Conditionals

In a playbook, you may want to execute different tasks, or have different goals, depending on the value of a fact (data about the remote system), a variable, or the result of a previous task. You may want the value of some variables to depend on the value of other variables. Or you may want to create additional groups of hosts based on whether the hosts match other criteria. You can do all of these things with conditionals.

Ansible uses Jinja2 tests and filters in conditionals. Ansible supports all the standard tests and filters, and adds some unique ones as well.

Note

There are many options to control execution flow in Ansible. You can find more examples of supported conditionals at https://jinja.palletsprojects.com/en/master/templates/#comparisons.

Basic conditionals with when

The simplest conditional statement applies to a single task. Create the task, then add a when statement that applies a test. The when clause is a raw Jinja2 expression without double curly braces (see group_by – Create Ansible groups based on facts). When you run the task or playbook, Ansible evaluates the test for all hosts. On any host where the test passes (returns a value of True), Ansible runs that task. For example, if you are installing mysql on multiple machines, some of which have SELinux enabled, you might have a task to configure SELinux to allow mysql to run. You would only want that task to run on machines that have SELinux enabled:

tasks:
  - name: Configure SELinux to start mysql on any port
    seboolean: name=mysql_connect_any state=true persistent=yes
    when: ansible_selinux.status == "enabled"
    # all variables can be used directly in conditionals without double curly braces

Conditionals based on ansible_facts

Often you want to execute or skip a task based on facts. Facts are attributes of individual hosts, including IP address, operating system, the status of a filesystem, and many more. With conditionals based on facts:

  • You can install a certain package only when the operating system is a particular version.
  • You can skip configuring a firewall on hosts with internal IP addresses.
  • You can perform cleanup tasks only when a filesystem is getting full.

See Commonly-used facts for a list of facts that frequently appear in conditional statements. Not all facts exist for all hosts. For example, the ‘lsb_major_release’ fact used in an example below only exists when the lsb_release package is installed on the target host. To see what facts are available on your systems, add a debug task to your playbook:

- debug: var=ansible_facts

Here is a sample conditional based on a fact:

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

If you have multiple conditions, you can group them with parentheses:

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")

You can use logical operators to combine conditions. When you have multiple conditions that all need to be true (that is, a logical and), you can specify them 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"

If a fact or variable is a string, and you need to run a mathematical comparison on it, use a filter to ensure that Ansible reads the value as an integer:

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

Conditions based on registered variables

Often in a playbook you want to execute or skip a task based on the outcome of an earlier task. For example, you might want to configure a service after it is upgraded by an earlier task. To create a conditional based on a registered variable:

  1. Register the outcome of the earlier task as a variable.
  2. Create a conditional test based on the registered variable.

You create the name of the registered variable using the register keyword. A registered variable always contains the status of the task that created it as well as any output that task generated. You can use registered variables in templates and action lines as well as in conditional when statements. You can access the string contents of the registered variable using variable.stdout. For 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

You can use registered results in the loop of a task if the variable is a list. If the variable is not a list, you can convert it into a list, with either stdout_lines or with variable.stdout.split(). You can also split the lines 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() }}"

The string content of a registered variable can be empty. If you want to run another task only on hosts where the stdout of your registered variable is empty, 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 == ""

Ansible always registers something in a registered variable for every host, even on hosts where a task fails or Ansible skips a task because a condition is not met. To run a follow-up task on these hosts, query the registered variable for is skipped (not for “undefined” or “default”). See Registering variables for more information. Here are sample conditionals based on the success or failure of a task. Remember to ignore errors if you want Ansible to continue executing on a host when a failure occurs:

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

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

  - command: /bin/something_else
    when: result is succeeded

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

Note

Older versions of Ansible used success and fail, but succeeded and failed use the correct tense. All of these options are now valid.

Conditionals based on variables

You can also create conditionals based on variables defined in the playbooks or inventory. Because conditionals require boolean input (a test must evaluate as True to trigger the condition), you must apply the | bool filter to non boolean variables, such as string variables with content like ‘yes’, ‘on’, ‘1’, or ‘true’. You can define variables like this:

vars:
  epic: true
  monumental: "yes"

With the variables above, Ansible would run one of these tasks and skip the other:

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

    - 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 do not need to use {{ }} to use variables inside conditionals, as these are already implied.

Using conditionals in loops

If you combine a when statement with a loop, Ansible processes the condition separately for each item. This is by design, so you can execute the task on some items in the loop and skip it on other items. For example:

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

If you need to skip the whole task when the loop variable is undefined, use the |default filter to provide an empty iterator. For example, when looping over a list:

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

You can do the same thing when looping over a dict:

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

Loading custom facts

You can provide your own facts, as described 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'

Conditionals with re-use

You can use conditionals with re-usable tasks files, playbooks, or roles. Ansible executes these conditional statements differently for dynamic re-use (includes) and for static re-use (imports). See Re-using Ansible artifacts for more information on re-use in Ansible.

