RHCE Ansible Series #5: Ansible Loops

You may sometimes want to repeat a task multiple times. For example, you may want to create multiple users, start/stop multiple services, or change ownership on several files on your managed hosts.

In this tutorial, you will learn how to use Ansible loops to repeat a task multiple times without having to rewrite the whole task over and over again.

Before you look at loops in Ansible, I hope you have followed other chapters in this Ansible tutorial series. You should know the concept of Ansible playbooks, aware of the ad-hoc commands and know the basic terminology associated with Ansible like list, dictionaries etc.

Knowing the basics of YAML is also appreciated.

Looping over lists

Ansible uses the keywords loop to iterate over the elements of a list. To demonstrate, let’s create a very simple playbook named print-list.yml that shows you how to print the elements in a list:

[[email protected] plays]$ cat print-list.yml 
---
- name: print list
  hosts: node1
  vars:
    prime: [2,3,5,7,11]
  tasks:
    - name: Show first five prime numbers
      debug:
        msg: "{{ item }}"
      loop: "{{ prime }}"

Notice that I use the item variable with Ansible loops. The task would run five times which is equal to the number of elements in the prime list.

On the first run, the item variable will be set to first element in the prime array (2). On the second run, the item variable will be set to the second element in the prime array (3) and so on.

Go ahead and run the playbook to see all the elements of the prime list displayed:

[[email protected] plays]$ ansible-playbook print-list.yml 

PLAY [print list] **************************************************************

TASK [Gathering Facts] *********************************************************
ok: [node1]

TASK [Show first five prime numbers] *******************************************
ok: [node1] => (item=2) => {
    "msg": 2
}
ok: [node1] => (item=3) => {
    "msg": 3
}
ok: [node1] => (item=5) => {
    "msg": 5
}
ok: [node1] => (item=7) => {
    "msg": 7
}
ok: [node1] => (item=11) => {
    "msg": 11
}

PLAY RECAP *********************************************************************
node1                      : ok=2    changed=0    unreachable=0    failed=0

Now you apply loops to a real life application. For example, you can create an add-users.yml playbook that would add multiple users on all the hosts in the dbservers group:

[[email protected] plays]$ cat add-users.yml 
---
- name: Add multiple users
  hosts: dbservers
  vars:
    dbusers:
      - username: brad
        pass: pass1
      - username: david
        pass: pass2
      - username: jason
        pass: pass3
  tasks: 
    - name: Add users
      user:
        name: "{{ item.username }}"
        password: "{{ item.pass | password_hash('sha512') }}"
      loop: "{{ dbusers }}"

I first created a dbusers list which is basically a list of hashes/dictionaries. I then used the user module along with a loop to add the users and set the passwords for all users in the dbusers list.

Notice that I also used the dotted notation item.username and item.pass to access the keys values inside the hashes/dictionaries of the dbusers list.

It is also worth noting that I used the password_hash('sha512') filter to encrypt the user passwords with the sha512 hashing algorithm as the user module wouldn’t allow setting unencrypted user passwords.

Now let’s run the add-users.yml playbook: