Introduction to Ansible Replace Line in File
Ansible Replace Line in File is a module provided in Ansible package to replace a string pattern in a file. For Linux environment, where we have everything as a file. It becomes very important to manage all important configuration, library and scripts files. This is done by administrator by updating all such files regularly with all updates in infrastructure. Ansible provides modules like replace, blockinfile, copy, template and lineinfile to work with such situations. Selecting a module depends on the scope of tasks and number of iterations involved in same. So one should evaluate first then select a module to work with.
Working of Ansible Replace Line in File
In Ansible, like any other module, where we have a set of available parameters and possible acceptable values for same, in Ansible Replace module we have same as well.
Below list includes many important parameters if not all, which can be used in combination to generate desired results:
- backup: This parameter is used for taking a backup of a file with timestamp before updating it. So that we will be able to restore it when needed later. Default is “no”.
- attributes: This is used to set attributes of the resulting file. This can be checked by lsattrlater. Check the supported flags at man page of chattr on target systems.
- owner: Name of the user, owning the resulting file.
- group: Name of the group, owning the resulting file.
- after: This is used to specify that only content after this match will be removed/replaced. This can be used with before in a combination.
- before: This is used to specify that only content after this match will be removed/replaced. This can be used with after in a combination.
- mode: The permission of resulting file. Better practice is use 4 octal numbers inside single quotes to represent the permission like ‘0777’ or ‘0644’.
- path: The file’s path, which is our task’s target.
- regexp: The pattern to look for in every line of file. It uses python regular expression.
- selevel, serole, setype, seuser: These are used to update the selinux file context.
- validate: This is used to validate the command before running on target system shell. The file path to be validated is passed in via ‘%s’.
- encoding: The character encoding for a file. Default s utf-8.
- others: All arguments accepted by file also works here.
- replace: The string pattern which will replace the mated patterns. If this is not set, then matched regex/patterns are removed entirely.
Examples of Ansible Replace Line in File
We will see some examples, but before going there, we first understand our lab, we used for testing purpose. Here we have an Ansible control server named ansible-controller and two remotes hosts named host-one and host-two. We will create playbooks and run ansible commands on ansible-controller node and see the results on remote hosts.
Example #1
In this example, we have a sample file, in which we are trying to replace a string using Ansible. For this, we have a playbook like below, where we are using Ansible replace and debug modules to replace and display the outputs.
Code:
---
- hosts: all
gather_facts: no
tasks:
- name: Here we check the contents of /var/tmp/sample_file file on target machines.
shell: 'cat /var/tmp/sample_file'
register: var_output
- name: Here are the contents of /var/tmp/sample_file file on target machines before replace
debug:
var: var_output.stdout_lines
- name: Here we are replacing the string pattern
replace:
path: /var/tmp/sample_file
regexp: 'Hello World'
replace: "Namaste To All"
- name: Here we are again checking the contents of /var/tmp/sample_file file on target machines.
shell: 'cat /var/tmp/sample_file'
register: var_output_1
- name: Here are the contents of /var/tmp/sample_file file on target machines after replace
debug:
var: var_output_1.stdout_lines
Now executing this playbook like below:
ansible-playbook ansible_replace.yaml
We have below output where we can see the Hello World is replaced with Namaste To All.
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Output:
Example #2
In this example, we have a playbook like below where we are trying to remove the matched pattern from out sample file. For this we have a playbook like below, where we are not using replace parameter in replace module, which will result in removing the matched pattern ‘To All’ from target file.
Code:
---
- hosts: all
gather_facts: no
tasks:
- name: Here we check the contents of /var/tmp/sample_file file on target machines.
shell: 'cat /var/tmp/sample_file'
register: var_output
- name: Here are the contents of /var/tmp/sample_file file on target machines before replace
debug:
var: var_output.stdout_lines
- name: Here we are replacing the string pattern
replace:
path: /var/tmp/sample_file
regexp: 'To All'
- name: Here we are again checking the contents of /var/tmp/sample_file file on target machines.
shell: 'cat /var/tmp/sample_file'
register: var_output_1
- name: Here are the contents of /var/tmp/sample_file file on target machines after replace
debug:
var: var_output_1.stdout_lines
Now executing this playbook like below:
ansible-playbook ansible_replace.yaml
We have below output where we can see the ‘To All’ is removed from the file.
Output:
Example #3
In this example, we are going to replace a string in /etc/hosts and also will set ownership and permissions. For this we have a playbook like below.
Code:
---
- hosts: host-one
gather_facts: no
tasks:
- name: Here we check the contents of /etc/hosts
shell: 'cat /etc/hosts| grep "192.168.0.11"'
register: var_output
- name: Here are the contents of /etc/hosts file on target machines before replace
debug:
var: var_output.stdout_lines
- name: Here we are replacing the string pattern
replace:
backup: yes
path: /etc/hosts
regexp: '(\s+)host1\.example\.com(\s+.*)?$'
replace: '\1server1.example.com\2'
owner: root
group: root
mode: 0644
- name: Here we again check the contents of /etc/hosts
shell: 'cat /etc/hosts| grep "192.168.0.11"'
register: var_output
- name: Here are the contents of /etc/hosts file on target machines after replace
debug:
var: var_output.stdout_lines
Now running this playbook will give output like below where we can clearly see that string host1.example.com has been replaced by server1.example.com.
Also check the other parameters used to set permission and ownership of the file after changes.
ansible-playbook ansible_replace_IP.yaml --syntax-check
Output:
Conclusion
As we saw in this article, this module can replace a line in a file on target remote machines. Also it can set permissions, ownership etc. to a file. This module is easy to use and works with basics of Linux systems.
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