Conditionals with imports

When you add a conditional to an import statement, Ansible applies the condition to all tasks within the imported file. This behavior is the equivalent of Tag inheritance: adding tags to multiple tasks. Ansible applies the condition to every task, and evaluates each task separately. For example, you might have a playbook called main.yml and a tasks file called other_tasks.yml:

# all tasks within an imported file inherit the condition from the import statement
# main.yml
- import_tasks: other_tasks.yml # note "import"
  when: x is not defined

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

Ansible expands this at execution time to the equivalent of:

- set_fact:
    x: foo
  when: x is not defined
  # this task sets a value for x

- debug:
    var: x
  when: x is not defined
  # Ansible skips this task, because x is now defined

Thus if x is initially undefined, the debug task will be skipped. If this is not the behavior you want, use an include_* statement to apply a condition only to that statement itself.

You can apply conditions to import_playbook as well as to the other import_* statements. When you use this approach, Ansible returns a ‘skipped’ message for every task on every host that does not match the criteria, creating repetitive output. In many cases the group_by module can be a more streamlined way to accomplish the same objective; see Handling OS and distro differences.

Conditionals with includes

When you use a conditional on an include_* statement, the condition is applied only to the include task itself and not to any other tasks within the included file(s). To contrast with the example used for conditionals on imports above, look at the same playbook and tasks file, but using an include instead of an import:

# Includes let you re-use a file to define a variable when it is not already defined

# main.yml
- include_tasks: other_tasks.yml
  when: x is not defined

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

Ansible expands this at execution time to the equivalent of:

- include_tasks: other_tasks.yml
  when: x is not defined
  # if condition is met, Ansible includes other_tasks.yml

- set_fact:
    x: foo
  # no condition applied to this task, Ansible sets the value of x to foo

- debug:
    var: x
  # no condition applied to this task, Ansible prints the debug statement

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 Re-using Ansible artifacts.

Conditionals with roles

There are three ways to apply conditions to roles:

  • Add the same condition or conditions to all tasks in the role by placing your when statement under the roles keyword. See the example in this section.
  • Add the same condition or conditions to all tasks in the role by placing your when statement on a static import_role in your playbook.
  • Add a condition or conditions to individual tasks or blocks within the role itself. This is the only approach that allows you to select or skip some tasks within the role based on your when statement. To select or skip tasks within the role, you must have conditions set on individual tasks or blocks, use the dynamic include_role in your playbook, and add the condition or conditions to the include. When you use this approach, Ansible applies the condition to the include itself plus any tasks in the role that also have that when statement.

When you incorporate a role in your playbook statically with the roles keyword, Ansible adds the conditions you define to all the tasks in the role. For example:

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

Selecting variables, files, or templates based on facts

Sometimes the facts about a host determine the values you want to use for certain variables or even the file or template you want to select for that host. For example, the names of packages are different on CentOS and on Debian. The configuration files for common services are also different on different OS flavors and versions. To load different variables file, templates, or other files based on a fact about the hosts you are managing:

# Name your vars files, templates, or files to match the Ansible fact that differentiates them # Select the correct vars file, template, or file for each host with a variable based on that Ansible fact

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

Selecting variables files based on facts

You can create a playbook that works on multiple platforms and OS versions with a minimum of syntax by placing your variable values in vars files and conditionally importing them. If you want to install Apache on some CentOS and some Debian servers, create variables files with YAML keys and values. For example:

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

Then import those variables files based on the facts you gather on the hosts in your playbook:

---
- hosts: webservers
  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

Ansible gathers facts on the hosts in the webservers group, then interpolates the variable “ansible_facts[‘os_family’]” into a list of filenames. If you have hosts with Red Hat operating systems (‘CentOS’, for example), Ansible looks for ‘vars/RedHat.yml’. If that file does not exist, Ansible attempts to load ‘vars/os_defaults.yml’. For Debian hosts, Ansible first looks for ‘vars/Debian.yml’, before falling back on ‘vars/os_defaults.yml’. If no files in the list are found, Ansible raises an error.

Selecting files and templates based on facts

You can use the same approach when different OS flavors or versions require different configuration files or templates. Select the appropriate file or template based on the variables assigned to each host. This approach is often much cleaner than putting a lot of conditionals into a single template to cover multiple OS or package versions.

For example, you can template out a configuration file that is 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/']

Commonly-used facts

The following Ansible facts are frequently used in conditionals.

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’]

The major version of the operating system. For example, the value is 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

See also

Working With Playbooks
An introduction to playbooks
Roles
Playbook organization by roles
Tips and tricks
Best practices in playbooks
Using Variables
All about variables
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