How to capture stderr?

To capture standard error (stderr) in Ansible, you can use the command or shell module along with the register keyword, just like you do for standard output. The captured output will include both stdout and stderr, which you can access separately.

Step-by-Step Example

1. Create a Playbook: Start by creating a new YAML file for your playbook. For example, capture_stderr.yml.

   nano capture_stderr.yml

2. Define the Playbook Structure: Add the following content to your playbook:

   ---
   - name: Capture stderr example
     hosts: localhost
     tasks:
       - name: Run a command that may produce an error
         command: /bin/false  # This command will fail
         register: command_output
         ignore_errors: yes  # Continue even if the command fails
   
       - name: Display captured stdout
         debug:
           msg: "Standard Output: {{ command_output.stdout }}"
   
       - name: Display captured stderr
         debug:
           msg: "Standard Error: {{ command_output.stderr }}"
   
       - name: Display return code
         debug:
           msg: "Return Code: {{ command_output.rc }}"

Explanation of the Playbook

• command: /bin/false: This command is intentionally chosen to fail, which will generate an error message.
• register: command_output: This captures the output of the command, including both stdout and stderr, and stores it in the variable command_output.
• ignore_errors: yes: This allows the playbook to continue executing even if the command fails.
• Accessing Outputs:
  • {{ command_output.stdout }}: Captures standard output.
  • {{ command_output.stderr }}: Captures standard error.
  • {{ command_output.rc }}: Captures the return code of the command.

Running the Playbook

To execute the playbook, run the following command in your terminal:

ansible-playbook capture_stderr.yml

Output

You should see output similar to this:

TASK [Display captured stdout] ****************************************************
ok: [localhost] => {
    "msg": "Standard Output: "
}

TASK [Display captured stderr] ****************************************************
ok: [localhost] => {
    "msg": "Standard Error: /bin/false: command not found"
}

TASK [Display return code] *******************************************************
ok: [localhost] => {
    "msg": "Return Code: 1"
}

Additional Notes

• Using shell Module: If you need to run a command that requires shell features (like pipes or redirection), use the shell module instead of command.

• Error Handling: You can implement additional error handling based on the return code or the content of stderr to make your playbook more robust.

This method allows you to effectively capture and utilize standard error output in your Ansible playbooks. If you have any further questions or need more examples, feel free to ask!