jq 中文手册(v1.5)
jq 中文手册(v1.5) 本文档为 stedolan.github.io/jq/manual/v1.5/ 的中文翻译，旨在推广 jq 的国内使用 (翻译说明) jq 程序就像一个过滤器：接收输入，并产
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jq 中文手册(v1.5)
本文档为 stedolan.github.io/jq/manual/v1.5/ 的中文翻译，旨在推广 jq 的国内使用 (翻译说明)
jq 程序就像一个过滤器：接收输入，并产生输出。有许多内置的过滤器，提取一个对象的特定字段、或是把数字转成字符串，或是大量的其他的标准任务。
可以通过多种方法结合这些过滤器 － 可以用管道将一个过滤器的输出连到另一个的输入上，或者是把一个过滤器的输出收集到一个数组里。
一些过滤器产生多个结果，比如就有一个可以展开输入的数组把每个元素都给出来的的过滤器，用管道把这个连到第二个过滤器上就使第二个过滤器在数组的每一个元素上作用一遍。通常在其他语言里用循环或者迭代的任务在jq中通过结合过滤器来完成。
切记每个过滤器都有一个输入和一个输出。即使像”hello”或者42这样的常量都是过滤器－他们接受输入但是只产生同样的常量作为输出罢了。操作符可以结合两个过滤器，比如 加 , 一般是给两个过滤器同样的输入，并把结果连接起来。所以你可以实现一个求平均过滤器，即add/length － 把输入数组分给add过滤器和length过滤器，然后做了一个除法。
但是这个可能有些超前了。:),接着看一些简单的:
内容:
调用jq
基本过滤器
类型和值
内置操作符和函数
条件和比较
正则表达式(PCRE)
高级特性
数学(Math)
IO
流(Streaming)
赋值(Assignment)
模块(Modules)
调用jq
jq 过滤器运行在JSON 数据流上。jq的输入被解析为一系列由空格分隔的JSON 值，它们一次一个地通过jq 的过滤器。过滤器的输出被写入标准输出，再次作为一系列由空格分隔的JSON数据。
注意: 注意: 一定要当心 shell 的 quote 规则。作为一般规则，最好总是为 jq 程序带上引号(使用单引号)，因为太多对jq有特殊意义的字符也是 shell 的元字符。比如 jq "foo"，在大多数的Unix shell中将会失败，因为这会被当做jq foo 来执行，而通常会报 foo is not defined 这样的错误。当使用Windows 的命令行 shell（cmd.exe）时，最好在命令行上给 jq 程序带上双引号(而不是-f program-file选项)，不过这样的话，jq 程序里面的双引号就需要反斜杠转义了。
你可以使用一些命令行选项来影响 jq 如何读写输入和输出：
--version:
输出 jq 的版本并以 exit 0 退出;
--seq:
使用 application/json-seq MIME类型格式分隔 jq 输入和输出中的 JSON 文本。这意味着会在每个输出值前打印一个ASCII RS（记录分隔符）字符，并在每次输出后打印一个ASCII LF（换行符）。输入无法解析的JSON文本会被忽略（但会被警告），直到下一个RS丢弃所有后续输入。
这样另外也可以解析 jq 不使用 -- seq选项时的输出。This more also parses the output of jq without the --seq option.（译者注：存疑，不懂这句。）
--stream:
以流方式解析输入，输出路径和叶子上的值（标量和空数组或空字典）。比如：
"a" becomes [[],"a"], and [[],"a",["b"]] becomes [[0],[]],[[1],"a"], and [[1,0],"b"].
这对于处理非常大的输入非常有用。 将此选项与过滤以及 reduce和foreach语法结合使用，可逐渐减少大量输入。
--slurp / -s:
不需要为输入中的每个JSON对象运行过滤器，只需将整个输入流读入为一个大型数组，然后只运行一次过滤器。
--raw-input / -R:
不要将输入解析为JSON。 相反，每行文本都以字符串形式传递给过滤器。如果与--slurp结合使用，则整个输入将作为单个长字符串传递给过滤器。
--null-input / -n:
根本不读任何输入！而是，过滤器使用null作为输入运行一次。 将jq用作简单计算器或从头开始构建JSON数据时，这很有用
--compact-output / -c:
默认情况下, jq 会 pretty-prints JSON 输出。使用这个选项可以把每一个 JSON 对象在单行内更紧凑的输出出来
--tab:
每个缩进将会使用 tab 而不是两个空格
--indent n:
指定缩进使用的空格数(不能超过 8)
--color-output / -C and --monochrome-output / -M:
默认情况下，如果是写入到终端，jq 会输出 colored JSON 。也可以使用 -C 强制输出彩色的JSON 到管道或者文件。也可以使用 -M 禁掉输出 colored JSON。
--ascii-output / -a:
jq 一般将 非-ASCII 的 Unicode 字符使用 UTF-8 输出，即使输入的时候可能是转义后的序列(如 “\u03bc”)。使用这个选项，可以强制 jq 生成纯 ASCII 输出，其中每个 非-ASCII 字符将用等价的转义序列替换。
--unbuffered:
Flush the output after each JSON object is printed (useful if you’re piping a slow data source into jq and piping jq’s output elsewhere). (译者: 很难翻译)
--sort-keys / -S:
将每个 JSON object 的各个字段按照 key 排序的顺序输出
--raw-output / -r:
在开启这个选项的情况下，如果 过滤器 的结果是 string，就会直接写入标准输出而不是以 JSON string 的格式输出。这在 jq 过滤器和其他处理 非-JSON 系统交互时比较有用。
--join-output / -j:
和 -r 作用一样，但是不会在每个输出的末尾打印一个换行。
-f filename / --from-file filename:
从文件中读取 filter 而不是从命令行中，类似 awk 的 -f 选项。文件中同样可以使用 # 来写注释
-Ldirectory / -L directory:
Prepend directory to the search list for modules. If this option is used then no builtin search list is used. See the section on modules below.
-e / --exit-status:
设置 jq 的退出状态, 如果最后的输出值既不是 false 也不是 null则 exit 0，如果最后的输出值是 false 或 null则 exit 1，如果没有输出有效的结果则 exit 4，
正常情况下如果有 Usage 问题或者系统错误则 exit 2，如果是 jq 程序编译出错则 exit 3，jq 程序正常跑起来则 exit 0
--arg name value:
这个选项向 jq 程序传递一个值作为一个预定义的变量。如果以 --arg foo bar 运行 jq 程序，那么在程序中 $foo就是一个值为 "bar"的变量。需要注意的是 value 只会被当做 string 处理，如 --arg foo 123 会提供 $foo 变量，值为"123"。
--argjson name JSON-text:
这个选项向 jq 程序传递一个 JSON 编码的值作为一个预定义的变量。
如果以 --argjson foo 123 运行 jq，那么程序中 $foo 就是一个值为 123 的变量。
--slurpfile variable-name filename:
这个选项会读取名为 filename 文件里面所有的 JSON 文本并将解析后的所有 JSON 组成一个 array 作为名为 variable-name 的全局变量的值。如果以 --argfile foo bar 运行 jq, 那么在程序中变量 $foo 就是一个 array，array 中的每个元素都对应着文件 bar 中的 JSON 文本。
--argfile variable-name filename:
Do not use. Use --slurpfile instead.
(这个选项和 --slurpfile 类似，不过当文件只有一个 JSON 文本时用这个，其他多个 JSON 文本用起来和 --slurpfile)
--run-tests [filename]:
Runs the tests in the given file or standard input. This must be the last option given and does not honor all preceding options. The input consists of comment lines, empty lines, and program lines followed by one input line, as many lines of output as are expected (one per output), and a terminating empty line. Compilation failure tests start with a line containing only “%%FAIL”, then a line containing the program to compile, then a line containing an error message to compare to the actual.
Be warned that this option can change backwards-incompatibly.
(译者: 暂不翻译)
基本过滤器
.
绝对最简单（也最平常）的过滤器是 `.｀，这是一个接收输入并原样输出的过滤器。
因为jq默认会优美打印所有的输出，这个小程序可以用来格式化一些JSON输出，比如curl。
Example
jq ‘.’
Input
“Hello, world!”
Output
“Hello, world!”
.foo,.foo.bar
最简单的有用过滤器是 .foo。给定一个JSON Object (即字典或hash)做输入，它会给出”foo”键的值，如果没有这个key则给出null.
如果键里含有关键字符，就要用双引号括起来，比如:.”foo$”.
一个形如.foo.bar的过滤器是.foo|.bar的等效写法。
Examples
jq '.foo'
--------------------
Input {"foo": 42, "bar": "less interesting data"}
Output 42
jq '.foo'
--------------------
Input {"notfoo": true, "alsonotfoo": false}
Output null
jq '.["foo"]'
--------------------
Input {"foo": 42}
Output 42
.foo?
就跟.foo差不多,但是当.不是一个数组或一个对象而报错时，不会输出。
Examples
jq '.foo?'
--------------------
Input {"foo": 42, "bar": "less interesting data"}
Output 42
jq '.foo?'
--------------------
Input {"notfoo": true, "alsonotfoo": false}
Output null
jq '.["foo"]?'
--------------------
Input {"foo": 42}
Output 42
jq '[.foo?]'
--------------------
Input [1,2]
Output []
.[
也可以使用类似 .["foo"] 的语法来查找 JSON Object 的一些元素 (上面的 .foo 是这个的一个速记版本)。如果 key 是数字的话，这种用法在 array 的情况下也可以有效。array 是以 0 为基的(类似 javascript)，因此 .[2] 返回 array 的第三个元素。
.[10:15] 这种语法可以用来返回一个数组的子数组，或者一个字符串的子字符串。.[10:15] 返回的数组长为 5，包含了索引从 10(包含)到 15(不包含)的元素。索引可以是负数的(这种情况下会从 array 的尾部开始倒着计数) 或者可以忽略(这种情况下指向数组的头部或者尾部)。
.[2] 这种语法用来返回数组的指定索引的元素。负索引也是可以的，-1 表示最后一个元素，-2 表示倒数第二个元素，以此类推。
.foo 这种语法仅对简单的 key 有效，即 key 仅包含字母或数字字符(alphanumeric)。
.[
? 操作符(“operator”) 也可以在这种切片操作(slice operator)下使用。例如 .[10:15]? 可以在输入是可进行切片操作(slice-able)的时候输出一些值。
Examples
jq '.[0]'
--------------------
Input [{"name":"JSON", "good":true}, {"name":"XML", "good":false}]
Output {"name":"JSON", "good":true}
jq '.[2]'
--------------------
Input [{"name":"JSON", "good":true}, {"name":"XML", "good":false}]
Output null
jq '.[2:4]'
--------------------
Input ["a","b","c","d","e"]
Output ["c", "d"]
jq '.[2:4]'
--------------------
Input "abcdefghi"
Output "cd"
jq '.[:3]'
--------------------
Input ["a","b","c","d","e"]
Output ["a", "b", "c"]
jq '.[-2:]'
--------------------
Input ["a","b","c","d","e"]
Output ["d", "e"]
jq '.[-2]'
--------------------
Input [1,2,3]
Output 2
.[]
如果使用 .[index] 这种语法，但完全省略 index，他就会返回 array 的 所有 元素。
对于输入 [1,2,3] 运行 .[] 就会生出 3 个独立的结果，而不是一个单个数组。
也可以在 object 上使用它，它将返回 object 的所有 value
Examples
jq '.[]'
--------------------
Input [{"name":"JSON", "good":true}, {"name":"XML", "good":false}]
Output {"name":"JSON", "good":true}
{"name":"XML", "good":false}
jq '.[]'
--------------------
Input []
Output
jq '.[]'
--------------------
Input {"a": 1, "b": 1}
Output [1, 1]
.[]?
