EX294: Use conditionals to control play execution

official doc: conditionals

0 Basic idea

A task can be executed conditionally based on the value of a  variable which may be a normal variable, a fact, inventory variable, or register variable.

e.g. a "yum" module may only be run on RedHat systems but not on Debian distributions.

1 Example

1.1 "when" based on ansible_facts

---

- hosts: localhost

  tasks:

        - name: print version of CentOS

          shell: cat /etc/redhat-release

          register: version

          when: ansible_facts['distribution'] == "CentOS"

        - debug: var=version.stdout

1.2 "when" based on register variables

This play will create a user named "user01" if it doesn't exist yet.

(NOTE: Ansible has a user management module to do such a job.)

---

- hosts: vm1

  tasks:

        - name: check if user user01 exists

          shell: 'cat /etc/passwd | grep ^user01'

          ignore_errors: yes

          register: result

        - debug: var=result

        - name: create user01 if not existing

          become: yes

          shell: 'useradd user01'

          when: result.rc != 0

1.3 "when" based on special variables

---

- hosts: all

  tasks:

        - name: only run on inentory_hostname

          command: 'hostname'

          when: inventory_hostname == 'vm1'

1.4 "when" based on normal variables

---

- hosts: all

  vars:

          foo: "bar"

  tasks:

        - command: 'uname'

          when: foo == "bar"

2 Key points

• the value of "when" is already in JinJa2 context, don't add {{}} any more.
• different versions of Ansible support different facts. Ansible 2.4 uses "facts" while 2.8 uses "anisble_facts" as the variable name.