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在 when 条件中对于变量而言不需要加 {{ }}

在 when 中可以使用 ==, <, <=, >, >=, !=, [not] yyy, xxx is [not] defined, mmm in nnn.
yyy 表示布尔变量 true， 1、True、yes 都等价于 true。
not yyy 表示布尔变量 false， 0、False、no 都等价于 false。
mmm in nnn 表示一个某个元素在集合中。

示例1：比较表达式的使用

---
- name: install httpd if not installed
  hosts: web
  tasks:
    - name: check the httpd if installed
      shell: rpm -qa | grep httpd
      register: cfop
      ignore_errors: yes
    - name: install httpd if not installed
      yum:
        name: httpd
        state: installed
      when: cfop.rc != 0

示例2：布尔变量的使用

---
- name: install httpd if not installed
  hosts: web
  vars:
    run_my_task: true
  tasks:
    - name: install httpd if not installed
      yum:
        name: httpd
        state: installed
      when: run_my_task

示例3：以是否定义为条件

# cat demo.yml
---
- name: install httpd if not installed
  hosts: web
  vars:
    my_service: httpd
  tasks:
    - name: install {{ my_service }} if not installed
      yum:
        name: "{{ my_service }}"
        state: installed
      when: my_service is defined

示例4：集合运算

# cat demo.yml 
---
- name: install httpd if not installed
  hosts: web
  vars:
    supported_distros:
      - RedHat
      - Fedora
  tasks:
    - name: install httpd using yum
      yum:
        name: httpd
        state: installed
      when: ansible_facts['distribution'] in supported_distros

我们还可以将多个条件用逻辑 and 与 or 结合起来：

# cat demo.yml 
---
- name: install httpd if not installed
  hosts: web
  tasks:
    - name: install httpd using yum
      yum:
        name: httpd
        state: installed
      when: ansible_facts['distribution'] == "RedHat" or ansible_facts['distribution'] == "Fedora"      
      #when: ansible_facts['distribution'] == "RedHat" and distribution_major_version == "8"

如果条件很复杂，可以使用 > 将一行写成多行：

when: >
  (ansible_facts['distribution'] == "RedHat" and
   ansible_facts['distribution_major_version'] == "8")
  or
  (ansible_facts['distribution'] == "RedHat" and
   ansible_facts['distribution_major_version'] == "28")

当以列表方式列出条件时，表示 and 运算：

when:
  - ansible_facts['distribution'] == "RedHat"
  - ansible_facts['distribution_major_version'] == "8"

条件表达式与 loop 结合使用：

---
- name: install mariadb if enough space on root
  hosts: web
  tasks:
    - name: install mariadb using yum
      yum:
        name: mariadb-server
        state: installed
      loop: "{{ ansible_facts['mounts'] }}"
      when: item.mount == "/" and item.size_available > 5000000000

When you use when with loop for a task, the when statement is checked for each item.