类似 .[], 不过当 . 不是 array 或者 object 是不会输出 errors
Like .[], but no errors will be output if . is not an array
or object.
,
如果用逗号分隔两个过滤器，输入就会被每个过滤器处理，并依次输出多个结果:
首先是第一个过滤器的生成的所有输出，然后是第二个过滤器的生成的所有输出。
如 .foo, .bar 生成 “foo” 字段和 “bar” 字段的值。
Examples
jq '.foo, .bar'
--------------------
Input {"foo": 42, "bar": "something else", "baz": true}
Output [42, "something else"]
jq '.user, .projects[]'
--------------------
Input {"user":"stedolan", "projects": ["jq", "wikiflow"]}
Output ["stedolan", "jq", "wikiflow"]
jq '.[4,2]'
--------------------
Input ["a","b","c","d","e"]
Output ["e", "c"]
|
| 运算符结合左右两个过滤器 Filter, 把左边的输出 output 投递到右边的输入 input。如果你用过 Unix shell 的管道 pipe, |几乎和它是一样的。
如果左边的过滤器产生多个结果，则将为每个结果运行右侧过滤器。 所以表达式.[] | .foo检索输入数组里每个元素的 “foo” 字段。
Examples
jq '.[] | .name'
--------------------
Input [{"name":"JSON", "good":true}, {"name":"XML", "good":false}]
Output ["JSON", '"XML"']
Types and Values
jq 支持与 JSON 相同的一组数据类型集合 – 数字 numbers、 字符串 strings、布尔值 booleans、数组 arrays、对象 objects（在 JSON 中称做是仅有字符串键的哈希值 hashes）和 null 。
布尔值 booleans 、空值 null、字符串 strings 和数字 numbers 的书写方式与在 JavaScript 中相同。和 jq 中其他的内容一样，这些简单的值也被看做接收一个输入, 并且产生一个输出。如, 42 是一个合法的 jq 表达式, 忽略接收的输入, 并输出 42。
数组构造(array) - []
与 JSON 一样，[] 用于构造数组，如 [1,2,3]。 数组的元素可以是任何 jq 表达式。所有表达式产生的所有结果都被收集到一个大数组中。你可以使用它从已知数量的值中构造一个数组（如 [.foo, .bar, .baz] ）或将过滤器的所有结果”收集”到一个数组中（如 [.items[].name]）
一旦你理解了 , 运算符，你就可以从不同的角度看待 jq 的数组语法：表达式 [1,2,3] 不是使用逗号分隔数组的内置语法，而是在对表达式 1,2,3(输出 3 个值) 应用 [] 运算符(收集结果)。
如果你有一个过滤器 X 产生四个结果，那么表达式 [X] 将产生一个结果，即一个包含四个元素的数组。
Examples
jq '[.user, .projects[]]'
-------------------------------------
Input {"user":"stedolan", "projects": ["jq", "wikiflow"]}
Output ["stedolan", "jq", "wikiflow"]
Objects - {}
与 JSON 一样，{} 用于构建对象(又叫字典 dictionary 或哈希 hash)，如：{"a": 42, "b": 17}。
如果 Key 是 “合理的(sensible)” (由所有字母字符组成)，则引号可以省略。Value 可以是任何表达式(如果比较复杂, 可以使用括号包起来)，表达式会将 {} 表达式的输入作为输入(每个过滤器都有一个输入和输出)。
{foo: .bar}
如果输入是 {"bar":42, "baz":43}, 那么表达式的输出为 {"foo": 42}。
可以用来筛选一个 object 的特定字段：如果输入对象有 “user”、”title”、”id”、”content” 字段，而只需要 “user”、”title”，则可以这样写
{user: .user, title: .title}
因为这种用法很常见，所以有一个快捷语法：{user, title}。
如果其中一个表达式生成多个结果，那么表达式将生成多个词典。如果输入是
{"user":"stedolan","titles":["JQ Primer", "More JQ"]}
那么表达式
{user, title: .titles[]}
将会生成输出
{"user":"stedolan", "title": "JQ Primer"}
{"user":"stedolan", "title": "More JQ"}
使用括号包裹 Key ，意味着它将被当做表达式来计算 key 。使用与上述相同的输入，
表达式
{(.user): .titles}
会输出
{"stedolan": ["JQ Primer", "More JQ"]}
Examples
jq '{user, title: .titles[]}'
-------------------------------------
Input {"user":"stedolan","titles":["JQ Primer", "More JQ"]}
Output {"user":"stedolan", "title": "JQ Primer"}
{"user":"stedolan", "title": "More JQ"}
jq '{(.user): .titles}'
-------------------------------------
Input {"user":"stedolan","titles":["JQ Primer", "More JQ"]}
Output {"stedolan": ["JQ Primer", "More JQ"]}
TODO
title: Builtin operators and functions
body: |
Some jq operator (for instance, +) do different things
depending on the type of their arguments (arrays, numbers,
etc.). However, jq never does implicit type conversions. If you
try to add a string to an object you’ll get an error message and
no result.
entries:
title: Addition - +
body: |
The operator + takes two filters, applies them both
to the same input, and adds the results together. What
“adding” means depends on the types involved:
Numbers are added by normal arithmetic.
Arrays are added by being concatenated into a larger array.
Strings are added by being joined into a larger string.
Objects are added by merging, that is, inserting all
the key-value pairs from both objects into a single
combined object. If both objects contain a value for the
same key, the object on the right of the + wins. (For
recursive merge use the * operator.)
null can be added to any value, and returns the other
value unchanged.
examples:
program: ‘.a + 1’
input: ‘{“a”: 7}’
output: [‘8’]
program: ‘.a + .b’
input: ‘{“a”: [1,2], “b”: [3,4]}’
output: [‘[1,2,3,4]’]
program: ‘.a + null’
input: ‘{“a”: 1}’
output: [‘1’]
program: ‘.a + 1’
input: ‘{}’
output: [‘1’]
program: ‘{a: 1} + {b: 2} + {c: 3} + {a: 42}’
input: ‘null’
output: [’{“a”: 42, “b”: 2, “c”: 3}’]
title: Subtraction - -
body: |
As well as normal arithmetic subtraction on numbers, the -
operator can be used on arrays to remove all occurrences of
the second array’s elements from the first array.
examples:
program: ‘4 - .a’
input: ‘{“a”:3}’
output: [‘1’]
program: . - [“xml”, “yaml”]
input: ‘[“xml”, “yaml”, “json”]’
output: [’[“json”]’]
title: Multiplication, division, modulo - *, /, and %
body: |
These infix operators behave as expected when given two numbers.
Division by zero raises an error. x % y computes x modulo y.
Multiplying a string by a number produces the concatenation of
that string that many times. "x" * 0 produces null.
Dividing a string by another splits the first using the second
as separators.
Multiplying two objects will merge them recursively: this works
like addition but if both objects contain a value for the
same key, and the values are objects, the two are merged with
the same strategy.
examples:
program: ‘10 / . * 3’
input: 5
output: [6]
program: ‘. / “, “’
input: ‘“a, b,c,d, e”’
output: [’[“a”,”b,c,d”,”e”]’]
program: ‘{“k”: {“a”: 1, “b”: 2}} * {“k”: {“a”: 0,”c”: 3}}’
input: ‘null’
output: [’{“k”: {“a”: 0, “b”: 2, “c”: 3}}’]
program: ‘.[] | (1 / .)?’
input: ‘[1,0,-1]’
output: [‘1’, ‘-1’]
title: “length”
body: |
The builtin function length gets the length of various
different types of value:
The length of a string is the number of Unicode
codepoints it contains (which will be the same as its
JSON-encoded length in bytes if it’s pure ASCII).
The length of an array is the number of elements.
The length of an object is the number of key-value pairs.
The length of null is zero.
examples:
program: ‘.[] | length’
input: ‘[[1,2], “string”, {“a”:2}, null]’
output: [2, 6, 1, 0]
title: “keys, keys_unsorted”
body: |
The builtin function keys, when given an object, returns
its keys in an array.
The keys are sorted “alphabetically”, by unicode codepoint
order. This is not an order that makes particular sense in
any particular language, but you can count on it being the
same for any two objects with the same set of keys,
regardless of locale settings.
When keys is given an array, it returns the valid indices
for that array: the integers from 0 to length-1.
The keys_unsorted function is just like keys, but if
the input is an object then the keys will not be sorted,
instead the keys will roughly be in insertion order.
examples:
program: ‘keys’
input: ‘{“abc”: 1, “abcd”: 2, “Foo”: 3}’
output: [’[“Foo”, “abc”, “abcd”]’]
program: ‘keys’
input: ‘[42,3,35]’
output: [‘[0,1,2]’]
title: “has(key)”
body: |
The builtin function has returns whether the input object
has the given key, or the input array has an element at the
given index.
has($key) has the same effect as checking whether $key
is a member of the array returned by keys, although has
will be faster.
examples:
program: ‘map(has(“foo”))’
input: ‘[{“foo”: 42}, {}]’
output: [‘[true, false]’]
program: ‘map(has(2))’
input: ‘[[0,1], [“a”,”b”,”c”]]’
output: [‘[false, true]’]
title: “in”
body: |
The builtin function in returns whether or not the input key is in the
given object, or the input index corresponds to an element
in the given array. It is, essentially, an inversed version
of has.
examples:
program: ‘.[] | in({“foo”: 42})’
input: ‘[“foo”, “bar”]’
output: [‘true’, ‘false’]
program: ‘map(in([0,1]))’
input: ‘[2, 0]’
output: [‘[false, true]’]
title: “path(path_expression)”
body: |
Outputs array representations of the given path expression
in .. The outputs are arrays of strings (object keys)
and/or numbers (array indices).
