Mastering Ansible: Capture Task Output with Register and Reuse It

In Ansible you can capture a task's output with the register keyword, storing it in a variable for later use.

Below is a playbook that runs the whoami shell command, registers the result in the variable user, and then prints the variable with the debug module.

---
- name: Check the user
  hosts: servera
  tasks:
    - name: View the logged in user name
      shell: whoami
      register: user
    - debug:
        var: user

Running the playbook produces the following output, where the debug task displays the captured result.

it@workstation:~/ansible$ ansible-playbook test.yml
BECOME password:

PLAY [Check the user] *****************************************************************

TASK [Gathering Facts] **************************************************************
ok: [servera]

TASK [View the logged in user name] *************************************************
changed: [servera]

TASK [debug] ***********************************************************************
ok: [servera] => {
    "user": {
        "changed": true,
        "cmd": "whoami",
        "delta": "0:00:00.002775",
        "end": "2021-01-19 08:53:22.611335",
        "failed": false,
        "rc": 0,
        "start": "2021-01-19 08:53:22.608560",
        "stderr": "",
        "stderr_lines": [],
        "stdout": "root",
        "stdout_lines": ["root"]
    }
}

PLAY RECAP **************************************************************************
servera : ok=3 changed=1 unreachable=0 failed=0 skipped=0 rescued=0 ignored=0

The result dictionary contains several useful fields:

changed : true if the command was executed.

cmd : the command that was run.

delta : time taken to execute the command.

start : timestamp when the command started.

end : timestamp when the command finished.

failed : false indicates success; true indicates failure.

rc : return code (0 means success).

stderr and stderr_lines : standard error output.

stdout and stdout_lines : standard output.

You can reference any of these fields in later tasks. For example, to print a friendly message using the captured username:

---
- name: Check the user
  hosts: servera
  tasks:
    - name: View the logged in user name
      shell: whoami
      register: user
    - debug:
        msg: "Logged in as user {{ user.stdout }}"

Running this playbook yields:

it@workstation:~/ansible$ ansible-playbook test.yml
BECOME password:

PLAY [Check the user] *****************************************************************

TASK [Gathering Facts] **************************************************************
ok: [servera]

TASK [View the logged in user name] *************************************************
changed: [servera]

TASK [debug] ***********************************************************************
ok: [servera] => {"msg": "Logged in as user root"}

PLAY RECAP **************************************************************************
servera : ok=3 changed=1 unreachable=0 failed=0 skipped=0 rescued=0 ignored=0