In Ansible, you can define conditions that will be evaluated before a task is executed. When a condition is not met, the task is then skipped. This is done with the when
keyword, which accepts expressions that are typically based on a variable or a fact.
The following example defines two variables: create_user_file
and user
. When the create_user_file
is evaluated to true
, a new file will be created in the home directory of the user defined by the user
variable:
Create a new file called playbook-04.yml
in your ansible-practice
directory:
- nano ~/ansible-practice/playbook-04.yml
Then add the following lines to the new playbook file:
---
- hosts: all
vars:
- create_user_file: yes
- user: sammy
tasks:
- name: create file for user
file:
path: /home/{{ user }}/myfile
state: touch
when: create_user_file
Save and close the file when you’re done editing its contents.
To execute this playbook on servers from your inventory file, run ansible-playbook
with the same connection arguments you’ve used before when running other playbooks in this series. Again, we’ll be using an inventory file named inventory
and the sammy user to connect to the remote servers:
- ansible-playbook -i inventory playbook-04.yml -u sammy
When the condition is met, you’ll see a changed
status in the play output:
Output...
TASK [create file for user] *****************************************************************************
changed: [203.0.113.10]
...
If you change the value of create_user_file
to no
, the condition will be evaluated to false
. In this case, you’ll see a skipping
status in the play output, indicating that the task was not executed:
Output...
TASK [create file for user] *****************************************************************************
skipping: [203.0.113.10]
...
A common use for conditionals in the context of Ansible playbooks is to combine them with register
, a keyword that creates a new variable and assigns it with the output obtained from a command. This way, you can use any external command to evaluate the execution of a task.
One important thing to notice is that, by default, Ansible will interrupt a play if the command you’re using to evaluate a condition fails. For that reason, you’ll need to include an ignore_errors
directive set to yes
in said task, and this will make Ansible move on to the next task and continue the play.
The following example will only create a new file in the user home directory in case that file doesn’t exist yet, which we’ll test with an ls
command. If the file exists, however, we’ll show a message using the debug
module.
Create a new file called playbook-05.yml
in your ansible-practice
directory:
- nano ~/ansible-practice/playbook-05.yml
Then add the following content to the new playbook file:
---
- hosts: all
vars:
- user: sammy
tasks:
- name: Check if file already exists
command: ls /home/{{ user }}/myfile
register: file_exists
ignore_errors: yes
- name: create file for user
file:
path: /home/{{ user }}/myfile
state: touch
when: file_exists is failed
- name: show message if file exists
debug:
msg: The user file already exists.
when: file_exists is succeeded
Save and close the file when you’re done.
Then, run ansible-playbook
with the same connection arguments from the previous examples. Here, we’re using an inventory file named inventory
and a user named sammy
, but you should change these values accordingly:
- ansible-playbook -i inventory playbook-05.yml -u sammy
The first time you run this playbook, the command will fail because the file doesn’t exist in that path. The task that creates the file will then be executed, while the last task will be skipped:
...
[secondary_label Output]
TASK [Check if file already exists] *********************************************************************
fatal: [203.0.113.10]: FAILED! => {"changed": true, "cmd": ["ls", "/home/sammy/myfile"], "delta": "0:00:00.004258", "end": "2020-10-22 13:10:12.680074", "msg": "non-zero return code", "rc": 2, "start": "2020-10-22 13:10:12.675816", "stderr": "ls: cannot access '/home/sammy/myfile': No such file or directory", "stderr_lines": ["ls: cannot access '/home/sammy/myfile': No such file or directory"], "stdout": "", "stdout_lines": []}
...ignoring
TASK [create file for user] *****************************************************************************
changed: [203.0.113.10]
TASK [show message if file exists] **********************************************************************
skipping: [203.0.113.10]
...
From the output, you can see that the create file for user
task caused a change in the server, which means the file was created. Now, run the playbook again and you’ll get a different result:
- ansible-playbook -i inventory playbook-05.yml -u sammy
Output...
TASK [Check if file already exists] *********************************************************************
changed: [203.0.113.10]
TASK [create file for user] *****************************************************************************
skipping: [203.0.113.10]
TASK [show message if file exists] **********************************************************************
ok: [203.0.113.10] => {
"msg": "The user file already exists."
}
...
If you’d like to learn more about using conditionals in Ansible playbooks, please refer to the official documentation.
Thanks for learning with the DigitalOcean Community. Check out our offerings for compute, storage, networking, and managed databases.
Ansible is a modern configuration management tool that doesn’t require the use of an agent software on remote nodes, using only SSH and Python to communicate and execute commands on managed servers. This series will walk you through the main Ansible features that you can use to write playbooks for server automation. At the end, we’ll see a practical example of how to create a playbook to automate setting up a remote Nginx web server and deploy a static HTML website to it.
This textbox defaults to using Markdown to format your answer.
You can type !ref in this text area to quickly search our full set of tutorials, documentation & marketplace offerings and insert the link!
Sign up for Infrastructure as a Newsletter.
Working on improving health and education, reducing inequality, and spurring economic growth? We'd like to help.
Get paid to write technical tutorials and select a tech-focused charity to receive a matching donation.