Ansible基于条件调用缓存OS facts遇'system'未定义错误的排查

问题场景 #

我使用以下Playbook基于ansible_system事实收集特定OS信息：

---
- name: Gather facts from all target hosts
  hosts: all
  gather_facts: false
  tasks: 

    - name: Gather OS information for linux
      set_fact:
        os_version: "{{ ansible_facts[distribution] }} {{ ansible_facts[distribution_version] }}"
      when: ansible_facts[system] == "Linux"

    - name: Gather OS information for Windows
      set_fact:
        os_version: "{{ ansible_facts[ansible_os_name] }}"
      when: ansible_facts[system] is search("Win")     

    - name: Set unknowm if not linux or windows
      set_fact:
        os_version: 'unknown'
      when: 
      - not ansible_facts[system] is search("Win")  
      - not ansible_facts[system] == "Linux"

报错信息 #

所有清单内主机均出现如下错误：

"msg": "The conditional check 'ansible_facts[system] == "Linux"' failed. The error was: error while evaluating conditional (ansible_facts[system] == "Linux"): 'system' is undefined."

缓存事实示例 #

经核查，报错主机均存在缓存的ansible_system事实，示例如下：

"ansible_system": "Linux",
  "ansible_cmdline": {
    "ro": true,
    "root": "UUID=fcd2fe34-d0d2-4d03-8c17-0a5d8e61cfee",
    "BOOT_IMAGE": "/boot/vmlinuz-5.4.0-193-generic",
    "maybe-ubiquity": true
  }

疑问 #

我认为可以通过ansible_facts[system] == "Linux"或ansible_system == "Linux"两种方式访问该事实，请问是否是访问方式有误？

问题分析与修正 #

核心错误点 #

1. 语法错误：字典键的访问方式
   原Playbook中ansible_facts[system]的写法错误——这里的system会被当作变量解析，但你并未定义这个变量，因此触发“'system' is undefined”报错。
   正确的字典访问方式有两种：

   • 字符串键索引：ansible_facts['system']
   • 点语法：ansible_facts.system

2. gather_facts: false导致ansible_facts字典为空
   你在Playbook中设置了gather_facts: false，Ansible不会主动收集新的事实，此时ansible_facts字典是空的。而缓存的事实是以ansible_*开头的独立全局变量存在的（比如ansible_system、ansible_distribution），直接调用这些变量即可，无需通过ansible_facts字典。

3. 其他变量访问错误
   原Playbook中ansible_facts[distribution]、ansible_facts[distribution_version]、ansible_facts[ansible_os_name]均存在相同语法错误，需对应修正为独立变量或正确的ansible_facts访问方式。

修正后的Playbook #

方案一：使用缓存的事实（无需重新收集）

---
- name: Gather facts from all target hosts
  hosts: all
  gather_facts: false
  tasks: 

    - name: Gather OS information for Linux
      set_fact:
        os_version: "{{ ansible_distribution }} {{ ansible_distribution_version }}"
      when: ansible_system == "Linux"

    - name: Gather OS information for Windows
      set_fact:
        os_version: "{{ ansible_os_name }}"
      when: ansible_system is search("Win")     

    - name: Set unknown if not Linux or Windows
      set_fact:
        os_version: 'unknown'
      when: 
      - ansible_system is not search("Win")  
      - ansible_system != "Linux"

方案二：重新收集事实（去掉gather_facts: false）

---
- name: Gather facts from all target hosts
  hosts: all
  # 默认gather_facts: true，可省略
  tasks: 

    - name: Gather OS information for Linux
      set_fact:
        os_version: "{{ ansible_facts.distribution }} {{ ansible_facts.distribution_version }}"
      when: ansible_facts.system == "Linux"

    - name: Gather OS information for Windows
      set_fact:
        os_version: "{{ ansible_facts.os_name }}"
      when: ansible_facts.system is search("Win")     

    - name: Set unknown if not Linux or Windows
      set_fact:
        os_version: 'unknown'
      when: 
      - ansible_facts.system is not search("Win")  
      - ansible_facts.system != "Linux"

内容的提问来源于stack exchange，提问作者J1raya