what does "// /_" mean in bash scripting

Understanding // /_ in Bash Scripting: A Groovy/Jenkins Context

Explore the meaning and common usage of the // /_ comment-like construct in Bash, particularly when interacting with Groovy scripts in Jenkins pipelines.

When working with Bash scripts, especially in environments that integrate with other languages like Groovy (common in Jenkins pipelines), you might encounter unusual comment-like syntax such as // /_. This construct isn't standard Bash syntax for comments. Instead, it's a clever trick used to allow a single file to be interpreted correctly by multiple language parsers, specifically Bash and Groovy.

The Dual-Purpose Comment Trick

The // /_ sequence leverages how different languages handle comments. In Groovy (and Java, C#, C++), // denotes a single-line comment. Anything following // on that line is ignored by the Groovy interpreter. In Bash, however, // is not a comment. Bash interprets // as a path separator or part of a command, but the subsequent /_ part is crucial. Bash interprets /_ as a command or argument, which typically results in an error or a no-op depending on the context, effectively making the line inert or causing a controlled exit if not handled.

#!/usr/bin/env bash

// /_ This line is a comment in Groovy, ignored by Bash (or causes an error)

echo "Hello from Bash!"

// /_ def myVar = "Groovy Value";
// /_ println "Groovy says: ${myVar}";

Example of // /_ in a Bash script intended for dual interpretation.

Why Use // /_? The Shebang and Polyglot Scripts

The primary reason for this construct is to create 'polyglot' scripts – files that can be executed by more than one interpreter. This is particularly useful in scenarios where a script might be invoked directly by Bash, but also parsed or embedded within a Groovy context (like a Jenkinsfile). The shebang line #!/usr/bin/env bash tells the operating system to execute the file using Bash. However, if this file's content is later read and evaluated by a Groovy interpreter, the // /_ lines ensure that the Groovy-specific parts are treated as comments by Groovy, preventing syntax errors.

flowchart TD
    A[Script File] --> B{Shebang `#!/usr/bin/env bash`}
    B -->|Executed by OS| C[Bash Interpreter]
    C --> D{Line: `// /_ Groovy Code`}
    D -->|Bash sees `// /_`| E[Bash ignores or errors on ` /_`]
    A --> F{Read by Groovy Context}
    F --> G[Groovy Interpreter]
    G --> H{Line: `// /_ Groovy Code`}
    H -->|Groovy sees `//`| I[Groovy treats as comment]
    E & I --> J[Script behaves as intended in both contexts]

How // /_ enables dual interpretation in Bash and Groovy contexts.

Practical Application in Jenkins Groovy

In Jenkins, a Jenkinsfile is written in Groovy. Sometimes, you might want to include a shell script directly within your Jenkinsfile or have a separate script file that needs to be executable by Bash but also contain Groovy-specific metadata or logic that the Jenkins pipeline might parse. The // /_ pattern allows you to embed Groovy-specific comments or even inert Groovy code within a Bash-executable script without breaking Bash execution.

pipeline {
    agent any
    stages {
        stage('Run Bash Script') {
            steps {
                script {
                    def myBashScript = """
#!/usr/bin/env bash

// /_ This is a Groovy comment within a Bash script
// /_ def pipelineVar = 'Hello from Jenkins!';

echo "Running a shell command."
echo "The current user is: $(whoami)"

// /_ println "Groovy variable: ${pipelineVar}";
                    """
                    sh myBashScript
                }
            }
        }
    }
}

Embedding a Bash script with // /_ comments inside a Jenkinsfile.

Alternatives and Best Practices

While // /_ is a clever hack, it's not always the clearest solution. Consider these alternatives:

1. Separate Files: The most straightforward approach is to keep Bash scripts and Groovy scripts in separate files. Jenkins can easily execute external shell scripts.
2. Heredoc for Embedded Scripts: For embedding multi-line shell scripts within Groovy, a heredoc (sh '''...''') is generally cleaner and safer, as the content within the heredoc is passed directly to the shell.
3. Conditional Logic: Use conditional logic within your scripts to detect the interpreter if you absolutely need a single file to behave differently based on the execution context (though this is rarely necessary for simple comment handling).

Bash Interpretation

#!/usr/bin/env bash

This is a standard Bash comment

// /_ This line is seen by Bash as a command or argument, likely an error or no-op.

echo "Bash output."

Groovy Interpretation

// This is a standard Groovy comment

// /_ This entire line is a comment in Groovy.

def message = "Groovy output." println message