Ansible Replace String In File

MIster jashon

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·

Oct 10, 2024

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How to Replace Strings in Files with Ansible

Ansible is a powerful automation tool that allows you to manage and configure your infrastructure. One of its key features is the ability to manipulate files, including replacing strings within them. This can be incredibly useful for tasks such as updating configuration files, patching code, or simply making quick changes to your system.

This article will guide you through the process of using Ansible to replace strings in files, covering different approaches and scenarios.

The "Replace" Module

The cornerstone of Ansible's file manipulation is the replace module. This module takes a file as input and replaces occurrences of a given string with a new string.

Here's a basic example:

---
- hosts: all
  tasks:
    - name: Replace "old_string" with "new_string" in file
      replace:
        path: /path/to/file.txt
        regexp: 'old_string'
        replace: 'new_string'

Explanation:

• hosts: all: This line specifies that the task will be executed on all managed hosts.
• tasks: This section defines the tasks to be performed.
• name: A descriptive name for the task.
• replace: This indicates that the module will replace strings in a file.
• path: The path to the file where the replacement should occur.
• regexp: The string to be replaced. This can be a regular expression for more advanced matching.
• replace: The new string that will replace the existing string.

Important Note: The regexp parameter uses regular expression syntax. This allows you to match complex patterns beyond simple strings. For example, you can use regexp: "old_[0-9]+" to replace any string starting with "old_" followed by one or more numbers.

Handling Multiple Occurrences

The replace module by default replaces all occurrences of the target string within the file. If you want to control the number of replacements, you can use the count parameter:

---
- hosts: all
  tasks:
    - name: Replace "old_string" with "new_string" only the first 3 occurrences
      replace:
        path: /path/to/file.txt
        regexp: 'old_string'
        replace: 'new_string'
        count: 3

This will replace only the first three occurrences of "old_string" with "new_string".

Using Backreferences

Regular expressions can be powerful, and the replace module supports backreferences. This allows you to capture parts of the matched string and use them in the replacement.

---
- hosts: all
  tasks:
    - name: Replace "old_string" with "new_string" using backreference
      replace:
        path: /path/to/file.txt
        regexp: 'old_(.+?)_string'
        replace: 'new_\1_string' 

Explanation:

• The regex old_(.+?)_string captures the text between "old_" and "_string" and stores it in a group.
• The replace value uses \1 to reference the captured group from the regexp.

This example would replace "old_value_string" with "new_value_string", effectively substituting only the middle part.

Beyond Strings: Working with Files

The replace module is primarily used for string replacements. However, you can also use it for more complex operations:

• Inserting lines: You can insert lines at a specific position within a file by using a regular expression that matches the desired position.
• Deleting lines: Use a regular expression to match lines you want to remove.
• Appending lines: Add lines to the end of a file by using a regular expression that matches the end of the file.

Examples

1. Adding a line to a configuration file:

---
- hosts: all
  tasks:
    - name: Add a line to the end of the file
      replace:
        path: /etc/apache2/apache2.conf
        regexp: '^