1. JSONPath简介

JSON (JavaScript Object Notation) allows for easy interchange of data, often between a program and a database.

JSONPath is a query language for JSON, similar to XPath for XML.

如上，json是一种常用的数据格式，jsonpath是json的查询语言，类似于XPath和SQL。

jsonpath在线测试：JSONPath Online Evaluator

2. YAML和JSON

YAML和JSON可以相互转化，详情参考《YAML语言》。

3. JSONPath语法

摘自 json-path/JsonPath

3.1. 操作符

Operator	Description
`$`	The root element to query. This starts all path expressions.
`@`	The current node being processed by a filter predicate.
`*`	Wildcard. Available anywhere a name or numeric are required.
`..`	Deep scan. Available anywhere a name is required.
`.<name>`	Dot-notated child
`['<name>' (, '<name>')]`	Bracket-notated child or children
`[<number> (, <number>)]`	Array index or indexes
`[start:end]`	Array slice operator
`[?(<expression>)]`	Filter expression. Expression must evaluate to a boolean value.

3.2. 函数

Functions can be invoked at the tail end of a path - the input to a function is the output of the path expression.
The function output is dictated by the function itself.

Function	Description	Output
min()	Provides the min value of an array of numbers	Double
max()	Provides the max value of an array of numbers	Double
avg()	Provides the average value of an array of numbers	Double
stddev()	Provides the standard deviation value of an array of numbers	Double
length()	Provides the length of an array	Integer
sum()	Provides the sum value of an array of numbers	Double

3.3. 过滤器

Filters are logical expressions used to filter arrays. A typical filter would be [?(@.age > 18)] where @ represents the current item being processed. More complex filters can be created with logical operators && and ||. String literals must be enclosed by single or double quotes ([?(@.color == 'blue')] or [?(@.color == "blue")]).

Operator	Description
==	left is equal to right (note that 1 is not equal to ‘1’)
!=	left is not equal to right
<	left is less than right
<=	left is less or equal to right
>	left is greater than right
>=	left is greater than or equal to right
=~	left matches regular expression [?(@.name =~ /foo.*?/i)]
in	left exists in right [?(@.size in [‘S’, ‘M’])]
nin	left does not exists in right
subsetof	left is a subset of right [?(@.sizes subsetof [‘S’, ‘M’, ‘L’])]
anyof	left has an intersection with right [?(@.sizes anyof [‘M’, ‘L’])]
noneof	left has no intersection with right [?(@.sizes noneof [‘M’, ‘L’])]
size	size of left (array or string) should match right
empty	left (array or string) should be empty

3.4. DEMO

Given the json

{
    "store": {
        "book": [
            {
                "category": "reference",
                "author": "Nigel Rees",
                "title": "Sayings of the Century",
                "price": 8.95
            },
            {
                "category": "fiction",
                "author": "Evelyn Waugh",
                "title": "Sword of Honour",
                "price": 12.99
            },
            {
                "category": "fiction",
                "author": "Herman Melville",
                "title": "Moby Dick",
                "isbn": "0-553-21311-3",
                "price": 8.99
            },
            {
                "category": "fiction",
                "author": "J. R. R. Tolkien",
                "title": "The Lord of the Rings",
                "isbn": "0-395-19395-8",
                "price": 22.99
            }
        ],
        "bicycle": {
            "color": "red",
            "price": 19.95
        }
    },
    "expensive": 10
}

JsonPath (click link to try)	Result
$.store.book[*].author	The authors of all books
$..author	All authors
$.store.*	All things, both books and bicycles
$.store..price	The price of everything
$..book[2]	The third book
$..book[-2]	The second to last book
$..book[0,1]	The first two books
$..book[:2]	All books from index 0 (inclusive) until index 2 (exclusive)
$..book[1:2]	All books from index 1 (inclusive) until index 2 (exclusive)
$..book[-2:]	Last two books
$..book[2:]	Book number two from tail
$..book[?(@.isbn)]	All books with an ISBN number
$.store.book[?(@.price < 10)]	All books in store cheaper than 10
$..book[?(@.price <= $[‘expensive’])]	All books in store that are not “expensive”
$..book[?(@.author =~ /.*REES/i)]	All books matching regex (ignore case)
$..*	Give me every thing
$..book.length()	The number of books

4. kubectl + JSONPath

kubelet支持JSONPath，具体参考JSONPath 支持。
除了标准jsonpath语法外，kubernetes jsonpath模板还额外支持以下语法:

用""双引号来引用JSONPath表达式中的文本
使用range和end来遍历集合
使用负数来从尾部索引集合

例如查看node的cpu信息：

kubectl get nodes -o=jsonpath='{.items[*].metadata.name}'
kubectl get nodes -o=jsonpath='{.items[*].status.capacity.cpu}'
kubectl get nodes -o=jsonpath='{.items[*].metadata.name}{.items[*].status.capacity.cpu}'
kubectl get nodes -o=jsonpath='{.items[*].metadata.name}{"\n"}{.items[*].status.capacity.cpu}'
kubectl get nodes -o=jsonpath='{range .items[*]}{.metadata.name}{"\t"}{.status.capacity.cpu}{end}'
kubectl get nodes -o=custom-columns=NODE:.metadata.name,CPU:.status.capacity.cpu
kubectl get nodes --sort-by=.metadata.name
kubectl get nodes --sort-by=.status.capacity.cpu

5. 特殊说明

如果key中包含点.，例如key为config.yaml，那么在使用jsonpath的时候需要对key中的点.进行转义。

1	kubectl get cm test-config -n test -o jsonpath='{.data.config\.yaml}'

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JSONPath入门篇