jq select contains

House Authors

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16-12-2024

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jq is a powerful command-line JSON processor. One of its most useful features is the ability to select elements based on whether they contain specific values. This article explores various techniques for using jq to select JSON elements that contain a particular string or pattern. We'll cover different scenarios and provide clear examples to help you master this essential jq skill.

Selecting Objects Containing a Specific Value

Let's start with a simple example. Suppose you have a JSON array of objects, and you want to select only those objects where a specific field contains a certain value.

Imagine this JSON data:

[
  {"name": "Apple", "color": "Red"},
  {"name": "Banana", "color": "Yellow"},
  {"name": "Orange", "color": "Orange"},
  {"name": "Grape", "color": "Green"}
]

To select objects where the color field contains "Red", you would use the following jq command:

jq '.[] | select(.color == "Red")' data.json

This command does the following:

• '.[]': This iterates over each element in the array.
• '| select(.color == "Red")': This filters the array, keeping only the objects where the color field is equal to "Red".

The output will be:

{
  "name": "Apple",
  "color": "Red"
}

This approach works well for exact matches. However, what if you want to find objects where the name field contains a specific substring, such as "grape"?

Using contains for Substring Matching

The contains operator in jq provides a way to check if a string contains a substring. Let's modify our example to find fruits where the name contains "ape":

jq '.[] | select(.name | contains("ape"))' data.json

This command will output:

{
  "name": "Apple",
  "color": "Red"
}
{
  "name": "Grape",
  "color": "Green"
}

Both "Apple" and "Grape" contain "ape", so both objects are selected.

This demonstrates the power of combining iteration ('.[]') with selection (select) and the contains operator for flexible filtering.

Handling Different Data Types

The contains operator works primarily with strings. If you're dealing with numerical or other data types, you'll need to adjust your approach accordingly. For example, if you want to select objects based on a numerical range, you'd use comparison operators like > or <.

Complex Filtering with Multiple Conditions

You can combine multiple select statements or use logical operators (and, or, not) to create more complex filtering logic.

For example, to select fruits that are either red or have a name containing "a":

jq '.[] | select(.color == "Red" or .name | contains("a"))' data.json

Searching within Nested JSON

jq handles nested JSON structures gracefully. You can navigate through nested objects using dot notation and apply the contains operator as needed. For instance:

{
  "fruits": [
    {"name": "Apple", "details": {"origin": "USA"}},
    {"name": "Banana", "details": {"origin": "Ecuador"}}
  ]
}

To select fruits where the origin contains "USA":

jq '.fruits[] | select(.details.origin | contains("USA"))' data.json

This command will correctly select the "Apple" object.

Conclusion

jq's contains operator, coupled with its powerful selection capabilities, provides a versatile approach to filtering JSON data based on substring matches. By understanding how to combine iteration, selection, and logical operators, you can perform complex filtering tasks efficiently and effectively. Remember to adapt your approach based on the data type and structure of your JSON. With practice, you'll become proficient in using jq to extract the precise information you need from your JSON data.