Path expressions are jq expressions like .a, but also .[].
There are two types of path expressions: ones that can match
exactly, and ones that cannot. For example, .a.b.c is an
exact match path expression, while .a[].b is not.
path(exact_path_expression) will produce the array
representation of the path expression even if it does not
exist in ., if . is null or an array or an object.
path(pattern) will produce array representations of the
paths matching pattern if the paths exist in ..
Note that the path expressions are not different from normal
expressions. The expression
path(..|select(type=="boolean")) outputs all the paths to
boolean values in ., and only those paths.
examples:
program: ‘path(.a[0].b)’
input: ‘null’
output: [’[“a”,0,”b”]’]
program: ‘[path(..)]’
input: ‘{“a”:[{“b”:1}]}’
output: [’[[],[“a”],[“a”,0],[“a”,0,”b”]]’]
title: “del(path_expression)”
body: |
The builtin function del removes a key and its corresponding
value from an object.
examples:
program: ‘del(.foo)’
input: ‘{“foo”: 42, “bar”: 9001, “baz”: 42}’
output: [’{“bar”: 9001, “baz”: 42}’]
program: ‘del(.[1, 2])’
input: ‘[“foo”, “bar”, “baz”]’
output: [’[“foo”]’]
title: “to_entries, from_entries, with_entries”
body: |
These functions convert between an object and an array of
key-value pairs. If to_entries is passed an object, then
for each k: v entry in the input, the output array
includes {"key": k, "value": v}.
from_entries does the opposite conversion, and
with_entries(foo) is a shorthand for to_entries |
map(foo) | from_entries, useful for doing some operation to
all keys and values of an object. from_entries accepts key, Key,
Name, value and Value as keys.
examples:
program: ‘to_entries’
input: ‘{“a”: 1, “b”: 2}’
output: [’[{“key”:”a”, “value”:1}, {“key”:”b”, “value”:2}]’]
program: ‘from_entries’
input: ‘[{“key”:”a”, “value”:1}, {“key”:”b”, “value”:2}]’
output: [’{“a”: 1, “b”: 2}’]
program: ‘with_entries(.key |= “KEY_” + .)’
input: ‘{“a”: 1, “b”: 2}’
output: [’{“KEY_a”: 1, “KEY_b”: 2}’]
title: “select(boolean_expression)”
body: |
The function select(foo) produces its input unchanged if
foo returns true for that input, and produces no output
otherwise.
It’s useful for filtering lists: [1,2,3] | map(select(. >= 2))
will give you [2,3].
examples:
program: ‘map(select(. >= 2))’
input: ‘[1,5,3,0,7]’
output: [‘[5,3,7]’]
program: ‘.[] | select(.id == “second”)’
input: ‘[{“id”: “first”, “val”: 1}, {“id”: “second”, “val”: 2}]’
output: [’{“id”: “second”, “val”: 2}’]
title: “arrays, objects, iterables, booleans, numbers, normals, finites, strings, nulls, values, scalars”
body: |
These built-ins select only inputs that are arrays, objects,
iterables (arrays or objects), booleans, numbers, normal
numbers, finite numbers, strings, null, non-null values, and
non-iterables, respectively.
examples:
program: ‘.[]|numbers’
input: ‘[[],{},1,”foo”,null,true,false]’
output: [‘1’]
title: “empty”
body: |
empty returns no results. None at all. Not even null.
It’s useful on occasion. You’ll know if you need it :)
examples:
program: ‘1, empty, 2’
input: ‘null’
output: [1, 2]
program: ‘[1,2,empty,3]’
input: ‘null’
output: [‘[1,2,3]’]
title: “error(message)”
body: |
Produces an error, just like .a applied to values other than
null and objects would, but with the given message as the
error’s value.
title: “$__loc__”
body: |
Produces an object with a “file” key and a “line” key, with
the filename and line number where $__loc__ occurs, as
values.
examples:
program: ‘try error(“($loc)”) catch .’
input: ‘null’
output: [’”{"file":"\",\"line\":1}"']
title: “map(x), map_values(x)”
body: |
For any filter x, map(x) will run that filter for each
element of the input array, and return the outputs in a new
array. map(.+1) will increment each element of an array of numbers.
Similarly, map_values(x) will run that filter for each element,
but it will return an object when an object is passed.
map(x) is equivalent to [.[] | x]. In fact, this is how
it’s defined. Similarly, map_values(x) is defined as .[] |= x.
examples:
program: ‘map(.+1)’
input: ‘[1,2,3]’
output: [‘[2,3,4]’]
program: ‘map_values(.+1)’
input: ‘{“a”: 1, “b”: 2, “c”: 3}’
output: [’{“a”: 2, “b”: 3, “c”: 4}’]
title: “paths, paths(node_filter), leaf_paths”
body: |
paths outputs the paths to all the elements in its input
(except it does not output the empty list, representing .
itself).
paths(f) outputs the paths to any values for which f is true.
That is, paths(numbers) outputs the paths to all numeric
values.
leaf_paths is an alias of paths(scalars); leaf_paths is
deprecated and will be removed in the next major release.
examples:
program: ‘[paths]’
input: ‘[1,[[],{“a”:2}]]’
output: [’[[0],[1],[1,0],[1,1],[1,1,”a”]]’]
program: ‘[paths(scalars)]’
input: ‘[1,[[],{“a”:2}]]’
output: [’[[0],[1,1,”a”]]’]
title: “add”
body: |
The filter add takes as input an array, and produces as
output the elements of the array added together. This might
mean summed, concatenated or merged depending on the types
of the elements of the input array - the rules are the same
as those for the + operator (described above).
If the input is an empty array, add returns null.
examples:
program: add
input: ‘[“a”,”b”,”c”]’
output: [‘“abc”’]
program: add
input: ‘[1, 2, 3]’
output: [6]
program: add
input: ‘[]’
output: [“null”]
title: “any, any(condition), any(generator; condition)”
body: |
The filter any takes as input an array of boolean values,
and produces true as output if any of the elements of
the array are true.
If the input is an empty array, any returns false.
The any(condition) form applies the given condition to the
elements of the input array.
The any(generator; condition) form applies the given
condition to all the outputs of the given generator.
examples:
program: any
input: ‘[true, false]’
output: [“true”]
program: any
input: ‘[false, false]’
output: [“false”]
program: any
input: ‘[]’
output: [“false”]
title: “all, all(condition), all(generator; condition)”
body: |
The filter all takes as input an array of boolean values,
and produces true as output if all of the elements of
the array are true.
The all(condition) form applies the given condition to the
elements of the input array.
The all(generator; condition) form applies the given
condition to all the outputs of the given generator.
If the input is an empty array, all returns true.
examples:
program: all
input: ‘[true, false]’
output: [“false”]
program: all
input: ‘[true, true]’
output: [“true”]
program: all
input: ‘[]’
output: [“true”]
title: “flatten, flatten(depth)”
body: |
The filter flatten takes as input an array of nested arrays,
and produces a flat array in which all arrays inside the original
array have been recursively replaced by their values. You can pass
an argument to it to specify how many levels of nesting to flatten.
flatten(2) is like flatten, but going only up to two
levels deep.
examples:
program: flatten
input: ‘[1, [2], [[3]]]’
output: [“[1, 2, 3]”]
program: flatten(1)
input: ‘[1, [2], [[3]]]’
output: [“[1, 2, [3]]”]
program: flatten
input: ‘[[]]’
output: [”[]”]
program: flatten
input: ‘[{“foo”: “bar”}, [{“foo”: “baz”}]]’
output: [’[{“foo”: “bar”}, {“foo”: “baz”}]’]
title: “range(upto), range(from;upto) range(from;upto;by)”
body: |
The range function produces a range of numbers. range(4;10)
produces 6 numbers, from 4 (inclusive) to 10 (exclusive). The numbers
are produced as separate outputs. Use [range(4;10)] to get a range as
an array.
The one argument form generates numbers from 0 to the given
number, with an increment of 1.
The two argument form generates numbers from from to upto
with an increment of 1.
The three argument form generates numbers from to upto
with an increment of by.
examples:
program: ‘range(2;4)’
input: ‘null’
output: [‘2’, ‘3’]
program: ‘[range(2;4)]’
input: ‘null’
output: [‘[2,3]’]
program: ‘[range(4)]’
input: ‘null’
output: [‘[0,1,2,3]’]
program: ‘[range(0;10;3)]’
input: ‘null’
output: [‘[0,3,6,9]’]
program: ‘[range(0;10;-1)]’
input: ‘null’
output: [’[]’]
program: ‘[range(0;-5;-1)]’
input: ‘null’
output: [‘[0,-1,-2,-3,-4]’]
title: “floor”
body: |
The floor function returns the floor of its numeric input.
examples:
program: ‘floor’
input: ‘3.14159’
output: [‘3’]
title: “sqrt”
body: |
The sqrt function returns the square root of its numeric input.
examples:
program: ‘sqrt’
input: ‘9’
output: [‘3’]
title: “tonumber”
body: |
The tonumber function parses its input as a number. It
will convert correctly-formatted strings to their numeric
equivalent, leave numbers alone, and give an error on all other input.
examples:
program: ‘.[] | tonumber’
input: ‘[1, “1”]’
output: [1, 1]
title: “tostring”
body: |
The tostring function prints its input as a
string. Strings are left unchanged, and all other values are
JSON-encoded.
examples:
program: ‘.[] | tostring’
input: ‘[1, “1”, [1]]’
output: [‘“1”’, ‘“1”’, ‘“[1]”’]
title: “type”
body: |
The type function returns the type of its argument as a
string, which is one of null, boolean, number, string, array
or object.
examples:
program: ‘map(type)’
input: ‘[0, false, [], {}, null, “hello”]’
output: [’[“number”, “boolean”, “array”, “object”, “null”, “string”]’]
title: “infinite, nan, isinfinite, isnan, isfinite, isnormal”
body: |
Some arithmetic operations can yield infinities and “not a
number” (NaN) values. The isinfinite builtin returns true
if its input is infinite. The isnan builtin returns true
if its input is a NaN. The infinite builtin returns a
positive infinite value. The nan builtin returns a NaN.
