Conditionals ¶

Topics

Conditionals

Often the result of a play may depend on the value of a variable, fact (something learned about the remote system), or previous task result. In some cases, the values of variables may depend on other variables. Additional groups can be created to manage hosts based on whether the hosts match other criteria. This topic covers how conditionals are used in playbooks.

Note

There are many options to control execution flow in Ansible. More examples of supported conditionals can be located here: http://jinja.pocoo.org/docs/dev/templates/#comparisons.

The When Statement ¶

Sometimes you will want to skip a particular step on a particular host. This could be something as simple as not installing a certain package if the operating system is a particular version, or it could be something like performing some cleanup steps if a filesystem is getting full.

This is easy to do in Ansible with the when clause, which contains a raw Jinja2 expression without double curly braces (see group_by – Create Ansible groups based on facts). It’s actually pretty simple:

tasks:
  - name: "shut down Debian flavored systems"
    command: /sbin/shutdown -t now
    when: ansible_facts['os_family'] == "Debian"
    # note that all variables can be used directly in conditionals without double curly braces

You can also use parentheses to group conditions:

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")

Multiple conditions that all need to be true (a logical ‘and’) can also be specified as a list:

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

A number of Jinja2 “tests” and “filters” can also be used in when statements, some of which are unique and provided by Ansible. Suppose we want to ignore the error of one statement and then decide to do something conditionally based on success or failure:

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

Note

both success and succeeded work (fail/failed, etc).

To see what facts are available on a particular system, you can do the following in a playbook:

- debug: var=ansible_facts

Tip: Sometimes you’ll get back a variable that’s a string and you’ll want to do a math operation comparison on it. You can do this like so:

tasks:
  - shell: echo "only on Red Hat 6, derivatives, and later"
    when: ansible_facts['os_family'] == "RedHat" and ansible_facts['lsb']['major_release']|int >= 6

Note

the above example requires the lsb_release package on the target host in order to return the ‘lsb major_release’ fact.

Variables defined in the playbooks or inventory can also be used, just make sure to apply the |bool filter to non boolean variables (ex: string variables with content like ‘yes’, ‘on’, ‘1’, ‘true’). An example may be the execution of a task based on a variable’s boolean value:

vars:
  epic: true
  monumental: "yes"

Then a conditional execution might look like:

tasks:
    - shell: echo "This certainly is epic!"
      when: epic or monumental|bool

or:

tasks:
    - shell: echo "This certainly isn't epic!"
      when: not epic

If a required variable has not been set, you can skip or fail using Jinja2’s defined test. For example:

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

This is especially useful in combination with the conditional import of vars files (see below). As the examples show, you don’t need to use {{ }} to use variables inside conditionals, as these are already implied.

Loops and Conditionals ¶

Combining when with loops (see Loops), be aware that the when statement is processed separately for each item. This is by design:

tasks:
    - command: echo {{ item }}
      loop: [ 0, 2, 4, 6, 8, 10 ]
      when: item > 5

If you need to skip the whole task depending on the loop variable being defined, used the |default filter to provide an empty iterator:

- command: echo {{ item }}
  loop: "{{ mylist|default([]) }}"
  when: item > 5

If using a dict in a loop:

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

Loading in Custom Facts ¶

It’s also easy to provide your own facts if you want, which is covered in Should you develop a module?. To run them, just make a call to your own custom fact gathering module at the top of your list of tasks, and variables returned there will be accessible to future tasks:

tasks:
    - name: gather site specific fact data
      action: site_facts
    - command: /usr/bin/thingy
      when: my_custom_fact_just_retrieved_from_the_remote_system == '1234'

Applying ‘when’ to roles, imports, and includes ¶

Note that if you have several tasks that all share the same conditional statement, you can affix the conditional to a task include statement as below. All the tasks get evaluated, but the conditional is applied to each and every task:

- import_tasks: tasks/sometasks.yml
  when: "'reticulating splines' in output"

Note

In versions prior to 2.0 this worked with task includes but not playbook includes. 2.0 allows it to work with both.

Or with a role:

- hosts: webservers
  roles:
     - role: debian_stock_config
       when: ansible_facts['os_family'] == 'Debian'

You will note a lot of ‘skipped’ output by default in Ansible when using this approach on systems that don’t match the criteria. In many cases the group_by module can be a more streamlined way to accomplish the same thing; see Operating System and Distribution Variance.

