Скачать или смотреть Mastering sed: How to Replace Multiple Lines in Bash Scripts

Learn how to replace multiple lines using `sed` in Bash without using `tr`. Understand pattern and hold space for efficient text manipulation.
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This video is based on the question https://stackoverflow.com/q/70382671/ asked by the user 'alex2020' ( https://stackoverflow.com/u/15508947/ ) and on the answer https://stackoverflow.com/a/70382750/ provided by the user 'Shawn' ( https://stackoverflow.com/u/9952196/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Mastering sed: How to Replace Multiple Lines in Bash Scripts

Good day, fellow Bash enthusiasts! If you've ever found yourself wrestling with the sed command to make changes to multiple lines in a text file or output, you're not alone. Manipulating text with sed can seem daunting, but once you understand how to utilize its hold space and pattern space, you can make quick work of text replacements. In this guide, we'll walk through how to replace multiple lines using sed in a clean and effective way.

Understanding sed: The Basics

Before diving into the solution, let’s clarify two crucial concepts in sed:

Pattern Space: This is where sed reads the input data. It holds the current line being operated on.

Hold Space: This is a secondary storage area where sed can store and retrieve lines temporarily.

The Goal: Replace Multiple Lines

Imagine you have the following lines of text:

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You want to replace the first two lines (aa and bb) with NEW STRING, resulting in the following output:

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Solution: Replacing Lines with sed

Step 1: Replace Lines by Line Range

One of the simplest ways to replace multiple lines using sed is by specifying a range. For the above example, you can use the following command:

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The output will be:

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In this command:

1,2 specifies that we want to operate on lines 1 to 2.

s/.*/NEW STRING/ replaces the entire content of the line with NEW STRING.

Step 2: Using Change Command

Additionally, you can also use the change command c for a cleaner replacement:

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This will produce the same output after replacing lines 1 and 2 with NEW STRING.

A More Dynamic Approach: Unknown Line Numbers

Now, what if you have a scenario where you do not know the exact line numbers, but you know that aa comes after bb? Here’s a clever solution using a conditional structure in sed:

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Explanation of the Command:

Pattern Match: The command begins with looking for the line containing aa.

Appending Next Line: If aa is found, N appends the next line (bb) to the current pattern space.

Conditional Replacement: If the pattern space exactly matches aa\nbb:

It replaces the matched string with two NEW STRING lines,

Prints out the modified pattern space,

Deletes it and begins anew.

Handling Other Text: If the pattern does not match, it prints everything before the newline, deletes that portion from the pattern space, and continues processing with the following lines.

Conclusion

Mastering the use of sed can greatly enhance your scripting and text manipulation skills. Being able to replace multiple lines with efficiency and clarity can save you time and effort. Whether you're working with known line numbers or need a more flexible approach based on the data context, the methods discussed in this post should provide you with the tools you need to tackle text replacement confidently.

Feel free to experiment with these commands and integrate them into your Bash scripts. Happy scripting!