The isnormal builtin returns true if its input is a normal
number.
Note that division by zero raises an error.
Currently most arithmetic operations operating on infinities,
NaNs, and sub-normals do not raise errors.
examples:
program: ‘.[] | (infinite * .) < 0’
input: ‘[-1, 1]’
output: [‘true’, ‘false’]
program: ‘infinite, nan | type’
input: ‘null’
output: [‘“number”’, ‘“number”’]
title: “sort, sort_by(path_expression)”
body: |
The sort functions sorts its input, which must be an
array. Values are sorted in the following order:
null
false
true
numbers
strings, in alphabetical order (by unicode codepoint value)
arrays, in lexical order
objects
The ordering for objects is a little complex: first they’re
compared by comparing their sets of keys (as arrays in
sorted order), and if their keys are equal then the values
are compared key by key.
sort may be used to sort by a particular field of an
object, or by applying any jq filter.
sort_by(foo) compares two elements by comparing the result of
foo on each element.
examples:
program: ‘sort’
input: ‘[8,3,null,6]’
output: [‘[null,3,6,8]’]
program: ‘sort_by(.foo)’
input: ‘[{“foo”:4, “bar”:10}, {“foo”:3, “bar”:100}, {“foo”:2, “bar”:1}]’
output: [’[{“foo”:2, “bar”:1}, {“foo”:3, “bar”:100}, {“foo”:4, “bar”:10}]’]
title: “group_by(path_expression)”
body: |
group_by(.foo) takes as input an array, groups the
elements having the same .foo field into separate arrays,
and produces all of these arrays as elements of a larger
array, sorted by the value of the .foo field.
Any jq expression, not just a field access, may be used in
place of .foo. The sorting order is the same as described
in the sort function above.
examples:
program: ‘group_by(.foo)’
input: ‘[{“foo”:1, “bar”:10}, {“foo”:3, “bar”:100}, {“foo”:1, “bar”:1}]’
output: [’[[{“foo”:1, “bar”:10}, {“foo”:1, “bar”:1}], [{“foo”:3, “bar”:100}]]’]
title: “min, max, min_by(path_exp), max_by(path_exp)”
body: |
Find the minimum or maximum element of the input array.
The min_by(path_exp) and max_by(path_exp) functions allow
you to specify a particular field or property to examine, e.g.
min_by(.foo) finds the object with the smallest foo field.
examples:
program: ‘min’
input: ‘[5,4,2,7]’
output: [‘2’]
program: ‘max_by(.foo)’
input: ‘[{“foo”:1, “bar”:14}, {“foo”:2, “bar”:3}]’
output: [’{“foo”:2, “bar”:3}’]
title: “unique, unique_by(path_exp)”
body: |
The unique function takes as input an array and produces
an array of the same elements, in sorted order, with
duplicates removed.
The unique_by(path_exp) function will keep only one element
for each value obtained by applying the argument. Think of it
as making an array by taking one element out of every group
produced by group.
examples:
program: ‘unique’
input: ‘[1,2,5,3,5,3,1,3]’
output: [‘[1,2,3,5]’]
program: ‘unique_by(.foo)’
input: ‘[{“foo”: 1, “bar”: 2}, {“foo”: 1, “bar”: 3}, {“foo”: 4, “bar”: 5}]’
output: [’[{“foo”: 1, “bar”: 2}, {“foo”: 4, “bar”: 5}]’]
program: ‘unique_by(length)’
input: ‘[“chunky”, “bacon”, “kitten”, “cicada”, “asparagus”]’
output: [’[“bacon”, “chunky”, “asparagus”]’]
title: “reverse”
body: |
This function reverses an array.
examples:
program: ‘reverse’
input: ‘[1,2,3,4]’
output: [‘[4,3,2,1]’]
title: “contains(element)”
body: |
The filter contains(b) will produce true if b is
completely contained within the input. A string B is
contained in a string A if B is a substring of A. An array B
is contained in an array A if all elements in B are
contained in any element in A. An object B is contained in
object A if all of the values in B are contained in the
value in A with the same key. All other types are assumed to
be contained in each other if they are equal.
examples:
program: ‘contains(“bar”)’
input: ‘“foobar”’
output: [‘true’]
program: ‘contains([“baz”, “bar”])’
input: ‘[“foobar”, “foobaz”, “blarp”]’
output: [‘true’]
program: ‘contains([“bazzzzz”, “bar”])’
input: ‘[“foobar”, “foobaz”, “blarp”]’
output: [‘false’]
program: ‘contains({foo: 12, bar: [{barp: 12}]})’
input: ‘{“foo”: 12, “bar”:[1,2,{“barp”:12, “blip”:13}]}’
output: [‘true’]
program: ‘contains({foo: 12, bar: [{barp: 15}]})’
input: ‘{“foo”: 12, “bar”:[1,2,{“barp”:12, “blip”:13}]}’
output: [‘false’]
title: “indices(s)”
body: |
Outputs an array containing the indices in . where s
occurs. The input may be an array, in which case if s is an
array then the indices output will be those where all elements
in . match those of s.
examples:
program: ‘indices(“, “)’
input: ‘“a,b, cd, efg, hijk”’
output: [‘[3,7,12]’]
program: ‘indices(1)’
input: ‘[0,1,2,1,3,1,4]’
output: [‘[1,3,5]’]
program: ‘indices([1,2])’
input: ‘[0,1,2,3,1,4,2,5,1,2,6,7]’
output: [‘[1,8]’]
title: “index(s), rindex(s)”
body: |
Outputs the index of the first (index) or last (rindex)
occurrence of s in the input.
examples:
program: ‘index(“, “)’
input: ‘“a,b, cd, efg, hijk”’
output: [‘3’]
program: ‘rindex(“, “)’
input: ‘“a,b, cd, efg, hijk”’
output: [‘12’]
title: “inside”
body: |
The filter inside(b) will produce true if the input is
completely contained within b. It is, essentially, an
inversed version of contains.
examples:
program: ‘inside(“foobar”)’
input: ‘“bar”’
output: [‘true’]
program: ‘inside([“foobar”, “foobaz”, “blarp”])’
input: ‘[“baz”, “bar”]’
output: [‘true’]
program: ‘inside([“foobar”, “foobaz”, “blarp”])’
input: ‘[“bazzzzz”, “bar”]’
output: [‘false’]
program: ‘inside({“foo”: 12, “bar”:[1,2,{“barp”:12, “blip”:13}]})’
input: ‘{“foo”: 12, “bar”: [{“barp”: 12}]}’
output: [‘true’]
program: ‘inside({“foo”: 12, “bar”:[1,2,{“barp”:12, “blip”:13}]})’
input: ‘{“foo”: 12, “bar”: [{“barp”: 15}]}’
output: [‘false’]
title: “startswith(str)”
body: |
Outputs true if . starts with the given string argument.
examples:
program: ‘[.[]|startswith(“foo”)]’
input: ‘[“fo”, “foo”, “barfoo”, “foobar”, “barfoob”]’
output: [‘[false, true, false, true, false]’]
title: “endswith(str)”
body: |
Outputs true if . ends with the given string argument.
examples:
program: ‘[.[]|endswith(“foo”)]’
input: ‘[“foobar”, “barfoo”]’
output: [‘[false, true]’]
title: “combinations, combinations(n)”
body: |
Outputs all combinations of the elements of the arrays in the
input array. If given an argument n, it outputs all combinations
of n repetitions of the input array.
examples:
program: ‘combinations’
input: ‘[[1,2], [3, 4]]’
output: [‘[1, 3]’, ‘[1, 4]’, ‘[2, 3]’, ‘[2, 4]’]
program: ‘combinations(2)’
input: ‘[0, 1]’
output: [‘[0, 0]’, ‘[0, 1]’, ‘[1, 0]’, ‘[1, 1]’]
title: “ltrimstr(str)”
body: |
Outputs its input with the given prefix string removed, if it
starts with it.
examples:
program: ‘[.[]|ltrimstr(“foo”)]’
input: ‘[“fo”, “foo”, “barfoo”, “foobar”, “afoo”]’
output: [’[“fo”,””,”barfoo”,”bar”,”afoo”]’]
title: “rtrimstr(str)”
body: |
Outputs its input with the given suffix string removed, if it
ends with it.
examples:
program: ‘[.[]|rtrimstr(“foo”)]’
input: ‘[“fo”, “foo”, “barfoo”, “foobar”, “foob”]’
output: [’[“fo”,””,”bar”,”foobar”,”foob”]’]
title: “explode”
body: |
Converts an input string into an array of the string’s
codepoint numbers.
examples:
program: ‘explode’
input: ‘“foobar”’
output: [‘[102,111,111,98,97,114]’]
title: “implode”
body: |
The inverse of explode.
examples:
program: ‘implode’
input: ‘[65, 66, 67]’
output: [‘“ABC”’]
title: “split”
body: |
Splits an input string on the separator argument.
examples:
program: ‘split(“, “)’
input: ‘“a, b,c,d, e, “’
output: [’[“a”,”b,c,d”,”e”,””]’]
title: “join(str)”
body: |
Joins the array of elements given as input, using the
argument as separator. It is the inverse of split: that is,
running split("foo") | join("foo") over any input string
returns said input string.
examples:
program: ‘join(“, “)’
input: ‘[“a”,”b,c,d”,”e”]’
output: [‘“a, b,c,d, e”’]
title: “ascii_downcase, ascii_upcase”
body: |
Emit a copy of the input string with its alphabetic characters (a-z and A-Z)
converted to the specified case.
example:
program: ‘ascii_upcase’
input: ‘“useful but not for é”’
output: ‘“USEFUL BUT NOT FOR é”’
title: “while(cond; update)”
body: |
The while(cond; update) function allows you to repeatedly
apply an update to . until cond is false.
Note that while(cond; update) is internally defined as a
recursive jq function. Recursive calls within while will
not consume additional memory if update produces at most one
output for each input. See advanced topics below.
examples:
program: ‘[while(.<100; .*2)]’
input: ‘1’
output: [‘[1,2,4,8,16,32,64]’]
title: “until(cond; next)”
body: |
The until(cond; next) function allows you to repeatedly
apply the expression next, initially to . then to its own
output, until cond is true. For example, this can be used
to implement a factorial function (see below).
