Advanced Jenkins

If I have a env variable. can I access it inside script block of stage and also inside sh block of script block of stage some me with examples

Yes, in a Jenkins pipeline, you can access environment variables inside a script block as well as inside an sh step. Environment variables can be accessed using the env prefix.

Here are some examples to illustrate this:

1. Accessing env variable inside a script block:

groovyCopy codepipeline {
    agent any
    environment {
        MY_ENV_VAR = 'Hello from env'
    }
    stages {
        stage('Demo Stage') {
            steps {
                script {
                    echo "Accessing env variable in script block: ${env.MY_ENV_VAR}"
                }
            }
        }
    }
}

2. Accessing env variable inside an sh step within a script block:

groovyCopy codepipeline {
    agent any
    environment {
        MY_ENV_VAR = 'Hello from env'
    }
    stages {
        stage('Demo Stage') {
            steps {
                script {
                    echo "Accessing env variable in script block: ${env.MY_ENV_VAR}"

                    // Accessing inside sh step
                    sh """
                    echo "Accessing env variable in sh block: $MY_ENV_VAR"
                    """
                }
            }
        }
    }
}

Note that when you access the environment variable inside the sh step, you don't need to use the env prefix. You can simply reference the variable by its name.

However, remember that this works because Groovy is interpolating the variable inside the double-quoted string ("""..."""). If you used single quotes ('''...'''), the variable would not be interpolated.

Can you elaborate more on this

Note that when you access the environment variable inside the sh step, you don't need to use the env prefix. You can simply reference the variable by its name.

Certainly!

In Jenkins pipelines, when you set an environment variable (either at the top-level using the environment directive or using the withEnv step), the variable is set in the build environment. This means it's available just like any regular environment variable you'd expect when running a script in a terminal or command prompt.

The env prefix in a Jenkins pipeline is specifically a Groovy mechanism to access environment variables. Groovy doesn't automatically interpolate or replace $VARIABLE_NAME syntax with the environment variable's value. Instead, you use the env object as a map to fetch the value of the environment variable. So, when you're in the Groovy context (like the script block), you would reference an environment variable as env.MY_ENV_VAR.

However, the sh step executes a shell script, which means we're now in the context of the shell (bash, sh, etc.), not Groovy. In a shell context, you reference environment variables directly using the $VARIABLE_NAME or ${VARIABLE_NAME} syntax.

Here's a breakdown:

1. In Groovy/script context:

groovyCopy codescript {
    echo "In Groovy: ${env.MY_ENV_VAR}" // You use env.MY_ENV_VAR to fetch the value.
}

2. In shell context (inside sh):

groovyCopy codesh """
echo "In shell: $MY_ENV_VAR"  // You can directly use $MY_ENV_VAR.
"""

or, if you want to ensure no adjacent characters interfere with the variable name:

groovyCopy codesh """
echo "In shell with braces: ${MY_ENV_VAR}"  // Using braces can help avoid ambiguity.
"""

The key takeaway is understanding the context. In the Groovy/script context, environment variables are accessed using the env object. In the shell context, you access environment variables as you typically would in any shell script.

what is the difference between declaring environment variables in parameters or environment block and inside any stage using env.varname?

In Jenkins Pipeline, environment variables can be set or manipulated in various sections of the pipeline. The behavior and scope of the variables differ depending on where and how they are defined. Let's discuss these differences:

1. Environment Block:

   • The environment block is usually declared at the top level of a Declarative Pipeline or within a stage block.

   • Variables defined in the top-level environment block are globally available to all stages and steps in the pipeline.

   • If defined within a stage, the environment variable is only applicable to that specific stage.

   • These variables can be accessed directly by their name in scripted parts or using env.VAR_NAME in Declarative Pipeline steps.

   • Example:

       groovyCopy codepipeline {
           agent any
           environment {
               GLOBAL_VAR = 'I am global'
           }
           stages {
               stage('Example') {
                   environment {
                       STAGE_VAR = 'I am local to this stage'
                   }
                   steps {
                       echo "${GLOBAL_VAR}"
                       echo "${STAGE_VAR}"
                   }
               }
           }
       }
     

2. Parameters:

   • The parameters block defines the input parameters for the pipeline. These are often used to provide choices or inputs when starting a build.

   • Once the build is triggered, these parameters become environment variables and can be accessed similarly to those set in the environment block.

   • Example:

       groovyCopy codepipeline {
           agent any
           parameters {
               string(name: 'USER_INPUT', defaultValue: 'default', description: 'Input from user')
           }
           stages {
               stage('Example') {
                   steps {
                       echo "${USER_INPUT}" // or echo "${env.USER_INPUT}"
                   }
               }
           }
       }
     

3. Inside a Stage using env.varname:

   • This is a more dynamic and scripted approach. It's used within the script block of a stage in a Declarative Pipeline or directly in a Scripted Pipeline.

   • Variables set in this manner are available for the remainder of the pipeline execution, unless they are overridden.

   • They can be accessed using env.VAR_NAME.

   • Example:

       groovyCopy codepipeline {
           agent any
           stages {
               stage('Set Variable') {
                   steps {
                       script {
                           env.DYNAMIC_VAR = 'Set from script block'
                       }
                   }
               }
               stage('Use Variable') {
                   steps {
                       echo "${env.DYNAMIC_VAR}"
                   }
               }
           }
       }
     

Key Differences:

• Scope: Variables in the environment block have a defined scope (either global or specific to a stage). Variables set using env.varname in a script block have a global scope from that point onward.

• Flexibility: Using env.varname in a script block offers more flexibility and allows for dynamic value assignments based on conditional logic or computations.

• Accessibility: All these variables, whether set in the environment block, parameters, or dynamically in a script block, become part of the env and can be accessed using env.VAR_NAME.

It's essential to choose the right place to define your variables based on your requirements concerning scope, flexibility, and readability.