| headline: jq Manual |
| body: | |
| |
| A jq program is a "filter": it takes an input, and produces an |
| output. There are a lot of builtin filters for extracting a |
| particular field of an object, or converting a number to a string, |
| or various other standard tasks. |
| |
| Filters can be combined in various ways - you can pipe the output of |
| one filter into another filter, or collect the output of a filter |
| into an array. |
| |
| Some filters produce multiple results, for instance there's one that |
| produces all the elements of its input array. Piping that filter |
| into a second runs the second filter for each element of the |
| array. Generally, things that would be done with loops and iteration |
| in other languages are just done by gluing filters together in jq. |
| |
| It's important to remember that every filter has an input and an |
| output. Even literals like "hello" or 42 are filters - they take an |
| input but always produce the same literal as output. Operations that |
| combine two filters, like addition, generally feed the same input to |
| both and combine the results. So, you can implement an averaging |
| filter as `add / length` - feeding the input array both to the `add` |
| filter and the `length` filter and dividing the results. |
| |
| But that's getting ahead of ourselves. :) Let's start with something |
| simpler: |
| |
| manpage_intro: | |
| jq(1) -- Command-line JSON processor |
| ==================================== |
| |
| ## SYNOPSIS |
| |
| `jq` [<options>...] <filter> [<files>...] |
| |
| `jq` can transform JSON in various ways, by selecting, iterating, |
| reducing and otherwise mangling JSON documents. For instance, |
| running the command `jq 'map(.price) | add'` will take an array of |
| JSON objects as input and return the sum of their "price" fields. |
| |
| By default, `jq` reads a stream of JSON objects (whitespace |
| separated) from `stdin`. One or more <files> may be specified, in |
| which case `jq` will read input from those instead. |
| |
| The <options> are described in the [INVOKING JQ] section, they |
| mostly concern input and output formatting. The <filter> is written |
| in the jq language and specifies how to transform the input |
| document. |
| |
| ## FILTERS |
| |
| manpage_epilogue: | |
| ## BUGS |
| |
| Presumably. Report them or discuss them at: |
| |
| https://github.com/stedolan/jq/issues |
| |
| ## AUTHOR |
| |
| Stephen Dolan `<mu@netsoc.tcd.ie>` |
| |
| sections: |
| - title: Invoking jq |
| body: | |
| |
| jq filters run on a stream of JSON data. The input to jq is |
| parsed as a sequence of whitespace-separated JSON values which |
| are passed through the provided filter one at a time. The |
| output(s) of the filter are written to standard out, again as a |
| sequence of whitespace-separated JSON data. |
| |
| You can affect how jq reads and writes its input and output |
| using some command-line options: |
| |
| * `--slurp`/`-s`: |
| |
| Instead of running the filter for each JSON object in the |
| input, read the entire input stream into a large array and run |
| the filter just once. |
| |
| * `--raw-input`/`-R`: |
| |
| Don't parse the input as JSON. Instead, each line of text is |
| passed to the filter as a string. If combined with `--slurp`, |
| then the entire input is passed to the filter as a single long |
| string. |
| |
| * `--null-input`/`-n`: |
| |
| Don't read any input at all! Instead, the filter is run once |
| using `null` as the input. This is useful when using jq as a |
| simple calculator or to construct JSON data from scratch. |
| |
| * `--compact-output` / `-c`: |
| |
| By default, jq pretty-prints JSON output. Using this option |
| will result in more compact output by instead putting each |
| JSON object on a single line. |
| |
| * `--colour-output` / `-C` and `--monochrome-output` / `-M`: |
| |
| By default, jq outputs colored JSON if writing to a |
| terminal. You can force it to produce color even if writing to |
| a pipe or a file using `-C`, and disable color with `-M`. |
| |
| * `--ascii-output` / `-a`: |
| |
| jq usually outputs non-ASCII Unicode codepoints as UTF-8, even |
| if the input specified them as escape sequences (like |
| "\u03bc"). Using this option, you can force jq to produce pure |
| ASCII output with every non-ASCII character replaced with the |
| equivalent escape sequence. |
| |
| * `--raw-output` / `-r`: |
| |
| With this option, if the filter's result is a string then it |
| will be written directly to standard output rather than being |
| formatted as a JSON string with quotes. This can be useful for |
| making jq filters talk to non-JSON-based systems. |
| |
| - title: Basic filters |
| entries: |
| - title: "`.`" |
| body: | |
| |
| The absolute simplest (and least interesting) filter |
| is `.`. This is a filter that takes its input and |
| produces it unchanged as output. |
| |
| Since jq by default pretty-prints all output, this trivial |
| program can be a useful way of formatting JSON output from, |
| say, `curl`. |
| |
| examples: |
| - program: '.' |
| input: '"Hello, world!"' |
| output: ['"Hello, world!"'] |
| |
| - title: "`.foo`" |
| body: | |
| |
| The simplest *useful* filter is .foo. When given a |
| JSON object (aka dictionary or hash) as input, it produces |
