Efficiently Store and Update JSON Over Time Using JQ

A practical guide on how to effectively read, modify, and write JSON data using jq in a shell script. Learn how to avoid common pitfalls and handle JSON safely!
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Storing and Updating JSON Over Time Using JQ

When working with JSON data in shell scripts, it's common to encounter challenges that can lead to errors. For instance, you might want to read some JSON data from a file, update it through a series of prompts, and eventually write it back to disk. This is a typical scenario for many developers and system administrators who need to manage configurations or data dynamically. Let's explore a recent issue someone faced and how they resolved it.

The Problem

A user was trying to accomplish the following tasks:

Read JSON data from a file into a variable.

Modify a property within the JSON structure based on user inputs.

Write the updated JSON back to the disk.

The user implemented the following code snippet to achieve this:

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

However, this resulted in a parse error: Invalid escape at line 1, column 18629. This error led them to suspect that their variable assignment was not in a format that jq could process effectively.

Understanding the Solution

The Issue: Quoting Parameters

In shell scripting, quoting is crucial. If parameters are not quoted when expanded, the shell may perform unwanted word splitting and glob expansion, leading to errors. In this scenario, the lack of quotes around UPDATED_JSON caused jq to parse the JSON incorrectly.

Correct Assignments

Here’s how they correctly updated their script to safely handle JSON content:

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

Key Changes Made:

Using quotes: By replacing echo $UPDATED_JSON with echo "$UPDATED_JSON", word splitting is avoided, ensuring the entire JSON structure is treated as a single string.

Initial Read: The JSON data was read into UPDATED_JSON directly using jq without unnecessary piping from cat, which simplifies the command.

Bonus Tip: Using printf

While echo may seem safe for printing JSON content, it’s a good practice to use printf for better stability and consistency in handling special characters. For example:

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

This approach helps prevent further potential parsing issues when dealing with more complex JSON data.

Conclusion

Manipulating JSON in a shell script can be highly effective when done correctly. Always ensure that you quote your variables to avoid issues with parsing. Should you find a need to make further modifications or write the upgraded JSON back to a file, remember the following suggestions:

Always handle JSON safely with jq commands.

Consider using printf to handle special characters in JSON effectively.

Be cautious of any properties within your JSON structure that may not always exist.

By understanding the importance of quoting and following best practices, you can safely and effectively store and update JSON data over time in your shell scripts.