When a conditional is used with include_* tasks instead of imports, it is applied only to the include task itself and not to any other tasks within the included file(s). A common situation where this distinction is important is as follows:

# 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

This expands at include time to the equivalent of:

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

Thus if x is initially undefined, the debug task will be skipped. By using include_tasks instead of import_tasks, both tasks from other_tasks.yml will be executed as expected.

For more information on the differences between include v import see Creating Reusable Playbooks.

Conditional Imports ¶

Note

This is an advanced topic that is infrequently used.

Sometimes you will want to do certain things differently in a playbook based on certain criteria. Having one playbook that works on multiple platforms and OS versions is a good example.

As an example, the name of the Apache package may be different between CentOS and Debian, but it is easily handled with a minimum of syntax in an Ansible Playbook:

---
- 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

Note

The variable “ansible_facts[‘os_family’]” is being interpolated into the list of filenames being defined for vars_files.

As a reminder, the various YAML files contain just keys and values:

---
# for vars/RedHat.yml
apache: httpd
somethingelse: 42

How does this work? For Red Hat operating systems (‘CentOS’, for example), the first file Ansible tries to import is ‘vars/RedHat.yml’. If that file does not exist, Ansible attempts to load ‘vars/os_defaults.yml’. If no files in the list were found, an error is raised.

On Debian, Ansible first looks for ‘vars/Debian.yml’ instead of ‘vars/RedHat.yml’, before falling back on ‘vars/os_defaults.yml’.

Ansible’s approach to configuration – separating variables from tasks, keeping your playbooks from turning into arbitrary code with nested conditionals - results in more streamlined and auditable configuration rules because there are fewer decision points to track.

Selecting Files And Templates Based On Variables ¶

Note

This is an advanced topic that is infrequently used. You can probably skip this section.

Sometimes a configuration file you want to copy, or a template you will use may depend on a variable. The following construct selects the first available file appropriate for the variables of a given host, which is often much cleaner than putting a lot of if conditionals in a template.

The following example shows how to template out a configuration file that was very different between, say, CentOS and Debian:

- name: template a file
  template:
      src: "{{ item }}"
      dest: /etc/myapp/foo.conf
  loop: "{{ query('first_found', { 'files': myfiles, 'paths': mypaths}) }}"
  vars:
    myfiles:
      - "{{ansible_facts['distribution']}}.conf"
      -  default.conf
    mypaths: ['search_location_one/somedir/', '/opt/other_location/somedir/']

Register Variables ¶

Often in a playbook it may be useful to store the result of a given command in a variable and access it later. Use of the command module in this way can in many ways eliminate the need to write site specific facts, for instance, you could test for the existence of a particular program.

Note

Registration happens even when a task is skipped due to the conditional. This way you can query the variable for `` is skipped`` to know if task was attempted or not.

The ‘register’ keyword decides what variable to save a result in. The resulting variables can be used in templates, action lines, or when statements. It looks like this (in an obviously trivial example):

- 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

As shown previously, the registered variable’s string contents are accessible with the ‘stdout’ value. The registered result can be used in the loop of a task if it is converted into a list (or already is a list) as shown below. “stdout_lines” is already available on the object as well though you could also call “home_dirs.stdout.split()” if you wanted, and could split by other fields:

- 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() }}"

As shown previously, the registered variable’s string contents are accessible with the ‘stdout’ value. You may check the registered variable’s string contents for emptiness:

- 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 == ""

Commonly Used Facts ¶

The following Facts are frequently used in Conditionals - see above for examples.

ansible_facts[‘distribution’]¶

Possible values (sample, not complete list):

Alpine
Altlinux
Amazon
Archlinux
ClearLinux
Coreos
CentOS
Debian
Fedora
Gentoo
Mandriva
NA
OpenWrt
OracleLinux
RedHat
Slackware
SMGL
SUSE
Ubuntu
VMwareESX

ansible_facts[‘distribution_major_version’]¶

This will be the major version of the operating system. For example, the value will be 16 for Ubuntu 16.04.

ansible_facts[‘os_family’]¶

Possible values (sample, not complete list):

AIX
Alpine
Altlinux
Archlinux
Darwin
Debian
FreeBSD
Gentoo
HP-UX
Mandrake
RedHat
SGML
Slackware
Solaris
Suse
Windows