Note that until(cond; next) is internally defined as a
recursive jq function. Recursive calls within until() will
not consume additional memory if next produces at most one
output for each input. See advanced topics below.
examples:
program: ‘[.,1]|until(.[0] < 1; [.[0] - 1, .[1] * .[0]])|.[1]’
input: ‘4’
output: [‘24’]
title: “recurse(f), recurse, recurse(f; condition), recurse_down”
body: |
The recurse(f) function allows you to search through a
recursive structure, and extract interesting data from all
levels. Suppose your input represents a filesystem:
{"name": "/", "children": [
{"name": "/bin", "children": [
{"name": "/bin/ls", "children": []},
{"name": "/bin/sh", "children": []}]},
{"name": "/home", "children": [
{"name": "/home/stephen", "children": [
{"name": "/home/stephen/jq", "children": []}]}]}]}
Now suppose you want to extract all of the filenames
present. You need to retrieve .name, .children[].name,
.children[].children[].name, and so on. You can do this
with:
recurse(.children[]) | .name
When called without an argument, recurse is equivalent to
recurse(.[]?).
recurse(f) is identical to recurse(f; . != null) and can be
used without concerns about recursion depth.
recurse(f; condition) is a generator which begins by
emitting . and then emits in turn .|f, .|f|f, .|f|f|f, … so long
as the computed value satisfies the condition. For example,
to generate all the integers, at least in principle, one
could write recurse(.+1; true).
For legacy reasons, recurse_down exists as an alias to
calling recurse without arguments. This alias is considered
deprecated and will be removed in the next major release.
The recursive calls in recurse will not consume additional
memory whenever f produces at most a single output for each
input.
examples:
program: ‘recurse(.foo[])’
input: ‘{“foo”:[{“foo”: []}, {“foo”:[{“foo”:[]}]}]}’
output:
’{“foo”:[{“foo”:[]},{“foo”:[{“foo”:[]}]}]}’
’{“foo”:[]}’
’{“foo”:[{“foo”:[]}]}’
’{“foo”:[]}’
program: ‘recurse’
input: ‘{“a”:0,”b”:[1]}’
output:
’{“a”:0,”b”:[1]}’
‘0’
‘[1]’
‘1’
program: ‘recurse(. * .; . < 20)’
input: 2
output:
- 2
- 4
- 16
title: “..”
body: |
Short-hand for recurse without arguments. This is intended
to resemble the XPath // operator. Note that ..a does not
work; use ..|a instead. In the example below we use
..|.a? to find all the values of object keys “a” in any
object found “below” ..
examples:
program: ‘..|.a?’
input: ‘[[{“a”:1}]]’
output: [‘1’]
title: “env”
body: |
Outputs an object representing jq’s environment.
examples:
program: ‘env.PAGER’
input: ‘null’
output: [‘“less”’]
title: “transpose”
body: |
Transpose a possibly jagged matrix (an array of arrays).
Rows are padded with nulls so the result is always rectangular.
examples:
program: ‘transpose’
input: ‘[[1], [2,3]]’
output: [’[[1,2],[null,3]]’]
title: “bsearch(x)”
body: |
bsearch(x) conducts a binary search for x in the input
array. If the input is sorted and contains x, then
bsearch(x) will return its index in the array; otherwise, if
the array is sorted, it will return (-1 - ix) where ix is an
insertion point such that the array would still be sorted
after the insertion of x at ix. If the array is not sorted,
bsearch(x) will return an integer that is probably of no
interest.
examples:
program: ‘bsearch(0)’
input: ‘[0,1]’
output: [‘0’]
program: ‘bsearch(0)’
input: ‘[1,2,3]’
output: [‘-1’]
program: ‘bsearch(4) as $ix | if $ix < 0 then .[-(1+$ix)] = 4 else . end’
input: ‘[1,2,3]’
output: [‘[1,2,3,4]’]
title: “String interpolation - \\(foo)”
body: |
Inside a string, you can put an expression inside parens
after a backslash. Whatever the expression returns will be
interpolated into the string.
examples:
program: ‘“The input was (.), which is one less than (.+1)”’
input: ‘42’
output: [‘“The input was 42, which is one less than 43”’]
title: “Convert to/from JSON”
body: |
The tojson and fromjson builtins dump values as JSON texts
or parse JSON texts into values, respectively. The tojson
builtin differs from tostring in that tostring returns strings
unmodified, while tojson encodes strings as JSON strings.
examples:
program: ‘[.[]|tostring]’
input: ‘[1, “foo”, [“foo”]]’
output: [’[“1”,”foo”,”["foo"]”]’]
program: ‘[.[]|tojson]’
input: ‘[1, “foo”, [“foo”]]’
output: [’[“1”,”"foo"”,”["foo"]”]’]
program: ‘[.[]|tojson|fromjson]’
input: ‘[1, “foo”, [“foo”]]’
output: [‘[1,”foo”,[“foo”]]’]
title: “Format strings and escaping”
body: |
The @foo syntax is used to format and escape strings,
which is useful for building URLs, documents in a language
like HTML or XML, and so forth. @foo can be used as a
filter on its own, the possible escapings are:
@text:
Calls tostring, see that function for details.
@json:
Serializes the input as JSON.
@html:
Applies HTML/XML escaping, by mapping the characters
<>&'" to their entity equivalents <, >,
&, ', ".
@uri:
Applies percent-encoding, by mapping all reserved URI
characters to a %XX sequence.
@csv:
The input must be an array, and it is rendered as CSV
with double quotes for strings, and quotes escaped by
repetition.
@tsv:
The input must be an array, and it is rendered as TSV
(tab-separated values). Each input array will be printed as
a single line. Fields are separated by a single
tab (ascii 0x09). Input characters line-feed (ascii 0x0a),
carriage-return (ascii 0x0d), tab (ascii 0x09) and
backslash (ascii 0x5c) will be output as escape sequences
\n, \r, \t, \\ respectively.
@sh:
The input is escaped suitable for use in a command-line
for a POSIX shell. If the input is an array, the output
will be a series of space-separated strings.
@base64:
The input is converted to base64 as specified by RFC 4648.
This syntax can be combined with string interpolation in a
useful way. You can follow a @foo token with a string
literal. The contents of the string literal will not be
escaped. However, all interpolations made inside that string
literal will be escaped. For instance,
@uri "https://www.google.com/search?q=\(.search)"
will produce the following output for the input
{"search":"what is jq?"}:
"https://www.google.com/search?q=what%20is%20jq%3F"
Note that the slashes, question mark, etc. in the URL are
not escaped, as they were part of the string literal.
examples:
program: ‘@html’
input: ‘“This works if x < y”’
output: [‘“This works if x < y”’]
- program: ‘@html “Anonymous said: (.)”’
input: ‘“”’
output: [“Anonymous said: ”]
- program: '@sh "echo \(.)"'
input: "\"O'Hara's Ale\""
output: ["\"echo 'O'\\\\''Hara'\\\\''s Ale'\""]
- title: "Dates"
body: |
jq provides some basic date handling functionality, with some
high-level and low-level builtins. In all cases these
builtins deal exclusively with time in UTC.
The `fromdateiso8601` builtin parses datetimes in the ISO 8601
format to a number of seconds since the Unix epoch
(1970-01-01T00:00:00Z). The `todateiso8601` builtin does the
inverse.
The `fromdate` builtin parses datetime strings. Currently
`fromdate` only supports ISO 8601 datetime strings, but in the
future it will attempt to parse datetime strings in more
formats.
The `todate` builtin is an alias for `todateiso8601`.
The `now` builtin outputs the current time, in seconds since
the Unix epoch.
Low-level jq interfaces to the C-library time functions are
also provided: `strptime`, `strftime`, `mktime`, and `gmtime`.
Refer to your host operating system's documentation for the
format strings used by `strptime` and `strftime`. Note: these
are not necessarily stable interfaces in jq, particularly as
to their localization functionality.
The `gmtime` builtin consumes a number of seconds since the
Unix epoch and outputs a "broken down time" representation of
time as an array of numbers representing (in this order): the
year, the month (zero-based), the day of the month, the hour
of the day, the minute of the hour, the second of the minute,
the day of the week, and the day of the year -- all one-based
unless otherwise stated.
The `mktime` builtin consumes "broken down time"
representations of time output by `gmtime` and `strptime`.
The `strptime(fmt)` builtin parses input strings matching the
`fmt` argument. The output is in the "broken down time"
representation consumed by `gmtime` and output by `mktime`.
The `strftime(fmt)` builtin formats a time with the given
format.
The format strings for `strptime` and `strftime` are described
in typical C library documentation. The format string for ISO
8601 datetime is `"%Y-%m-%dT%H:%M:%SZ"`.
jq may not support some or all of this date functionality on
some systems.
examples:
- program: 'fromdate'
input: '"2015-03-05T23:51:47Z"'
output: ['1425599507']
- program: 'strptime("%Y-%m-%dT%H:%M:%SZ")'
input: '"2015-03-05T23:51:47Z"'
output: ['[2015,2,5,23,51,47,4,63]']
- program: 'strptime("%Y-%m-%dT%H:%M:%SZ")|mktime'
input: '"2015-03-05T23:51:47Z"'
output: ['1425599507']
title: Conditionals and Comparisons
entries:
title: “==, !=”
body: |
The expression ‘a == b’ will produce ‘true’ if the result of a and b
are equal (that is, if they represent equivalent JSON documents) and
‘false’ otherwise. In particular, strings are never considered equal
to numbers. If you’re coming from Javascript, jq’s == is like
Javascript’s === - considering values equal only when they have the
same type as well as the same value.
!= is “not equal”, and ‘a != b’ returns the opposite value of ‘a == b’
examples:
program: ‘.[] == 1’
input: ‘[1, 1.0, “1”, “banana”]’
output: [‘true’, ‘true’, ‘false’, ‘false’]
title: if-then-else
body: |
if A then B else C end will act the same as B if A
produces a value other than false or null, but act the same
as C otherwise.
Checking for false or null is a simpler notion of
“truthiness” than is found in Javascript or Python, but it
means that you’ll sometimes have to be more explicit about
the condition you want: you can’t test whether, e.g. a
string is empty using if .name then A else B end, you’ll
need something more like if (.name | length) > 0 then A else
B end instead.
If the condition A produces multiple results, then B is evaluated
once for each result that is not false or null, and C is evaluated
once for each false or null.
More cases can be added to an if using elif A then B syntax.
examples:
program: |-
if . == 0 then
“zero”
elif . == 1 then
“one”
else
“many”
end
input: 2
output: [‘“many”’]
title: “>, >=, <=, <”
body: |
The comparison operators >, >=, <=, < return whether
their left argument is greater than, greater than or equal
to, less than or equal to or less than their right argument
(respectively).
