类似于编程语言的if
有时候用户有可能需满足特定条件才执行某一个特定的步骤。例如，在某个特定版本的系统上装包，或者只在磁盘空间满了的文件系统上执行清理操作。这些操作在Playbook中用when语句实现。
主机为Debian Linux立刻关机
tasks:
- name: "shutdown Debian flavored systems"
command: /sbin/shutdown -t now
when: ansible_os_family == "Debian"
根据action的执行结果，来决定接下来执行的action。
tasks:
- command: /bin/false
register: result
ignore_errors: True
- command: /bin/something
when: result|failed
- command: /bin/something_else
when: result|success
- command: /bin/still/something_else
when: result|skipped
远程中的系统变量facts变量作为when的条件，用“|int”还可以转换返回值的类型：
---
- hosts: web
tasks:
- debug: msg="only on Red Hat 7, derivatives, and later"
when: ansible_os_family == "RedHat" and ansible_lsb.major_release|int >= 6
vars:
epic: true
基本款
tasks:
- shell: echo "This certainly is epic!"
when: epic
否定款：
tasks:
- shell: echo "This certainly isn't epic!"
when: not epic
变量定义款
tasks:
- shell: echo "I've got '\{\{ foo \}\}' and am not afraid to use it!"
when: foo is defined
- fail: msg="Bailing out. this play requires 'bar'"
when: bar is not defined
数值表达款
tasks:
- command: echo \{\{ item \}\}
with_items: [ 0, 2, 4, 6, 8, 10 ]
when: item > 5
- include: tasks/sometasks.yml
when: "'reticulating splines' in output"
- hosts: webservers
roles:
- { role: debian_stock_config, when: ansible_os_family == 'Debian' }