| the value at the key "foo", or null if there\'s none present. |
| |
| examples: |
| - program: '.foo' |
| input: '{"foo": 42, "bar": "less interesting data"}' |
| output: [42] |
| - program: '.foo' |
| input: '{"notfoo": true, "alsonotfoo": false}' |
| output: ['null'] |
| |
| - title: "`.[foo]`" |
| body: | |
| |
| You can also look up fields of an object using syntax like |
| `.["foo"]` (.foo above is a shorthand version of this). This |
| one works for arrays as well, if the key is an |
| integer. Arrays are zero-based (like javascript), so .[2] |
| returns the third element of the array. |
| |
| examples: |
| - program: '.[0]' |
| input: '[{"name":"JSON", "good":true}, {"name":"XML", "good":false}]' |
| output: ['{"name":"JSON", "good":true}'] |
| |
| - program: '.[2]' |
| input: '[{"name":"JSON", "good":true}, {"name":"XML", "good":false}]' |
| output: ['null'] |
| |
| - title: "`.[]`" |
| body: | |
| |
| If you use the `.[foo]` syntax, but omit the index |
| entirely, it will return *all* of the elements of an |
| array. Running `.[]` with the input `[1,2,3]` will produce the |
| numbers as three seperate results, rather than as a single |
| array. |
| |
| examples: |
| - program: '.[]' |
| input: '[{"name":"JSON", "good":true}, {"name":"XML", "good":false}]' |
| output: |
| - '{"name":"JSON", "good":true}' |
| - '{"name":"XML", "good":false}' |
| |
| - program: '.[]' |
| input: '[]' |
| output: [] |
| |
| - title: "`,`" |
| body: | |
| |
| If two filters are separated by a comma, then the |
| input will be fed into both and there will be multiple |
| outputs: first, all of the outputs produced by the left |
| expression, and then all of the outputs produced by the |
| right. For instance, filter `.foo, .bar`, produces |
| both the "foo" fields and "bar" fields as separate outputs. |
| |
| examples: |
| - program: '.foo, .bar' |
| input: '{"foo": 42, "bar": "something else", "baz": true}' |
| output: ['42', '"something else"'] |
| |
| - program: ".user, .projects[]" |
| input: '{"user":"stedolan", "projects": ["jq", "wikiflow"]}' |
| output: ['"stedolan"', '"jq"', '"wikiflow"'] |
| |
| - program: '.[4,2]' |
| input: '["a","b","c","d","e"]' |
| output: ['"e"', '"c"'] |
| |
| - title: "`|`" |
| body: | |
| The | operator combines two filters by feeding the output(s) of |
| the one on the left into the input of the one on the right. It\'s |
| pretty much the same as the Unix shell\'s pipe, if you\'re used to |
| that. |
| |
| If the one on the left produces multiple results, the one on |
| the right will be run for each of those results. So, the |
| expression `.[] | .foo` retrieves the "foo" field of each |
| element of the input array. |
| |
| examples: |
| - program: '.[] | .name' |
| input: '[{"name":"JSON", "good":true}, {"name":"XML", "good":false}]' |
| output: ['"JSON"', '"XML"'] |
| |
| - title: Types and Values |
| body: | |
| |
| jq supports the same set of datatypes as JSON - numbers, |
| strings, booleans, arrays, objects (which in JSON-speak are |
| hashes with only string keys), and "null". |
| |
| Booleans, null, strings and numbers are written the same way as |
| in javascript. Just like everything else in jq, these simple |
| values take an input and produce an output - `42` is a valid jq |
| expression that takes an input, ignores it, and returns 42 |
| instead. |
| |
| entries: |
| - title: Array construction - `[]` |
| body: | |
| |
| As in JSON, `[]` is used to construct arrays, as in |
| `[1,2,3]`. The elements of the arrays can be any jq |
| expression. All of the results produced by all of the |
| expressions are collected into one big array. You can use it |
| to construct an array out of a known quantity of values (as |
| in `[.foo, .bar, .baz]`) or to "collect" all the results of a |
| filter into an array (as in `[.items[].name]`) |
| |
| Once you understand the "," operator, you can look at jq\'s array |
| syntax in a different light: the expression [1,2,3] is not using a |
| built-in syntax for comma-separated arrays, but is instead applying |
| the `[]` operator (collect results) to the expression 1,2,3 (which |
| produces three different results). |
| |
| If you have a filter `X` that produces four results, |
| then the expression `[X]` will produce a single result, an |
| array of four elements. |
| |
| examples: |
| - program: "[.user, .projects[]]" |
| input: '{"user":"stedolan", "projects": ["jq", "wikiflow"]}' |
| output: ['["stedolan", "jq", "wikiflow"]'] |
| - title: Objects - `{}` |
| body: | |
| |
| Like JSON, `{}` is for constructing objects (aka |
| dictionaries or hashes), as in: `{"a": 42, "b": 17}`. |
| |
| If the keys are "sensible" (all alphabetic characters), then |
| the quotes can be left off. The value can be any expression |
| (although you may need to wrap it in parentheses if it\'s a |
| complicated one), which gets applied to the {} expression\'s |
| input (remember, all filters have an input and an |
| output). |
| |
| {foo: .bar} |
| |
| will produce the JSON object `{"foo": 42}` if given the JSON |
| object `{"bar":42, "baz":43}`. You can use this to select |
| particular fields of an object: if the input is an object |
| with "user", "title", "id", and "content" fields and you |
| just want "user" and "title", you can write |