The ordering is the same as that described for sort, above.
examples:
program: ‘. < 5’
input: 2
output: [‘true’]
title: and/or/not
body: |
jq supports the normal Boolean operators and/or/not. They have the
same standard of truth as if expressions - false and null are
considered “false values”, and anything else is a “true value”.
If an operand of one of these operators produces multiple
results, the operator itself will produce a result for each input.
not is in fact a builtin function rather than an operator,
so it is called as a filter to which things can be piped
rather than with special syntax, as in .foo and .bar |
not.
These three only produce the values “true” and “false”, and
so are only useful for genuine Boolean operations, rather
than the common Perl/Python/Ruby idiom of
“value_that_may_be_null or default”. If you want to use this
form of “or”, picking between two values rather than
evaluating a condition, see the “//” operator below.
examples:
program: ‘42 and “a string”’
input: ‘null’
output: [‘true’]
program: ‘(true, false) or false’
input: ‘null’
output: [‘true’, ‘false’]
- program: ‘(true, false) and (true, false)’
input: ‘null’
output: [‘true’, ‘false’, ‘false’, ‘false’]
program: ‘(true, true) and (true, false)’
input: ‘null’
output: [‘true’, ‘false’, ‘true’, ‘false’]
program: ‘[true, false | not]’
input: ‘null’
output: [‘[false, true]’]
title: Alternative operator - //
body: |
A filter of the form a // b produces the same
results as a, if a produces results other than false
and null. Otherwise, a // b produces the same results as b.
This is useful for providing defaults: .foo // 1 will
evaluate to 1 if there’s no .foo element in the
input. It’s similar to how or is sometimes used in Python
(jq’s or operator is reserved for strictly Boolean
operations).
examples:
program: ‘.foo // 42’
input: ‘{“foo”: 19}’
output: [19]
program: ‘.foo // 42’
input: ‘{}’
output: [42]
title: try-catch
body: |
Errors can be caught by using try EXP catch EXP. The first
expression is executed, and if it fails then the second is
executed with the error message. The output of the handler,
if any, is output as if it had been the output of the
expression to try.
The try EXP form uses empty as the exception handler.
examples:
program: ‘try .a catch “. is not an object”’
input: ‘true’
output: [’”. is not an object”’]
program: ‘[.[]|try .a]’
input: ‘[{}, true, {“a”:1}]’
output: [‘[null, 1]’]
program: ‘try error(“some exception”) catch .’
input: ‘true’
output: [‘“some exception”’]
title: Breaking out of control structures
body: |
A convenient use of try/catch is to break out of control
structures like reduce, foreach, while, and so on.
For example:
# Repeat an expression until it raises "break" as an
# error, then stop repeating without re-raising the error.
# But if the error caught is not "break" then re-raise it.
try repeat(exp) catch .=="break" then empty else error;
jq has a syntax for named lexical labels to “break” or “go (back) to”:
label $out | ... break $out ...
The break $label_name expression will cause the program to
to act as though the nearest (to the left) label $label_name
produced empty.
The relationship between the break and corresponding label
is lexical: the label has to be “visible” from the break.
To break out of a reduce, for example:
label $out | reduce .[] as $item (null; if .==false then break $out else ... end)
The following jq program produces a syntax error:
break $out
because no label $out is visible.
title: “? operator”
body: |
The ? operator, used as EXP?, is shorthand for try EXP.
examples:
program: ‘[.[]|(.a)?]’
input: ‘[{}, true, {“a”:1}]’
output: [‘[null, 1]’]
title: Regular expressions (PCRE)
body: |
jq uses the Oniguruma regular expression library, as do php,
ruby, TextMate, Sublime Text, etc, so the description here
will focus on jq specifics.
The jq regex filters are defined so that they can be used using
one of these patterns:
STRING | FILTER( REGEX )
STRING | FILTER( REGEX; FLAGS )
STRING | FILTER( [REGEX] )
STRING | FILTER( [REGEX, FLAGS] )
where:
STRING, REGEX and FLAGS are jq strings and subject to jq string interpolation;
REGEX, after string interpolation, should be a valid PCRE regex;
FILTER is one of test, match, or capture, as described below.
FLAGS is a string consisting of one of more of the supported flags:
g - Global search (find all matches, not just the first)
i - Case insensitive search
m - Multi line mode (‘.’ will match newlines)
n - Ignore empty matches
p - Both s and m modes are enabled
s - Single line mode (‘^’ -> ‘\A’, ‘$’ -> ‘\Z’)
l - Find longest possible matches
x - Extended regex format (ignore whitespace and comments)
To match whitespace in an x pattern use an escape such as \s, e.g.
test( “a\sb”, “x” ).
Note that certain flags may also be specified within REGEX, e.g.
jq -n ‘(“test”, “TEst”, “teST”, “TEST”)
test( “(?i)te(?-i)st” )’
evaluates to: true, true, false, false.
entries:
title: “test(val), test(regex; flags)”
body: |
Like match, but does not return match objects, only true or false
for whether or not the regex matches the input.
examples:
program: ‘test(“foo”)’
input: ‘“foo”’
output: [‘true’]
program: ‘.[] | test(“a b c # spaces are ignored”; “ix”)’
input: ‘[“xabcd”, “ABC”]’
output: [‘true’, ‘true’]
title: “match(val), match(regex; flags)”
body: |
match outputs an object for each match it finds. Matches have
the following fields:
offset - offset in UTF-8 codepoints from the beginning of the input
length - length in UTF-8 codepoints of the match
string - the string that it matched
captures - an array of objects representing capturing groups.
Capturing group objects have the following fields:
offset - offset in UTF-8 codepoints from the beginning of the input
length - length in UTF-8 codepoints of this capturing group
string - the string that was captured
name - the name of the capturing group (or null if it was unnamed)
Capturing groups that did not match anything return an offset of -1
examples:
program: ‘match(“(abc)+”; “g”)’
input: ‘“abc abc”’
output:
’{“offset”: 0, “length”: 3, “string”: “abc”, “captures”: [{“offset”: 0, “length”: 3, “string”: “abc”, “name”: null}]}’
’{“offset”: 4, “length”: 3, “string”: “abc”, “captures”: [{“offset”: 4, “length”: 3, “string”: “abc”, “name”: null}]}’
program: ‘match(“foo”)’
input: ‘“foo bar foo”’
output: [’{“offset”: 0, “length”: 3, “string”: “foo”, “captures”: []}’]
program: ‘match([“foo”, “ig”])’
input: ‘“foo bar FOO”’
output:
’{“offset”: 0, “length”: 3, “string”: “foo”, “captures”: []}’
’{“offset”: 8, “length”: 3, “string”: “FOO”, “captures”: []}’
program: ‘match(“foo (?bar)? foo"; "ig")'
input: '"foo bar foo foo foo"'
output:
’{“offset”: 0, “length”: 11, “string”: “foo bar foo”, “captures”: [{“offset”: 4, “length”: 3, “string”: “bar”, “name”: “bar123”}]}’
’{“offset”: 12, “length”: 8, “string”: “foo foo”, “captures”: [{“offset”: -1, “length”: 0, “string”: null, “name”: “bar123”}]}’
program: ‘[ match(“.”; “g”)] | length’
input: ‘“abc”’
output: [3]
title: “capture(val), capture(regex; flags)”
body: |
Collects the named captures in a JSON object, with the name
of each capture as the key, and the matched string as the
corresponding value.
examples:
program: ‘capture(“(?[a-z]+)-(?[0-9]+)")'
input: '"xyzzy-14"'
output: ['{ "a": "xyzzy", "n": "14" }']
title: “scan(regex), scan(regex; flags)”
body: |
Emit a stream of the non-overlapping substrings of the input
that match the regex in accordance with the flags, if any
have been specified. If there is no match, the stream is empty.
To capture all the matches for each input string, use the idiom
[ expr ], e.g. [ scan(regex) ].
example:
program: ‘scan(“c”)’
input: ‘“abcdefabc”’
output: [‘“c”’, ‘“c”’]
program: ‘scan(“b”)’
input: (“”, “”)
output: [’[]’, ‘[]’]
title: “split(regex; flags)”
body: |
For backwards compatibility, split splits on a string, not a regex.
example:
program: ‘split(“, *”; null)’
input: ‘“ab,cd, ef”’
output: [‘“ab”,”cd”,”ef”’]
title: “splits(regex), splits(regex; flags)”
body: |
These provide the same results as their split counterparts,
but as a stream instead of an array.
example:
program: ‘splits(“, *”)’
input: ‘(“ab,cd”, “ef, gh”)’
output: [‘“ab”’, ‘“cd”’, ‘“ef”’, ‘“gh”’]
title: “sub(regex; tostring) sub(regex; string; flags)”
body: |
Emit the string obtained by replacing the first match of regex in the
input string with tostring, after interpolation. tostring should
be a jq string, and may contain references to named captures. The
named captures are, in effect, presented as a JSON object (as
constructed by capture) to tostring, so a reference to a captured
variable named “x” would take the form: “(.x)”.
example:
program: ‘sub(“^[^a-z]*(?[a-z]*).*")'
input: '"123abc456"'
output: '"ZabcZabc"'
title: “gsub(regex; string), gsub(regex; string; flags)”
body: |
gsub is like sub but all the non-overlapping occurrences of the regex are
replaced by the string, after interpolation.
example:
program: ‘gsub(“(?.)[^a]*"; "+\(.x)-")'
input: '"Abcabc"'
output: '"+A-+a-"'
title: Advanced features
body: |
Variables are an absolute necessity in most programming languages, but
they’re relegated to an “advanced feature” in jq.
In most languages, variables are the only means of passing around
data. If you calculate a value, and you want to use it more than once,
you’ll need to store it in a variable. To pass a value to another part
of the program, you’ll need that part of the program to define a
variable (as a function parameter, object member, or whatever) in
which to place the data.
It is also possible to define functions in jq, although this is
is a feature whose biggest use is defining jq’s standard library
(many jq functions such as map and find are in fact written
in jq).
jq has reduction operators, which are very powerful but a bit
tricky. Again, these are mostly used internally, to define some
useful bits of jq’s standard library.
It may not be obvious at first, but jq is all about generators
(yes, as often found in other languages). Some utilities are
provided to help deal with generators.
Some minimal I/O support (besides reading JSON from standard
input, and writing JSON to standard output) is available.
