environment variables using the environment keyword:

stage('Build') {
    environment {
        AOEU = sh(returnStdout: true, script: 'echo aoeu').trim()
    }
    steps {
        sh 'env'
        sh 'echo $AOEU'
    }
}

sh(returnStdout: true, script: 'echo aoeu') executes the shell command and captures its output.
.trim() removes any leading or trailing whitespace.
The environment block defines the variable AOEU and its value.

4. Ant Build Scripts

If you are using Ant as part of your build process, you can access environment variables through the env property.

First, make sure the environment variable is set as described in one of the methods above. Then, within your build.xml file:

<property environment="env"/>
<property name="jmeter.home" value="${env.JMETER_HOME}"/>

<echo message="JMeter Home: ${jmeter.home}"/>

This allows you to dynamically access and utilize environment variables within your Ant build scripts.

Best Practices

Scope: Consider the scope of your environment variables. Global variables affect all jobs, while job-specific variables provide isolation.
Security: Be cautious about storing sensitive information in environment variables. Consider using Jenkins’ credential management features for secrets.
Clarity: Use descriptive variable names to improve readability and maintainability.
Testing: Always test your environment variable settings to ensure they are working as expected.