Conditionals — Ansible Documentation

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# Conditionals

Topics

- Conditionals
  - The When Statement
  - Loops and Conditionals
  - Loading in Custom Facts
  - Applying ‘when’ to roles and includes
  - Conditional Imports
  - Selecting Files And Templates Based On Variables
  - Register Variables

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. Further, additional groups can be created to manage hosts based on whether the hosts match other criteria. 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

Let’s dig into what they are.

## 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 Variables). It’s actually pretty simple:

    tasks:
      - name: "shut down Debian flavored systems"
        command: /sbin/shutdown -t now
        when: ansible_os_family == "Debian"
        # note that Ansible facts and vars like ansible_os_family 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_distribution == "CentOS" and ansible_distribution_major_version == "6") or
              (ansible_distribution == "Debian" and ansible_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_distribution == "CentOS"
          - ansible_distribution_major_version == "6"

A number of Jinja2 “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|failed

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

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

Note

the filters have been updated in 2.1 so both success and succeeded work (fail/failed, etc).

Note that was a little bit of foreshadowing on the ‘register’ statement. We’ll get to it a bit later in this chapter.

As a reminder, to see what facts are available on a particular system, you can do:

    ansible hostname.example.com -m setup

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_os_family == "RedHat" and ansible_lsb.major_release|int >= 6

Note

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

Variables defined in the playbooks or inventory can also be used. An example may be the execution of a task based on a variable’s boolean value:

    vars:
      epic: true

Then a conditional execution might look like:

    tasks:
        - shell: echo "This certainly is epic!"
          when: epic

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

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

If using with_dict which does not take a list:

    - command: echo {{ item.key }}
      with_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 Developing Modules. 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 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:

    - include: 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_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. Read up on the ‘group_by’ module in the About Modules docs for a more streamlined way to accomplish the same thing.

## Conditional Imports

Note

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

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_os_family }}.yml", "vars/os_defaults.yml" ]
      tasks:
      - name: make sure apache is started
        service: name={{ apache }} state=started

Note

The variable ‘ansible_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/CentOS.yml
    apache: httpd
    somethingelse: 42

How does this work? If the operating system was ‘CentOS’, the first file Ansible would try to import would be ‘vars/CentOS.yml’, followed by ‘/vars/os_defaults.yml’ if that file did not exist. If no files in the list were found, an error would be raised. On Debian, it would instead first look towards ‘vars/Debian.yml’ instead of ‘vars/CentOS.yml’, before falling back on ‘vars/os_defaults.yml’. Pretty simple.

To use this conditional import feature, you’ll need facter or ohai installed prior to running the playbook, but you can of course push this out with Ansible if you like:

    # for facter
    ansible -m yum -a "pkg=facter state=present"
    ansible -m yum -a "pkg=ruby-json state=present"

    # for ohai
    ansible -m yum -a "pkg=ohai state=present"

Ansible’s approach to configuration – separating variables from tasks, keeps your playbooks from turning into arbitrary code with ugly nested ifs, conditionals, and so on - and results in more streamlined & auditable configuration rules – especially because there are a minimum of 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
      with_first_found:
        - files:
           - {{ ansible_distribution }}.conf
           - default.conf
          paths:
           - 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.

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 “with_items” 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 with_items 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
            with_items: "{{ home_dirs.stdout_lines }}"
            # same as with_items: "{{ 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 == ""

See also

Playbooks
  An introduction to playbooks
Playbook Roles and Include Statements
  Playbook organization by roles
Best Practices
  Best practices in playbooks
Variables
  All about variables
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