jq startswith

Guri Bee

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20-10-2024

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Mastering jq: The "startswith" Technique for JSON Data Manipulation

jq, the lightweight and powerful command-line JSON processor, is a must-have tool for anyone working with JSON data. One of its most useful features is the ability to filter data based on specific patterns. This article explores the "startswith" technique in jq, empowering you to efficiently extract and manipulate data from JSON objects.

What is "startswith" in jq?

In jq, the startswith function allows you to check if a string begins with a specific substring. It returns a boolean value (true or false) based on the comparison. This powerful function can be used in various scenarios, such as:

• Filtering JSON objects based on specific prefixes in keys or values: For example, you might want to extract all objects where the key "name" starts with "John".
• Conditional statements: You can use startswith within if statements to execute code blocks only if a particular condition is met.
• Combining with other jq functions: startswith can be seamlessly combined with other jq functions like select, map, and reduce to perform complex data transformations.

Understanding the Syntax

The basic syntax for startswith in jq is:

.key | startswith("substring")

Breakdown:

• .key: Selects the specific key you want to check.
• |: Pipes the output to the startswith function.
• "substring": The substring you're looking for at the beginning of the string.

Practical Examples

Let's illustrate the use of startswith with some practical examples:

1. Filtering JSON based on key prefixes:

Suppose you have a JSON file containing a list of users, and you want to extract users whose names start with "A":

Example JSON (users.json):

[
  {"name": "Alice", "age": 25},
  {"name": "Bob", "age": 30},
  {"name": "Andrew", "age": 28},
  {"name": "Catherine", "age": 22}
]

jq command:

jq '.[] | select(.name | startswith("A"))' users.json

Output:

[
  {
    "name": "Alice",
    "age": 25
  },
  {
    "name": "Andrew",
    "age": 28
  }
]

2. Conditional statements:

Let's imagine you want to add a custom field "status" to each user in the JSON, assigning "VIP" if the username starts with "J" and "Standard" otherwise:

jq command:

jq '.[] | {
  (.name | startswith("J")) // "VIP" as $status,
  ( .name | startswith("J")) // "Standard" as $status,
  . + { status: $status } 
}' users.json

Output:

[
  {
    "name": "Alice",
    "age": 25,
    "status": "Standard"
  },
  {
    "name": "Bob",
    "age": 30,
    "status": "Standard"
  },
  {
    "name": "Andrew",
    "age": 28,
    "status": "Standard"
  },
  {
    "name": "Catherine",
    "age": 22,
    "status": "Standard"
  }
]

3. Combining with other jq functions:

Suppose you want to extract all users whose age is over 25 and their names start with "B":

jq command:

jq '.[] | select(.age > 25 and .name | startswith("B"))' users.json

Output:

{
  "name": "Bob",
  "age": 30
}

Beyond "startswith": Enhancing Your Data Manipulation

While startswith is a powerful tool, remember that jq offers many other functions for manipulating strings and data. Explore functions like endswith, contains, match, and capture to broaden your data filtering and processing capabilities.

Conclusion

jq's startswith function is a versatile tool for analyzing and transforming JSON data. By incorporating it into your jq workflows, you can filter, select, and manipulate JSON objects based on specific string prefixes. Whether you're working with user profiles, product catalogs, or any other JSON-based data, mastering "startswith" empowers you to perform intricate data operations with ease.