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when是ansible playbook中最常用的条件判断关键字，用于决定某个任务(task)、block、role或include_tasks是否执行。它的作用类似于编程语言中的if。

1.基本语法

- hosts: all tasks: - name: Install httpd yum: name: httpd state: present when: ansible_os_family == "RedHat"

备注：只有当目标主机的ansible_os_family等于“RedHat”时，才执行安装httpd。

[root@cent79-2 yaml-directory]# ansible-playbook-2 when.yaml PLAY [all] **************************************************************************************************** TASK [Gathering Facts] **************************************************************************************** ok: [192.168.10.100] TASK [Install httpd] ****************************************************************************************** ok: [192.168.10.100] PLAY RECAP **************************************************************************************************** 192.168.10.100 : ok=2 changed=0 unreachable=0 failed=0 skipped=0 rescued=0 ignored=0 [root@cent79-2 yaml-directory]#

2.when可以判断的内容

1.判断变量

- hosts: all vars: install_httpd: true tasks: - name: Install httpd yum: name: httpd state: present when: install_httpd

变量（install_httpd）为true-->执行

变量（install_httpd）为false-->跳过

上述when条件也可以使用如下方式

when: install_httpd == true

OR

when: install_httpd | bool

2.判断字符串

- hosts: all tasks: - name: Install httpd yum: name: httpd state: present when: ansible_os_family == "RedHat"

结果：

[root@cent79-2 yaml-directory]# ansible-playbook-2 when.yaml PLAY [all] **************************************************************************************************** TASK [Gathering Facts] **************************************************************************************** ok: [192.168.10.100] TASK [Install httpd] ****************************************************************************************** ok: [192.168.10.100] PLAY RECAP **************************************************************************************************** 192.168.10.100 : ok=2 changed=0 unreachable=0 failed=0 skipped=0 rescued=0 ignored=0 [root@cent79-2 yaml-directory]#

3.判断数字

- hosts: all tasks: - name: Install httpd yum: name