jq 1.5 Manual

.

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.

.foo, .foo.bar

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.

If the key contains special characters or starts with a digit, you need to surround it with double quotes like this: ."foo$".

A filter of the form .foo.bar is equivalent to .foo|.bar.

.foo?

Just like .foo, but does not output even an error when . is not an array or an object.

.[<string>], .[2], .[10:15]

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.

The .[10:15] syntax can be used to return a subarray of an array or substring of a string. The array returned by .[10:15] will be of length 5, containing the elements from index 10 (inclusive) to index 15 (exclusive). Either index may be negative (in which case it counts backwards from the end of the array), or omitted (in which case it refers to the start or end of the array).

The .[2] syntax can be used to return the element at the given index. Negative indices are allowed, with -1 referring to the last element, -2 referring to the next to last element, and so on.

The .foo syntax only works for simple keys i.e. keys that are all alphanumeric characters. .[<string>] works with keys that contain special characters such as colons and dots. For example .["foo::bar"] and .["foo.bar"] work while .foo::bar and .foo.bar would not.

The ? "operator" can also be used with the slice operator, as in .[10:15]?, which outputs values where the inputs are slice-able.

.[]

If you use the .[index] 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 separate results, rather than as a single array.

You can also use this on an object, and it will return all the values of the object.

.[]?

Like .[], but no errors will be output if . is not an array or object.

,

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.

|

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.

Array construction - []

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.

Objects - {}

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"]}

Addition - +

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.

Subtraction - -

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.

Multiplication, division, modulo - *, /, and %

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.

length

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.

keys, keys_unsorted

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.

has(key)

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.

in

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.

path(path_expression)

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.

del(path_expression)

The builtin function del removes a key and its corresponding value from an object.

to_entries, from_entries, with_entries

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.

select(boolean_expression)

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].

arrays, objects, iterables, booleans, numbers, normals, finites, strings, nulls, values, scalars

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.

empty

empty returns no results. None at all. Not even null.

It's useful on occasion. You'll know if you need it :)

error(message)

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.

$__loc__

Produces an object with a "file" key and a "line" key, with the filename and line number where $__loc__ occurs, as values.

map(x), map_values(x)

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.

paths, paths(node_filter), leaf_paths

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.

add

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.

any, any(condition), any(generator; condition)

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.

all, all(condition), all(generator; condition)

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.

flatten, flatten(depth)

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.

range(upto), range(from;upto) range(from;upto;by)

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.

floor

The floor function returns the floor of its numeric input.

sqrt

The sqrt function returns the square root of its numeric input.

tonumber

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.

tostring

The tostring function prints its input as a string. Strings are left unchanged, and all other values are JSON-encoded.

type

The type function returns the type of its argument as a string, which is one of null, boolean, number, string, array or object.

infinite, nan, isinfinite, isnan, isfinite, isnormal

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.

sort, sort_by(path_expression)

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.

group_by(path_expression)

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.

min, max, min_by(path_exp), max_by(path_exp)

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.

unique, unique_by(path_exp)

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.

reverse

This function reverses an array.

contains(element)

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.

indices(s)

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.

index(s), rindex(s)

Outputs the index of the first (index) or last (rindex) occurrence of s in the input.

inside

The filter inside(b) will produce true if the input is completely contained within b. It is, essentially, an inversed version of contains.

startswith(str)

Outputs true if . starts with the given string argument.

endswith(str)

Outputs true if . ends with the given string argument.

combinations, combinations(n)

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.

ltrimstr(str)

Outputs its input with the given prefix string removed, if it starts with it.

rtrimstr(str)

Outputs its input with the given suffix string removed, if it ends with it.

explode

Converts an input string into an array of the string's codepoint numbers.

implode

The inverse of explode.

split

Splits an input string on the separator argument.

join(str)

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.

ascii_downcase, ascii_upcase

Emit a copy of the input string with its alphabetic characters (a-z and A-Z) converted to the specified case.

while(cond; update)

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.

until(cond; next)

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.

recurse(f), recurse, recurse(f; condition), recurse_down

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.

..

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" ..

env

Outputs an object representing jq's environment.

transpose

Transpose a possibly jagged matrix (an array of arrays). Rows are padded with nulls so the result is always rectangular.

bsearch(x)

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.

String interpolation - \(foo)

Inside a string, you can put an expression inside parens after a backslash. Whatever the expression returns will be interpolated into the string.

Convert to/from JSON

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.

Format strings and escaping

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.

Dates

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.

==, !=

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'

if-then-else

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.

>, >=, <=, <

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.

and/or/not

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.

Alternative operator - //

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).

try-catch

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.

Breaking out of control structures

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.

? operator

The ? operator, used as EXP?, is shorthand for try EXP.

test(val), test(regex; flags)

Like match, but does not return match objects, only true or false for whether or not the regex matches the input.

match(val), match(regex; flags)

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

capture(val), capture(regex; flags)

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.

scan(regex), scan(regex; flags)

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) ].

split(regex; flags)

For backwards compatibility, split splits on a string, not a regex.

splits(regex), splits(regex; flags)

These provide the same results as their split counterparts, but as a stream instead of an array.

sub(regex; tostring) sub(regex; string; flags)

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)".

gsub(regex; string), gsub(regex; string; flags)

gsub is like sub but all the non-overlapping occurrences of the regex are replaced by the string, after interpolation.

Variables

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.

Defining Functions

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.

Reduce

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)

limit(n; exp)

The limit function extracts up to n outputs from exp.

first(expr), last(expr), nth(n; expr)

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.

first, last, nth(n)

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 ..

foreach

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).

Recursion

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;

Generators and iterators

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.

input

Outputs one new input.

inputs

Outputs all remaining inputs, one by one.

This is primarily useful for reductions over a program's inputs.

debug

Causes a debug message based on the input value to be produced. The jq executable wraps the input value with ["DEBUG:", <input-value>] and prints that and a newline on stderr, compactly. This may change in the future.

input_filename

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.

input_line_number

Returns the line number of the input currently being filtered.

truncate_stream(stream_expression)

Consumes a number as input and truncates the corresponding number of path elements from the left of the outputs of the given streaming expression.

fromstream(stream_expression)

Outputs values corresponding to the stream expression's outputs.

tostream

The tostream builtin outputs the streamed form of its input.

=

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.

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.

|=

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.

+=, -=, *=, /=, %=, //=

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.

Complex assignments

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."]

import RelativePathString as NAME [<metadata>];

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.

include RelativePathString [<metadata>];

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.

import RelativePathString as $NAME [<metadata>];

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.

module <metadata>;

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.

modulemeta

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.