How to use Ansible Loops ?

Ansible is an open-source tool from Red Hat Inc., which is used for automating many of the IT infrastructure tasks. It can be used for a wide range of tasks such as installing software packages, adding compute nodes, resource provisioning, and many others. It has a number of modules that can be used to manage multiple configuration-related tasks.

Ansible is agentless and is installed on a controlling host called a controller. The controller work is to manage remote machines via SSH and other supported communication mechanisms.

Loops are handy for performing multiple tasks in one shot, such as creating multiple users, installing multiple packages, or repeatedly executing a task until a condition is true or false.

Ansible uses YAML (YAML Ain't Markup Language), a data serialization language, for its playbooks. An Ansible code can be minimized by using Loops, i.e., iterating over sequences or mappings.

Here at LinuxAPT, we shall look into Loops in Ansible and examine a few examples of using them inside playbooks.

More about Loops in Ansible 

Ansible provides three keywords for performing a loop operation: loop, with_<lookup>, and until.

Loops are usually used to create multiple users, modifying the ownership for files and directories and iterating over an instruction until a condition is satisfied.

What is Standard or Simple Loops ?

Standard loops can be used for iterating over a simple list of strings that can be defined directly inside a task.

For example, examine the playbook below:

$ nano my-playbook
- name: Add multiple users
hosts: managed1
become: yes
- name: Add three users using Loops
name: "{{ item }}"
state: present
- myuser1
- myuser2

Here, Ansible creates multiple users in a single task using Loops. Also, it uses the "user" module to create the above users.

For every iteration of the loop, the values of the list are substituted in the item variable.

We further explained the Playbook:

In the "user" module, we have not given any name directly; instead, a variable {{ item }} is used with the "name" parameter.

Under the "loop" keyword, the user names to be used are listed. These usernames replace the {{ item }} when the playbook is actually executed.

While executing the task, it is listed only once, but three modifications are listed just below it.

How does Loops Over a List of Hashes work ?

Loops can also be used to iterate over hashes. 

For instance, if the users need to be assigned to distinct additional groups, we need to do it as:

- username: my_user1
groups: production
- username: my_user2
groups: development
- username: my_user3
groups: staging

Here, the groups are already present on the remote machines. We are just adding them to the users using Loops.

An optional parameter called "groups" is used by the "user" module for listing additional users. 

For a list of hashes, we can reference subkeys in a loop. For this purpose, we use the {{ item }} keyword. 

As an illustration, consider the below snippet of a playbook:

- name: Add multiple users
hosts: managed1
become: yes
- name: Add three users using Loops
name: "{{ item.username }}"
state: present
groups: "{{ item.groups }}"
- { username: 'my_user1', groups: 'production' }
- { username: 'my_user2', groups: 'development' }
- { username: 'my_user3', groups: 'staging' }

Ansible Loops With Conditionals

Conditionals can be used with loops to iterate over a sequence until a condition is true. For example, the "when" statement can be used for this purpose. In this case, each condition is separately processed by the Ansible. 

Using this technique, a task can be executed for some items and skipped for others; for example, consider the case below:

- name: Print numbers smaller than 46
ansible.builtin.command: echo {{ item }}
loop: [ 1, 5, 44, 56, 48, 9, 4 ]
when: item < 46

The task should print all the items, here numbers smaller than 46, when it is run and print the result on the terminal. The complete my-playbook.yml, in this case, will be:

- hosts: all
gather_facts: yes
become: true
- name: Print numbers smaller than 46
ansible.builtin.command: echo {{ item }}
loop: [ 1, 5, 44, 56, 48, 9, 4 ]
when: item < 46

Now run the execute the playbook using the command below:

$ ansible-playbook my_playbook.yml -i /path/to/inventory/file

Here echo command will run on those values of the items that are smaller than 46; this can be noted from the output on the terminal.

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This article covers Ansible Loops and examples of using Loops in Ansible. In fact, to avoid repeating the task several times in your playbook file, it's better to use loops instead. A loop allows you to repeat instructions, typically until a certain condition is met. Ansible offers different looping methods, with the loop keyword being the most recommended option for longer term compatibility.

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