Finally, there is a module/library system.
entries:
title: Variables
body: |
In jq, all filters have an input and an output, so manual
plumbing is not necessary to pass a value from one part of a program
to the next. Many expressions, for instance a + b, pass their input
to two distinct subexpressions (here a and b are both passed the
same input), so variables aren’t usually necessary in order to use a
value twice.
For instance, calculating the average value of an array of numbers
requires a few variables in most languages - at least one to hold the
array, perhaps one for each element or for a loop counter. In jq, it’s
simply add / length - the add expression is given the array and
produces its sum, and the length expression is given the array and
produces its length.
So, there’s generally a cleaner way to solve most problems in jq than
defining variables. Still, sometimes they do make things easier, so jq
lets you define variables using expression as $variable. All
variable names start with $. Here’s a slightly uglier version of the
array-averaging example:
length as $array_length | add / $array_length
We’ll need a more complicated problem to find a situation where using
variables actually makes our lives easier.
Suppose we have an array of blog posts, with “author” and “title”
fields, and another object which is used to map author usernames to
real names. Our input looks like:
{"posts": [{"title": "Frist psot", "author": "anon"},
{"title": "A well-written article", "author": "person1"}],
"realnames": {"anon": "Anonymous Coward",
"person1": "Person McPherson"}}
We want to produce the posts with the author field containing a real
name, as in:
{"title": "Frist psot", "author": "Anonymous Coward"}
{"title": "A well-written article", "author": "Person McPherson"}
We use a variable, $names, to store the realnames object, so that we
can refer to it later when looking up author usernames:
.realnames as $names | .posts[] | {title, author: $names[.author]}
The expression exp as $x | ... means: for each value of expression
exp, run the rest of the pipeline with the entire original input, and
with $x set to that value. Thus as functions as something of a
foreach loop.
Just as {foo} is a handy way of writing {foo: .foo}, so
{$foo} is a handy way of writing {foo:$foo}.
Multiple variables may be declared using a single as expression by
providing a pattern that matches the structure of the input
(this is known as “destructuring”):
. as {realnames: $names, posts: [$first, $second]} | ...
The variable declarations in array patterns (e.g., . as
[$first, $second]) bind to the elements of the array in from
the element at index zero on up, in order. When there is no
value at the index for an array pattern element, null is
bound to that variable.
Variables are scoped over the rest of the expression that defines
them, so
.realnames as $names | (.posts[] | {title, author: $names[.author]})
will work, but
(.realnames as $names | .posts[]) | {title, author: $names[.author]}
won’t.
For programming language theorists, it’s more accurate to
say that jq variables are lexically-scoped bindings. In
particular there’s no way to change the value of a binding;
one can only setup a new binding with the same name, but which
will not be visible where the old one was.
examples:
program: ‘.bar as $x | .foo | . + $x’
input: ‘{“foo”:10, “bar”:200}’
output: [‘210’]
program: ‘. as $i|[(.*2|. as $i| $i), $i]’
input: ‘5’
output: [‘[10,5]’]
program: ‘. as [$a, $b, {c: $c}] | $a + $b + $c’
input: ‘[2, 3, {“c”: 4, “d”: 5}]’
output: [‘9’]
program: ‘.[] as [$a, $b] | {a: $a, b: $b}’
input: ‘[[0], [0, 1], [2, 1, 0]]’
output: [’{“a”:0,”b”:null}’, ‘{“a”:0,”b”:1}’, ‘{“a”:2,”b”:1}’]
title: ‘Defining Functions’
body: |
You can give a filter a name using “def” syntax:
def increment: . + 1;
From then on, increment is usable as a filter just like a
builtin function (in fact, this is how some of the builtins
are defined). A function may take arguments:
def map(f): [.[] | f];
Arguments are passed as filters, not as values. The
same argument may be referenced multiple times with
different inputs (here f is run for each element of the
input array). Arguments to a function work more like
callbacks than like value arguments. This is important to
understand. Consider:
def foo(f): f|f;
5|foo(.*2)
The result will be 20 because f is .*2, and during the
first invocation of f . will be 5, and the second time it
will be 10 (5 * 2), so the result will be 20. Function
arguments are filters, and filters expect an input when
invoked.
If you want the value-argument behaviour for defining simple
functions, you can just use a variable:
def addvalue(f): f as $f | map(. + $f);
Or use the short-hand:
def addvalue($f): ...;
With either definition, addvalue(.foo) will add the current
input’s .foo field to each element of the array.
Multiple definitions using the same function name are allowed.
Each re-definition replaces the previous one for the same
number of function arguments, but only for references from
functions (or main program) subsequent to the re-definition.
examples:
program: ‘def addvalue(f): . + [f]; map(addvalue(.[0]))’
input: ‘[[1,2],[10,20]]’
output: [’[[1,2,1], [10,20,10]]’]
program: ‘def addvalue(f): f as $x | map(. + $x); addvalue(.[0])’
input: ‘[[1,2],[10,20]]’
output: [’[[1,2,1,2], [10,20,1,2]]’]
title: Reduce
body: |
The reduce syntax in jq allows you to combine all of the
results of an expression by accumulating them into a single
answer. As an example, we’ll pass [3,2,1] to this expression:
reduce .[] as $item (0; . + $item)
For each result that .[] produces, . + $item is run to
accumulate a running total, starting from 0. In this
example, .[] produces the results 3, 2, and 1, so the
effect is similar to running something like this:
0 | (3 as $item | . + $item) |
(2 as $item | . + $item) |
(1 as $item | . + $item)
examples:
program: ‘reduce .[] as $item (0; . + $item)’
input: ‘[10,2,5,3]’
output: [‘20’]
title: “limit(n; exp)”
body: |
The limit function extracts up to n outputs from exp.
examples:
program: ‘[limit(3;.[])]’
input: ‘[0,1,2,3,4,5,6,7,8,9]’
output: [‘[0,1,2]’]
title: “first(expr), last(expr), nth(n; expr)”
body: |
The first(expr) and last(expr) functions extract the first
and last values from expr, respectively.
The nth(n; expr) function extracts the nth value output by
expr. This can be defined as def nth(n; expr):
last(limit(n + 1; expr));. Note that nth(n; expr) doesn’t
support negative values of n.
examples:
program: ‘[first(range(.)), last(range(.)), nth(./2; range(.))]’
input: ‘10’
output: [‘[0,9,5]’]
title: “first, last, nth(n)”
body: |
The first and last functions extract the first
and last values from any array at ..
The nth(n) function extracts the nth value of any array at ..
examples:
program: ‘[range(.)]|[first, last, nth(5)]’
input: ‘10’
output: [‘[0,9,5]’]
title: “foreach”
body: |
The foreach syntax is similar to reduce, but intended to
allow the construction of limit and reducers that produce
intermediate results (see example).
The form is foreach EXP as $var (INIT; UPDATE; EXTRACT).
Like reduce, INIT is evaluated once to produce a state
value, then each output of EXP is bound to $var, UPDATE
is evaluated for each output of EXP with the current state
and with $var visible. Each value output by UPDATE
replaces the previous state. Finally, EXTRACT is evaluated
for each new state to extract an output of foreach.
This is mostly useful only for constructing reduce- and
limit-like functions. But it is much more general, as it
allows for partial reductions (see the example below).
examples:
program: ‘[foreach .[] as $item
([[],[]];
if $item == null then [[],.[0]] else [(.[0] + [$item]),[]] end;
if $item == null then .[1] else empty end)]’
input: ‘[1,2,3,4,null,”a”,”b”,null]’
output: [’[[1,2,3,4],[“a”,”b”]]’]
title: Recursion
body: |
As described above, recurse uses recursion, and any jq
function can be recursive. The while builtin is also
implemented in terms of recursion.
Tail calls are optimized whenever the expression to the left of
the recursive call outputs its last value. In practice this
means that the expression to the left of the recursive call
should not produce more than one output for each input.
For example:
def recurse(f): def r: ., (f | select(. != null) | r); r;
def while(cond; update):
def _while:
if cond then ., (update | _while) else empty end;
_while;
def repeat(exp):
def _repeat:
exp, _repeat;
_repeat;
title: Generators and iterators
body: |
Some jq operators and functions are actually generators in
that they can produce zero, one, or more values for each
input, just as one might expect in other programming
languages that have generators. For example, `.[]`
generates all the values in its input (which must be an
array or an object), `range(0; 10)` generates the integers
between 0 and 10, and so on.
Even the comma operator is a generator, generating first the
values generated by the expression to the left of the comma,
then for each of those, the values generate by the
expression on the right of the comma.
The `empty` builtin is the generator that produces zero
outputs. The `empty` builtin backtracks to the preceding
generator expression.
All jq functions can be generators just by using builtin
generators. It is also possible to define new generators
using only recursion and the comma operator. If the
recursive call(s) is(are) "in tail position" then the
generator will be efficient. In the example below the
recursive call by `_range` to itself is in tail position.
The example shows off three advanced topics: tail recursion,
generator construction, and sub-functions.
examples:
program: ‘def range(init; upto; by):
def _range:
if (by > 0 and . < upto) or (by < 0 and . > upto)
then ., ((.+by)|_range)
else . end;
if by == 0 then init else init|_range end |
select((by > 0 and . < upto) or (by < 0 and . > upto));
range(0; 10; 3)’
input: ‘null’
output: [‘0’, ‘3’, ‘6’, ‘9’]
program: ‘def while(cond; update):
def _while:
if cond then ., (update | _while) else empty end;
_while;
[while(.<100; .*2)]’
input: ‘1’
output: [‘[1,2,4,8,16,32,64]’]
title: ‘Math’
body: |
jq currently only has IEEE754 double-precision (64-bit) floating
point number support.
Besides simple arithmetic operators such as +, jq also has most
standard math functions from the C math library. C math functions
that take a single input argument (e.g., sin()) are available as
zero-argument jq functions. C math functions that take two input
arguments (e.g., pow()) are available as two-argument jq
functions that ignore ..
Availability of standard math functions depends on the
availability of the corresponding math functions in your operating
system and C math library. Unavailable math functions will be
defined but will raise an error.
title: ‘I/O’
body: |
At this time jq has minimal support for I/O, mostly in the
form of control over when inputs are read. Two builtins functions
are provided for this, input and inputs, that read from the
same sources (e.g., stdin, files named on the command-line) as
jq itself. These two builtins, and jq’s own reading actions, can
be interleaved with each other.
One builtin provides minimal output capabilities, debug.
