The command ajson is a tool that allows you to execute JSONPath expressions on JSON objects. JSONPath is a query language for JSON, similar to XPath for XML. It can be used to extract specific data from JSON documents.

Use case 1: Read JSON from a file and execute a specified JSONPath expression

Code:

ajson '$..json[?(@.path)]' path/to/file.json

Motivation: This use case is useful when you have a JSON file and want to extract specific data from it using a JSONPath expression. By specifying the JSONPath expression in the command, you can easily retrieve the desired data without the need for writing a separate script.

Explanation:

• ajson: This is the command used to execute JSONPath expressions.
• '$..json[?(@.path)]': This is the JSONPath expression that specifies the data to be extracted. In this example, it selects all objects with a “path” property.
• path/to/file.json: This is the path to the JSON file from which the data will be read.

Example output:

[
  {
    "path": "/example/path1",
    "value": "data1"
  },
  {
    "path": "/example/path2",
    "value": "data2"
  }
]

Use case 2: Read JSON from stdin and execute a specified JSONPath expression

Code:

cat path/to/file.json | ajson '$..json[?(@.path)]'

Motivation: This use case is useful when you want to process JSON data from an external source, such as the output of another command. By using stdin, you can easily integrate ajson into a pipeline of commands.

Explanation:

• cat path/to/file.json: This command reads the contents of the JSON file and outputs them to stdout.
• |: The pipe operator is used to redirect the output of the previous command to the input of the next command.
• ajson '$..json[?(@.path)]': This is the JSONPath expression that specifies the data to be extracted. In this example, it selects all objects with a “path” property.

Example output:

[
  {
    "path": "/example/path1",
    "value": "data1"
  },
  {
    "path": "/example/path2",
    "value": "data2"
  }
]

Use case 3: Read JSON from a URL and evaluate a specified JSONPath expression

Code:

ajson 'avg($..price)' 'https://example.com/api/'

Motivation: This use case is useful when you need to fetch JSON data from a remote API and calculate certain values from it. In this example, the JSONPath expression calculates the average value of the “price” field in the JSON data.

Explanation:

• ajson: This is the command used to execute JSONPath expressions.
• 'avg($..price)': This is the JSONPath expression that calculates the average value of the “price” field in the JSON data. The $..price part selects all “price” values from the JSON data, and avg() is a JSONPath function that calculates the average.
• 'https://example.com/api/': This is the URL from which the JSON data will be fetched.

Example output:

21.5

Use case 4: Read some simple JSON and calculate a value

Code:

echo '3' | ajson '2 * pi * $'

Motivation: This use case is useful when you want to perform simple calculations on JSON-like data. By specifying a mathematical expression in the JSONPath query, you can easily perform calculations using the data as variables.

Explanation:

• echo '3': This command prints the number 3 to stdout.
• |: The pipe operator is used to redirect the output of the previous command to the input of the next command.
• ajson '2 * pi * $': This is the JSONPath expression that calculates the result of the mathematical expression. In this example, it multiplies the number 3 by 2, pi, and the value from the previous command.

Example output:

18.8495559215

Conclusion:

The ajson command is a useful tool for executing JSONPath expressions on JSON objects. It allows you to extract specific data from JSON files, process JSON data from stdin, fetch JSON data from remote APIs, and perform calculations on JSON-like data. By understanding the syntax of JSONPath expressions and utilizing the ajson command, you can efficiently work with JSON data.