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Learn how to append a JSON object to an existing JSON file using shell scripting in Ubuntu Linux with the help of the `jq` command-line tool.
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This video is based on the question https://stackoverflow.com/q/68025031/ asked by the user 'codeLover' ( https://stackoverflow.com/u/485539/ ) and on the answer https://stackoverflow.com/a/68025193/ provided by the user 'chepner' ( https://stackoverflow.com/u/1126841/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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How to Append a JSON Object to a JSON File Using Shell Script on Linux

If you're working with JSON files in a Linux environment, you might encounter the need to append a new JSON object to an existing JSON file. This task can be accomplished efficiently using a command-line tool called jq. In this post, we’ll walk you through the process of appending a JSON object to the end of a JSON file in a shell script. We’ll use the example of modifying a Docker configuration file, ~/.docker/config.json, to demonstrate this functionality.

Problem Overview

Imagine you have the following JSON structure in your Docker configuration file (~/.docker/config.json):

[[See Video to Reveal this Text or Code Snippet]]

Now, you want to add a new section for proxies, which looks as follows:

[[See Video to Reveal this Text or Code Snippet]]

The final structure of your config.json should look like this:

[[See Video to Reveal this Text or Code Snippet]]

Solution with jq

The jq tool is perfect for manipulating JSON data because it allows you to filter and modify JSON files easily. Here’s how to use jq to append the new JSON object to config.json:

Write the Command: You can execute the following command in your terminal:

[[See Video to Reveal this Text or Code Snippet]]

Explanation of the Command:

The first part .proxies = {...} indicates that you are adding or modifying the proxies field in your JSON object.

The {...} encapsulates the new JSON object you wish to add under the proxies key.

The operation > tmp writes the output to a temporary file named tmp instead of altering config.json directly. This prevents loss of original data if anything goes wrong.

The mv tmp ~/.docker/config.json command renames the temporary file to replace the original config.json only after ensuring there was no error.

Why Use a Temporary File? Directly modifying files can lead to data loss if there’s an error in the command. By first writing to a temporary file, you can ensure data integrity and avoid potential issues.

Conclusion

Using jq to append a JSON object to an existing JSON file in Linux is a straightforward process that enhances your ability to manipulate configurations efficiently. The above command provides a robust solution, allowing you to integrate new settings without risking corruption of your original configuration file.

Feel free to try implementing this in your own environment and adjust the JSON objects according to your needs!