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/latest/templates/#comparisons.

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_module). 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:

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 the 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:

Here is a sample conditional based on a fact:

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

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:

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:

You can store Ansible facts as variables to use for conditional logic, as in the following example:

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:

Register the outcome of the earlier task as a variable.

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 the 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:

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:

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:

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:

Note

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

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:

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

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

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.

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:

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:

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

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 the variables returned there will be accessible for future tasks:

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

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, if you want to define and then display a variable that was not previously defined, you might have a playbook called main.yml and a tasks file called other_tasks.yml:

Ansible expands this at execution time to the equivalent of:

If x is initially defined, both tasks are skipped as intended. But if x is initially undefined, the debug task will be skipped since the conditional is evaluated for every imported task. The conditional will evaluate to true for the set_fact task, which will define the variable and cause the debug conditional to evaluate to false.

If this is not the behavior you want, use an include_* statement to apply a condition only to that statement itself.

Now if x is initially undefined, the debug task will not be skipped because the conditional is evaluated at the time of the include and does not apply to the individual tasks.

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.

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:

Ansible expands this at execution time to the equivalent of:

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.

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:

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 Debian. The configuration files for common services are also different on different OS flavors and versions. To load different variables files, templates, or other files based on a fact about the hosts:

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.

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:

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

Ansible gathers facts on the hosts in the webservers group, then interpolates the variable “ansible_facts[‘os_family’]” into a list of file names. 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.

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:

If your conditional when statement is not behaving as you intended, you can add a debug statement to determine if the condition evaluates to true or false. A common cause of unexpected behavior in conditionals is testing an integer as a string or a string as an integer. To debug a conditional statement, add the entire statement as the var: value in a debug task. Ansible then shows the test and how the statement evaluates. For example, here is a set of tasks and sample output:

The following Ansible facts are frequently used in conditionals.

Possible values (sample, not complete list):

The major version of the operating system. For example, the value is 16 for Ubuntu 16.04.

Possible values (sample, not complete list):

See also

An introduction to playbooks

Playbook organization by roles

Tips and tricks for playbooks

All about variables

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