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Discover a reliable method to replace an IP address in a file using Perl or Sed while avoiding unwanted changes.
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How to Change an IP Address Safely with Perl or Sed

Changing IP addresses in files can often be a tricky process, especially when you need to ensure that only specific instances are replaced without affecting similar patterns. In this guide, we will explore how to address an issue where using Perl or Sed might unintentionally alter additional, similar IP addresses—specifically, when replacing "10.135.10.11" can change "10.135.10.111" to "10.135.10.771". Let's dive into the problem and then walk through a robust solution.

The Problem at Hand

Imagine having a file full of IP addresses and subnet masks formatted like this:

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When you run commands like the following:

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or

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you'll find that every instance of .11 gets changed, turning 10.135.10.111 into 10.135.10.771, 10.135.10.772, etc. This is obviously not the desired outcome.

The Solution: Using Word Boundaries

To accurately change just the IP address in question without affecting any similar patterns, we can leverage GNU sed's word boundary features. Here’s how:

Step-by-Step Guidelines

Set Your Variables:
First, you'll want to define the original IP and the new IP address.

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Use sed with Word Boundaries:
Instead of using a straightforward replacement, utilize word boundary sequences to ensure that only the intended IP address is modified:

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Here, the < and > tell sed to only match the exact word, preventing alterations to similar patterns.

Example Input and Expected Output

If you have the input file structured as shown above, after running the modified sed command:

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You'll notice that only 10.135.10.11 has changed to 10.135.10.77, leaving other instances like 10.135.10.111 unaffected.

Conclusion

Replacing IP addresses in text files can lead to unintended consequences if not handled carefully. By utilizing word boundaries with sed, you can ensure precise replacements without altering similar patterns accidentally. If your version of sed doesn’t support word boundaries, check if you are using GNU sed by running sed --version.

With this approach, you can efficiently manage your IP addresses with confidence, maintaining the integrity of your file's content.