After completing this section, you should be able to:

Variable names must start with a letter and may only contain letters, numbers, and underscores.

The following are valid Ansible variable names:

The following are not valid Ansible variable names:

Where and how variables are defined affects their scope and precedence.

This is a simplified view of the relative precedence of different kinds of variables. There are many more types. Extra variables override vars_files variables, which override play variables, which override host facts, and so forth.

Ansible variable substitution uses the Jinja2 templating system. Variable names enclosed in curly braces are replaced during evaluation with values.

---
- name: trace the path from source (remote host) to trace_me
  hosts: cs01
  gather_facts: no

  vars:
    trace_me: 172.25.250.254

  tasks:
    - name: trace the path from {{ inventory_hostname }} to {{ trace_me }}
      ios_command:
        commands:
          - traceroute {{ trace_me }}
      register: path

    - name: show the path from {{ inventory_hostname }} to {{ trace_me }}
      debug:
        msg: "{{ path }}"

Ordinarily, YAML syntax does not care whether strings are quoted or not. The double braces used by Jinja2, though, are similar enough to a YAML construct for notating mappings to confuse the YAML parser when the variable is the first element of a value.

debug:
  msg: {{ output_from_show_command }}
^ here
We could be wrong, but this one looks like it might be an issue with
missing quotes. Always quote template expression brackets when they
start a value. For instance:
with_items:
- {{ foo }}
Should be written as:
with_items:
- "{{ foo }}"

If the value starts with a brace, quote the string. Avoid unwarranted quoting, which could cause quotes to be interpreted as data.

The connection modules used to connect to networking devices run Ansible's built-in, Python-based, fact-gathering system on the control node (localhost). You see that this is the case if you run an ad hoc command using the setup module: the data returned refers to the control node (localhost), not the network device.

[user@host ~]$ ansible -m setup network-device-host-inventory-identifier

When network devices are targeted by plays, turn off Ansible's built-in fact gathering. This disables fact gathering in a play:

- hosts: network-devices
  gather_facts: no

Facts are available for networking devices by way of platform specific *os_facts modules, such as ios_facts, iosxr_facts, nxos_facts, vyos_facts, junos_facts, and so forth.

Ansible provides some predefined variables that are automatically set. Some of the better known magic variables are listed below. For the full list, see http://docs.ansible.com/ansible/playbooks_variables.html.

The debug module prints messages while plays are executing. It can be useful for debugging variables or expressions.

- debug:
  msg: >
    "{{ansible_net_hostname}} is {{ansible_net_model}}
    running {{ansible_net_version}}"

The debug module could be used, for instance, to see values assumed by magic variables in a particular context in playbooks created for this purpose.

- debug:
  msg: "{{ role_names }} and {{ vars }}"

The register keyword is used in the context of a playbook task. It saves the result of a task to a variable. Like facts, registered variables have host scope.

---
- name: >
    play for ios devices
    that uses commands
    to examine indicators
  hosts: ios
  gather_facts: no
  tasks:
    - name: look at indicators
      ios_command:
        commands:
          - sh ver | include uptime
          - sh ip domain
          - sh clock
          - sh ip name-server
          - sh proc mem | include Total
      register: results

- name: show results
  debug:
    var: results.stdout

PLAY [play for ios devices that uses commands to examine indicators] *******

TASK [look at indicators] **************************************************
ok: [cs01]

TASK [show results] ********************************************************
ok: [cs01] => {
                "results.stdout": [
                "cs01 uptime is 3 hours, 12 minutes",
                "lab.example.com",
                "*01:09:01.866 UTC Sun Jun 3 2018",
                "255.255.255.255",
                "Processor Pool Total: 2202704640 Used:   250994304 Free:
1951710336\n lsmpi_io Pool Total: 6295128 Used:   6294296 Free:
                832\n                             257260704 Total"
]
}

PLAY RECAP *****************************************************************!
cs01                             : ok=2  changed=0  unreachable=0  failed=0

You are probably already familiar with the CLI for a given network OS, but *os_facts modules provide easy access to important facts. This example is for the ios_facts module, where interesting facts show up listed under # hardware.

$ ansible-doc ios_facts | sed -n '/^# hardware/,/^$/p'
# hardware
ansible_net_filesystems:
  description: All file system names available on the device
  returned: when hardware is configured
  type: list
ansible_net_memfree_mb:
  description: The available free memory on the remote device in Mb
  returned: when hardware is configured
  type: int
ansible_net_memtotal_mb:
  description: The total memory on the remote device in Mb
  returned: when hardware is configured
  type: int

The *os_facts modules handle variable registration for you. This example uses the hardware subset that you learned about from ansible-doc ios_facts.

---
- name: >
    play for ios devices
    that uses ios_facts
  hosts: ios
  gather_facts: no
  tasks:
  - name: look at some facts
    ios_facts:
      gather_subset:
        - hardware

  - name: show results
    debug:
      msg:
        - "filesystems: {{ ansible_net_filesystems }}"
        - "free mem MB: {{ ansible_net_memfree_mb }}"
        - "mem total MB: {{ ansible_net_memtotal_mb }}"

PLAY [play for ios devices that uses ios_facts] ****

TASK [look at some facts] **************************
ok: [cs01]

TASK [show results] ********************************
ok: [cs01] => {
    "msg": [
        "filesystems: [u'bootflash:']",
        "free mem MB: 1905270",
        "mem total MB: 2151078"
    ]
}

PLAY RECAP *****************************************
cs01      : ok=2 changed=0 unreachable=0 failed=0