| |
| {user: .user, title: .title} |
| |
| Because that\'s so common, there\'s a shortcut syntax: `{user, title}`. |
| |
| If one of the expressions produces multiple results, |
| multiple dictionaries will be produced. If the input\'s |
| |
| {"user":"stedolan","titles":["JQ Primer", "More JQ"]} |
| |
| then the expression |
| |
| {user, title: .titles[]} |
| |
| will produce two outputs: |
| |
| {"user":"stedolan", "title": "JQ Primer"} |
| {"user":"stedolan", "title": "More JQ"} |
| |
| Putting parentheses around the key means it will be evaluated as an |
| expression. With the same input as above, |
| |
| {(.user): .titles} |
| |
| produces |
| |
| {"stedolan": ["JQ Primer", "More JQ"]} |
| |
| examples: |
| - program: '{user, title: .titles[]}' |
| input: '{"user":"stedolan","titles":["JQ Primer", "More JQ"]}' |
| output: |
| - '{"user":"stedolan", "title": "JQ Primer"}' |
| - '{"user":"stedolan", "title": "More JQ"}' |
| - program: '{(.user): .titles}' |
| input: '{"user":"stedolan","titles":["JQ Primer", "More JQ"]}' |
| output: ['{"stedolan": ["JQ Primer", "More JQ"]}'] |
| |
| - 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. |
| |
| 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: 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 occurences 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 - `*` and `/` |
| body: | |
| |
| These operators only work on numbers, and do the expected. |
| |
| examples: |
| - program: '10 / . * 3' |
| input: 5 |
| output: [6] |
| |
| - 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` |
| 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. |
| |
| 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: `select` |
| 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 `[3]`. |
| |
| examples: |
| - program: 'map(select(. >= 2))' |
| input: '[1,5,3,0,7]' |
| output: ['[5,3,7]'] |
| |
| - 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: `map(x)` |
| body: | |
| |
| For any filter `x`, `map(x)` will run that filter for each |
| element of the input array, and produce the outputs a new |
| array. `map(.+1)` will increment each element of an array of numbers. |
| |
| `map(x)` is equivalent to `[.[] | x]`. In fact, this is how |
| it's defined. |
| |
| examples: |
| - program: 'map(.+1)' |
| input: '[1,2,3]' |
| output: ['[2,3,4]'] |
| |
| - 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: `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: `sort, sort_by` |
| 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_by` 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` |
| 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`, `max_by` |
| body: | |
| |
| Find the minimum or maximum element of the input array. The |
| `_by` versions 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` |
| body: | |
| |
| The `unique` function takes as input an array and produces |
| an array of the same elements, in sorted order, with |
| duplicates removed. |
| |
| examples: |
| - program: 'unique' |
| input: '[1,2,5,3,5,3,1,3]' |
| output: ['[1,2,3,5]'] |
| |
| - title: `contains` |
| 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 is 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: "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: 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 | count) > 0 then A else |
| B end' instead. |
| |
| If the condition A produces multiple results, it is |
| considered "true" if any of those results is not false or |
| null. If it produces zero results, it's considered false. |
| |
| 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, 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: Variables and Functions |
| 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). |
| |
| 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 `sum` expression is given the array and |
| produces its sum, and the `count` expression is given the array and |
| produces its length. |
| |
| So, there's generally a cleaner way to solve most problems in jq that |
| 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 "foo as $x" runs foo, puts the result in $x, |
| and returns the original input. Apart from the side-effect |
| of binding the variable, it has the same effect as ".". |
| |
| 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. |
| |
| examples: |
| - program: '.bar as $x | .foo | . + $x' |
| input: '{"foo":10, "bar":200}' |
| output: ['210'] |
| |
| - 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. |
| |
| If you want the value-argument behaviour for defining simple |
| functions, you can just use a variable: |
| |
| def addvalue(f): f as $value | map(, + $value); |
| |
| With that definition, `addvalue(.foo)` will add the current |
| input's `.foo` field to each element of the array. |
| |
| 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: 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). |
| |
| 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 |
| 1. 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. |
| |
| - 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 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}. |
| |
| - 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 langauges. 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."] |
