死磕ansible系列--when函数的作用

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when

可以使用when做条件判断

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tasks:
  - name: "shut down Debian flavored systems"
    command: /sbin/shutdown -t now
    when: ansible_facts['os_family'] == "Debian"

同时我们可以使用 or 来指定多个条件语句

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tasks:
  - name: "shut down CentOS 6 and Debian 7 systems"
    command: /sbin/shutdown -t now
    when: (ansible_facts['distribution'] == "CentOS" and ansible_facts['distribution_major_version'] == "6") or
          (ansible_facts['distribution'] == "Debian" and ansible_facts['distribution_major_version'] == "7")
````

我们可以写多行来实现and的情况

```yaml
tasks:
  - name: "shut down CentOS 6 systems"
    command: /sbin/shutdown -t now
    when:
      - ansible_facts['distribution'] == "CentOS"
      - ansible_facts['distribution_major_version'] == "6"

假设我们要忽略一个语句的错误，然后根据成功或失败决定有条件地执行某项操作

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tasks:
  - command: /bin/false
    register: result
    ignore_errors: True

  - command: /bin/something
    when: result is failed

  # In older versions of ansible use ``success``, now both are valid but succeeded uses the correct tense.
  - command: /bin/something_else
    when: result is succeeded

  - command: /bin/still/something_else
    when: result is skipped

var 可以使用 ‘yes’, ‘on’, ‘1’, ‘true’ 这些值设置bool值，然后进行规制判断。

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vars:
  epic: true
  monumental: "yes"
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tasks:
    - shell: echo "This certainly is epic!"
      when: epic or monumental|bool
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tasks:
    - shell: echo "This certainly isn't epic!"
      when: not epic

可以判断当前变量是否定义来执行相应的条件

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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 undefined

when函数可以结合loop函数一起使用

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tasks:
    - command: echo {{ item }}
      loop: [ 0, 2, 4, 6, 8, 10 ]
      when: item > 5
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- command: echo {{ item }}
  loop: "{{ mylist|default([]) }}"
  when: item > 5

loop 可以是个字典

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- command: echo {{ item.key }}
  loop: "{{ query('dict', mydict|default({})) }}"
  when: item.value > 5

when 条件还可以在 role incloud import 下使用

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- import_tasks: tasks/sometasks.yml
  when: "'reticulating splines' in output"
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- hosts: webservers
  roles:
     - role: debian_stock_config
       when: ansible_facts['os_family'] == 'Debian'

当我们使用incloud_*的时候，它仅适用于包含任务本身，而不适用于包含文件中的任何其他任务。例子如下

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# We wish to include a file to define a variable when it is not
# already defined

# main.yml
- import_tasks: other_tasks.yml # note "import"
  when: x is not defined

# other_tasks.yml
- set_fact:
    x: foo
- debug:
    var: x

上面的配置，等同于

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- set_fact:
    x: foo
  when: x is not defined
- debug:
    var: x
  when: x is not defined

因此，如果最初未定义x，则将跳过调试任务。通过使用include_tasks而不是import_tasks，other_tasks.yml中的两个任务都将按预期执行。

有些时候我们希望一个playbook可以应用多个操作系统。我们可以这是配置

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---
- hosts: all
  remote_user: root
  vars_files:
    - "vars/common.yml"
    - [ "vars/{{ ansible_facts['os_family'] }}.yml", "vars/os_defaults.yml" ]
  tasks:
  - name: make sure apache is started
    service: name={{ apache }} state=started
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---
# for vars/RedHat.yml
apache: httpd
somethingelse: 42

Register Variables 可以和when一起使用

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- name: test play
  hosts: all

  tasks:

      - shell: cat /etc/motd
        register: motd_contents

      - shell: echo "motd contains the word hi"
        when: motd_contents.stdout.find('hi') != -1
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- hosts: webservers
  roles:
     - { role: debian_stock_config, when: ansible_os_family == 'Debian' }

register 中有个stdout_lines的变量，可以帮你将结果按照split做切割。

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- name: registered variable usage as a loop list
  hosts: all
  tasks:

    - name: retrieve the list of home directories
      command: ls /home
      register: home_dirs

    - name: add home dirs to the backup spooler
      file:
        path: /mnt/bkspool/{{ item }}
        src: /home/{{ item }}
        state: link
      loop: "{{ home_dirs.stdout_lines }}"
      # same as loop: "{{ home_dirs.stdout.split() }}"

也可以判断 stdout 是否为空

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- name: check registered variable for emptiness
  hosts: all

  tasks:

      - name: list contents of directory
        command: ls mydir
        register: contents

      - name: check contents for emptiness
        debug:
          msg: "Directory is empty"
        when: contents.stdout == ""
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- name: test play
  hosts: mfs
  tasks:
      - shell: cat /etc/motd
        register: motd_contents

      - shell: echo "motd contains the word hi" > /tmp/test2
        when: motd_contents.stdout.find('hi') != -1

changed_when

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tasks:
  - shell: /usr/bin/billybass --mode="take me to the river"
    register: bass_result
    changed_when: "bass_result.rc != 2"

  # this will never report 'changed' status
  - shell: wall 'beep'
    changed_when: False

failed_when

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- name: this command prints FAILED when it fails
  command: /usr/bin/example-command -x -y -z
  register: command_result
  failed_when: "'FAILED' in command_result.stderr"
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  - name: this command prints FAILED when it fails
  command: /usr/bin/example-command -x -y -z
  register: command_result
  ignore_errors: True

- name: fail the play if the previous command did not succeed
  fail: msg="the command failed"
  when: "'FAILED' in command_result.stderr"