(Recall that a jq program’s output values are always output as
JSON texts on stdout.) The debug builtin can have
application-specific behavior, such as for executables that use
the libjq C API but aren’t the jq executable itself.
entries:
title: “input”
body: |
Outputs one new input.
title: “inputs”
body: |
Outputs all remaining inputs, one by one.
This is primarily useful for reductions over a program’s
inputs.
title: “debug”
body: |
Causes a debug message based on the input value to be
produced. The jq executable wraps the input value with
["DEBUG:",
stderr, compactly. This may change in the future.
title: “input_filename”
body: |
Returns the name of the file whose input is currently being
filtered. Note that this will not work well unless jq is
running in a UTF-8 locale.
title: “input_line_number”
body: |
Returns the line number of the input currently being filtered.
title: ‘Streaming’
body: |
With the --stream option jq can parse input texts in a streaming
fashion, allowing jq programs to start processing large JSON texts
immediately rather than after the parse completes. If you have a
single JSON text that is 1GB in size, streaming it will allow you
to process it much more quickly.
However, streaming isn’t easy to deal with as the jq program will
have [
Several builtins are provided to make handling streams easier.
The examples below use the streamed form of [0,[1]], which
is [[0],0],[[1,0],1],[[1,0]],[[1]].
Streaming forms include [
scalar value, empty array, or empty object), and [
indicate the end of an array or object). Future versions of jq
run with --stream and -seq may output additional forms such as
["error message"] when an input text fails to parse.
entries:
title: “truncate_stream(stream_expression)”
body: |
Consumes a number as input and truncates the corresponding
number of path elements from the left of the outputs of the
given streaming expression.
examples:
program: ‘[1|truncate_stream([[0],1],[[1,0],2],[[1,0]],[[1]])]’
input: ‘1’
output: [’[[[0],2],[[0]]]’]
title: “fromstream(stream_expression)”
body: |
Outputs values corresponding to the stream expression’s
outputs.
examples:
program: ‘fromstream(1|truncate_stream([[0],1],[[1,0],2],[[1,0]],[[1]]))’
input: ‘null’
output: [‘[2]’]
title: “tostream”
body: |
The tostream builtin outputs the streamed form of its input.
examples:
program: ‘. as $dot|fromstream($dot|tostream)|.==$dot’
input: ‘[0,[1,{“a”:1},{“b”:2}]]’
output: [‘true’]
title: Assignment
body: |
Assignment works a little differently in jq than in most
programming languages. jq doesn’t distinguish between references
to and copies of something - two objects or arrays are either
equal or not equal, without any further notion of being “the
same object” or “not the same object”.
If an object has two fields which are arrays, .foo and .bar,
and you append something to .foo, then .bar will not get
bigger. Even if you’ve just set .bar = .foo. If you’re used to
programming in languages like Python, Java, Ruby, Javascript,
etc. then you can think of it as though jq does a full deep copy
of every object before it does the assignment (for performance,
it doesn’t actually do that, but that’s the general idea).
All the assignment operators in jq have path expressions on the
left-hand side.
entries:
title: “=”
body: |
The filter .foo = 1 will take as input an object
and produce as output an object with the “foo” field set to
There is no notion of “modifying” or “changing” something
in jq - all jq values are immutable. For instance,
.foo = .bar
.foo.baz = 1
will not have the side-effect of setting .bar.baz to be set
to 1, as the similar-looking program in Javascript, Python,
Ruby or other languages would. Unlike these languages (but
like Haskell and some other functional languages), there is
no notion of two arrays or objects being “the same array” or
“the same object”. They can be equal, or not equal, but if
we change one of them in no circumstances will the other
change behind our backs.
This means that it’s impossible to build circular values in
jq (such as an array whose first element is itself). This is
quite intentional, and ensures that anything a jq program
can produce can be represented in JSON.
Note that the left-hand side of ‘=’ refers to a value in ..
Thus $var.foo = 1 won’t work as expected ($var.foo is not
a valid or useful path expression in .); use $var | .foo =
1 instead.
If the right-hand side of ‘=’ produces multiple values, then
for each such value jq will set the paths on the left-hand
side to the value and then it will output the modified ..
For example, (.a,.b)=range(2) outputs {"a":0,"b":0}, then
{"a":1,"b":1}. The “update” assignment forms (see below) do
not do this.
Note too that .a,.b=0 does not set .a and .b, but
(.a,.b)=0 sets both.
title: “|=”
body: |
As well as the assignment operator ‘=’, jq provides the “update”
operator ‘|=’, which takes a filter on the right-hand side and
works out the new value for the property of . being assigned
to by running the old value through this expression. For
instance, .foo |= .+1 will build an object with the “foo”
field set to the input’s “foo” plus 1.
This example should show the difference between ‘=’ and ‘
=’:
Provide input ‘{“a”: {“b”: 10}, “b”: 20}’ to the programs:
.a = .b
.a |= .b
The former will set the “a” field of the input to the “b” field of the
input, and produce the output {“a”: 20}. The latter will set the “a”
field of the input to the “a” field’s “b” field, producing {“a”: 10}.
The left-hand side can be any general path expression; see path().
Note that the left-hand side of ‘|=’ refers to a value in ..
Thus $var.foo |= . + 1 won’t work as expected ($var.foo is
not a valid or useful path expression in .); use $var |
.foo |= . + 1 instead.
If the right-hand side outputs multiple values, only the last
one will be used.
examples:
program: ‘(..|select(type==”boolean”)) |= if . then 1 else 0 end’
input: ‘[true,false,[5,true,[true,[false]],false]]’
output: [‘[1,0,[5,1,[1,[0]],0]]’]
title: “+=, -=, *=, /=, %=, //=”
body: |
jq has a few operators of the form a op= b, which are all
equivalent to a |= . op b. So, += 1 can be used to increment values.
examples:
program: .foo += 1
input: ‘{“foo”: 42}’
output: [’{“foo”: 43}’]
title: Complex assignments
body: |
Lots more things are allowed on the left-hand side of a jq assignment
than in most languages. We’ve already seen simple field accesses on
the left hand side, and it’s no surprise that array accesses work just
as well:
.posts[0].title = "JQ Manual"
What may come as a surprise is that the expression on the left may
produce multiple results, referring to different points in the input
document:
.posts[].comments |= . + ["this is great"]
That example appends the string “this is great” to the “comments”
array of each post in the input (where the input is an object with a
field “posts” which is an array of posts).
When jq encounters an assignment like ‘a = b’, it records the “path”
taken to select a part of the input document while executing a. This
path is then used to find which part of the input to change while
executing the assignment. Any filter may be used on the
left-hand side of an equals - whichever paths it selects from the
input will be where the assignment is performed.
This is a very powerful operation. Suppose we wanted to add a comment
to blog posts, using the same “blog” input above. This time, we only
want to comment on the posts written by “stedolan”. We can find those
posts using the “select” function described earlier:
.posts[] | select(.author == "stedolan")
The paths provided by this operation point to each of the posts that
“stedolan” wrote, and we can comment on each of them in the same way
that we did before:
(.posts[] | select(.author == "stedolan") | .comments) |=
. + ["terrible."]
title: Modules
body: |
jq has a library/module system. Modules are files whose names end
in .jq.
Modules imported by a program are searched for in a default search
path (see below). The import and include directives allow the
importer to alter this path.
Paths in the a search path are subject to various substitutions.
For paths starting with “~/”, the user’s home directory is
substituted for “~”.
For paths starting with “$ORIGIN/”, the path of the jq executable
is substituted for “$ORIGIN”.
For paths starting with “./” or paths that are “.”, the path of
the including file is substituted for “.”. For top-level programs
given on the command-line, the current directory is used.
Import directives can optionally specify a search path to which
the default is appended.
The default search path is the search path given to the -L
command-line option, else ["~/.jq", "$ORIGIN/../lib/jq",
"$ORIGIN/../lib"].
Null and empty string path elements terminate search path
processing.
A dependency with relative path “foo/bar” would be searched for in
“foo/bar.jq” and “foo/bar/bar.jq” in the given search path. This
is intended to allow modules to be placed in a directory along
with, for example, version control files, README files, and so on,
but also to allow for single-file modules.
Consecutive components with the same name are not allowed to avoid
ambiguities (e.g., “foo/foo”).
For example, with -L$HOME/.jq a module foo can be found in
$HOME/.jq/foo.jq and $HOME/.jq/foo/foo.jq.
If “$HOME/.jq” is a file, it is sourced into the main program.
entries:
title: “import RelativePathString as NAME [
body: |
Imports a module found at the given path relative to a
directory in a search path. A “.jq” suffix will be added to
the relative path string. The module’s symbols are prefixed
with “NAME::”.
The optional metadata must be a constant jq expression. It
should be an object with keys like “homepage” and so on. At
this time jq only uses the “search” key/value of the metadata.
The metadata is also made available to users via the
modulemeta builtin.
The “search” key in the metadata, if present, should have a
string or array value (array of strings); this is the search
path to be prefixed to the top-level search path.
title: “include RelativePathString [
body: |
Imports a module found at the given path relative to a
directory in a search path as if it were included in place. A
“.jq” suffix will be added to the relative path string. The
module’s symbols are imported into the caller’s namespace as
if the module’s content had been included directly.
The optional metadata must be a constant jq expression. It
should be an object with keys like “homepage” and so on. At
this time jq only uses the “search” key/value of the metadata.
The metadata is also made available to users via the
modulemeta builtin.
title: “import RelativePathString as $NAME [
body: |
Imports a JSON file found at the given path relative to a
directory in a search path. A “.json” suffix will be added to
the relative path string. The file’s data will be available
as $NAME::NAME.
The optional metadata must be a constant jq expression. It
should be an object with keys like “homepage” and so on. At
this time jq only uses the “search” key/value of the metadata.
The metadata is also made available to users via the
modulemeta builtin.
The “search” key in the metadata, if present, should have a
string or array value (array of strings); this is the search
path to be prefixed to the top-level search path.
title: “module
body: |
This directive is entirely optional. It’s not required for
proper operation. It serves only the purpose of providing
metadata that can be read with the modulemeta builtin.
The metadata must be a constant jq expression. It should be
an object with keys like “homepage”. At this time jq doesn’t
use this metadata, but it is made available to users via the
modulemeta builtin.
title: “modulemeta”
body: |
Takes a module name as input and outputs the module’s metadata
as an object, with the module’s imports (including metadata)
as an array value for the “deps” key.
Programs can use this to query a module’s metadata, which they
could then use to, for example, search for, download, and
install missing dependencies.
jq-manual-cn is maintained by alingse.
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