How to debug script execution?

Debugging Ansible Script Execution

Debugging Ansible script execution is a crucial skill for Ansible users, as it helps identify and resolve issues that may arise during playbook execution. In this response, we'll explore various techniques and tools to effectively debug Ansible scripts.

Understanding Ansible Execution Flow

Before diving into the debugging process, it's essential to understand the general execution flow of an Ansible playbook. When you run an Ansible playbook, the following steps occur:

1. Inventory Gathering: Ansible collects information about the target hosts, such as their IP addresses, connection details, and other metadata.
2. Variable Evaluation: Ansible resolves and processes all the variables defined in the playbook and its associated files.
3. Task Execution: Ansible executes the tasks defined in the playbook, one by one, on the target hosts.
4. Gathering Facts: Ansible collects information about the target hosts, such as their operating system, installed packages, and other system details.
5. Playbook Completion: Ansible completes the execution of the playbook and reports the results.

Understanding this execution flow can help you identify where potential issues may occur and focus your debugging efforts accordingly.

Debugging Techniques

1. Verbose Output: One of the simplest ways to debug an Ansible playbook is to use the -v or -vv (more verbose) flags when running the playbook. This will provide more detailed output, including information about the tasks being executed, the variables being used, and any errors or warnings that occur.

ansible-playbook playbook.yml -v

2. Debug Module: The debug module in Ansible allows you to print information to the console during playbook execution. You can use this module to display the values of variables, the output of commands, or any other information that can help you understand what's happening in your playbook.

- name: Print a message
  debug:
    msg: "This is a debug message"

- name: Print a variable
  debug:
    var: my_variable

3. Ansible Callbacks: Ansible supports various callback plugins that can modify the behavior of the output during playbook execution. One useful callback is the stderr callback, which redirects the standard error output to a separate file, making it easier to identify and analyze error messages.

To enable the stderr callback, add the following to your Ansible configuration file (typically located at ~/.ansible.cfg or /etc/ansible/ansible.cfg):

[defaults]
stdout_callback = default
stderr_callback = stderr

4. Ansible-Playbook Flags: Ansible provides several flags that can be used to control the execution of a playbook and aid in debugging:

   • --step: This flag allows you to execute the playbook one task at a time, giving you the opportunity to inspect the state of the system and the variables before proceeding to the next task.
   • --start-at-task: This flag allows you to start the playbook execution at a specific task, which can be useful when you're trying to reproduce a specific issue.
   • --list-tasks: This flag displays a list of all the tasks in the playbook, which can help you understand the overall structure and flow of the playbook.

5. Ansible Logs: Ansible generates log files that can provide valuable information for debugging. By default, Ansible logs are written to the /var/log/ansible/ directory (on Linux systems). You can also configure Ansible to log to a different location by modifying the log_path option in the Ansible configuration file.

6. Ansible-Playbook Dry Run: The --check flag allows you to perform a "dry run" of the playbook, which means that Ansible will simulate the execution of the playbook without actually making any changes to the target hosts. This can be helpful in identifying potential issues before applying the changes.

ansible-playbook playbook.yml --check

7. Ansible Vault: If your playbook uses sensitive data, such as passwords or API keys, you can use Ansible Vault to encrypt and decrypt this information. This can be useful for debugging, as it allows you to inspect the encrypted values without exposing the sensitive data.

ansible-playbook playbook.yml --ask-vault-pass

8. Ansible Debugger: Ansible provides a built-in debugger that allows you to step through the execution of a playbook, inspect variables, and set breakpoints. To use the debugger, you can add the debug task to your playbook and then run the playbook with the --start-at-task flag.

- name: Debug task
  debug:
    var: my_variable
  when: my_variable is defined

ansible-playbook playbook.yml --start-at-task "Debug task"

By using these various debugging techniques, you can effectively identify and resolve issues in your Ansible scripts, ensuring that your playbooks execute as expected.