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Discover how to correctly concatenate string variables in Bash without losing line feeds. Learn effective methods to manage newlines in your scripts.
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This video is based on the question https://stackoverflow.com/q/78140667/ asked by the user 'Robin Hsu' ( https://stackoverflow.com/u/11954790/ ) and on the answer https://stackoverflow.com/a/78140686/ provided by the user 'KamilCuk' ( https://stackoverflow.com/u/9072753/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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How to Concatenate String Variables in Bash While Preserving Line Feeds

Programming in Bash can sometimes lead you to unexpected challenges, especially when dealing with string variables and formatting. One common issue developers face is preserving line feeds while concatenating strings. In this post, we will explore a specific example that illustrates this problem and provide a straightforward solution.

The Problem

Imagine you have a Bash variable, A, containing two lines: 33 and 44. It looks like this:

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When you display the contents of A, it appears correctly with the two line feeds:

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However, when you try to concatenate another value, 55, to this variable:

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You notice that the output is different. Instead of getting three lines as expected, you end up with only two:

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What's Happening?

The unexpected behavior arises because the command substitution $(...) removes all trailing newlines. Essentially:

The second newline, after 44, is eliminated during the concatenation process.

The variable A ends up storing only a single newline character instead of the expected two.

The Solution

Now that we understand the problem, let’s address it. Here’s how you can effectively concatenate string variables in Bash while retaining the line feeds.

1. Appending a Newline

If you want to add a newline followed by 55, you can do this by:

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This will ensure that 55 is placed on a new line.

2. Adding a Trailing Newline

If your intention is to have a trailing newline in your variable, you can modify your assignment like this:

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Using $'\n' appends a newline character explicitly to the variable A.

3. Debugging Variables

It's also helpful to inspect what your variable contains at any time. You can do this using the declare -p command:

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This will show you how the variable A is structured, which helps in diagnosing issues with line feeds.

Fun Fact

Interestingly, these two lines of code are equivalent when defining A:

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Both lines will correctly initialize A with 33 and 44, followed by a newline.

Conclusion

Concatenating string variables in Bash can be tricky, especially when it comes to preserving formatting like line feeds. By understanding how Bash handles newlines and using the proper techniques to append or maintain newlines, you can avoid common pitfalls in your scripting. The key takeaway is to always be mindful of how newlines are managed in your variables, so you can ensure your output remains as intended!