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.
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. 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:
- name: Configure SELinux to start mysql on any port
ansible.posix.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.
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:
ansible.builtin.debug:
var: ansible_facts
Here is a sample conditional based on a fact:
- name: Shut down Debian flavored systems
ansible.builtin.command: /sbin/shutdown -t now
when: ansible_facts['os_family'] == "Debian"
If you have multiple conditions, you can group them with parentheses:
- name: Shut down CentOS 6 and Debian 7 systems
ansible.builtin.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:
- name: Shut down CentOS 6 systems
ansible.builtin.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:
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:
- 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 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.
hosts: all
tasks:
ansible.builtin.shell: cat /etc/motd
register: motd_contents
- name: Use the variable in conditional statement
ansible.builtin.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:
hosts: all
tasks:
- name: Retrieve the list of home directories
ansible.builtin.command: ls /home
register: home_dirs
- name: Add home dirs to the backup spooler
ansible.builtin.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:
hosts: all
tasks:
- name: List contents of directory
ansible.builtin.command: ls mydir
register: contents
- name: Check contents for emptiness
ansible.builtin.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”). 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:
- name: Register a variable, ignore errors and continue
ansible.builtin.command: /bin/false
register: result
ignore_errors: true
- name: Run only if the task that registered the "result" variable fails
ansible.builtin.command: /bin/something
when: result is failed
- name: Run only if the task that registered the "result" variable succeeds
ansible.builtin.command: /bin/something_else
when: result is succeeded
- name: Run only if the task that registered the "result" variable is skipped
ansible.builtin.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:
epic: true
monumental: "yes"
With the variables above, Ansible would run one of these tasks and skip the other:
- name: Run the command if "epic" or "monumental" is true
ansible.builtin.shell: echo "This certainly is epic!"
when: epic or monumental | bool
- name: Run the command if "epic" is false
ansible.builtin.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:
- name: Run the command if "foo" is defined
ansible.builtin.shell: echo "I've got '{{ foo }}' and am not afraid to use it!"
when: foo is defined
- name: Fail if "bar" is undefined
ansible.builtin.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:
- name: Run with items greater than 5
ansible.builtin.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:
ansible.builtin.command: echo {{ item }}
loop: "{{ mylist|default([]) }}"
when: item > 5
You can do the same thing when looping over a dict:
ansible.builtin.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:
- name: Gather site specific fact data
action: site_facts
- name: Use a custom fact
ansible.builtin.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:
# main.yml
- import_tasks: other_tasks.yml # note "import"
when: x is not defined
# other_tasks.yml
- name: Set a variable
ansible.builtin.set_fact:
x: foo
- name: Print a variable
ansible.builtin.debug:
var: x
Ansible expands this at execution time to the equivalent of:
ansible.builtin.set_fact:
x: foo
when: x is not defined
# this task sets a value for x
- name: Do the task if "x" is not defined
ansible.builin.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.
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:
# main.yml
- include_tasks: other_tasks.yml
when: x is not defined
# other_tasks.yml
- name: Set a variable
ansible.builtin.set_fact:
x: foo
- name: Print a variable
ansible.builtin.debug:
var: x
Ansible expands this at execution time to the equivalent of:
# main.yml
- include_tasks: other_tasks.yml
when: x is not defined
# if condition is met, Ansible includes other_tasks.yml
# other_tasks.yml
- name: Set a variable
ansible.builtin.set_fact:
x: foo
# no condition applied to this task, Ansible sets the value of x to foo
- name: Print a variable
ansible.builtin.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.
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:
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:
- 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" ]
- name: Make sure apache is started
ansible.builtin.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:
ansible.builtin.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):
Altlinux
Amazon
Archlinux
ClearLinux
Coreos
CentOS
Debian
Fedora
Gentoo
Mandriva
NA
OpenWrt
OracleLinux
RedHat
Slackware
SLES
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):
Alpine
Altlinux
Archlinux
Darwin
Debian
FreeBSD
Gentoo
HP-UX
Mandrake
RedHat
SGML
Slackware
Solaris
Suse
Windows
No comments:
Post a Comment