jq Manual (development version)

For released versions, see jq 1.7 , jq 1.6 , jq 1.5 , jq 1.4 or jq 1.3 .

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 then performing the division.

But that's getting ahead of ourselves. :) Let's start with something simpler:

Invoking jq

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 output, as a sequence of newline-separated JSON data.

The simplest and most common filter (or jq program) is . , which is the identity operator, copying the inputs of the jq processor to the output stream. Because the default behavior of the jq processor is to read JSON texts from the input stream, and to pretty-print outputs, the . program's main use is to validate and pretty-print the inputs. The jq programming language is quite rich and allows for much more than just validation and pretty-printing.

Note: it is important to mind the shell's quoting rules. As a general rule it's best to always quote (with single-quote characters on Unix shells) the jq program, as too many characters with special meaning to jq are also shell meta-characters. For example, jq "foo" will fail on most Unix shells because that will be the same as jq foo , which will generally fail because foo is not defined . When using the Windows command shell (cmd.exe) it's best to use double quotes around your jq program when given on the command-line (instead of the -f program-file option), but then double-quotes in the jq program need backslash escaping. When using the Powershell ( powershell.exe ) or the Powershell Core ( pwsh / pwsh.exe ), use single-quote characters around the jq program and backslash-escaped double-quotes ( \" ) inside the jq program.

Unix shells: jq '.["foo"]'

Powershell: jq '.[\"foo\"]'

Windows command shell: jq ".[\"foo\"]"

Note: jq allows user-defined functions, but every jq program must have a top-level expression.

You can affect how jq reads and writes its input and output using some command-line options:

--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.

--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.

--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.

--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.

--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.

--raw-output0 :

Like -r but jq will print NUL instead of newline after each output. This can be useful when the values being output can contain newlines. When the output value contains NUL, jq exits with non-zero code.

--join-output / -j :

Like -r but jq won't print a newline after each output.

--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.

--sort-keys / -S :

Output the fields of each object with the keys in sorted order.

--color-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 . When the NO_COLOR environment variable is not empty, jq disables colored output by default, but you can enable it by -C .

Colors can be configured with the JQ_COLORS environment variable (see below).

--tab :

Use a tab for each indentation level instead of two spaces.

--indent n :

Use the given number of spaces (no more than 7) for indentation.

--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).

--stream :

Parse the input in streaming fashion, outputting arrays of path and leaf values (scalars and empty arrays or empty objects). For example, "a" becomes [[],"a"] , and [[],"a",["b"]] becomes [[0],[]] , [[1],"a"] , and [[2,0],"b"] .

This is useful for processing very large inputs. Use this in conjunction with filtering and the reduce and foreach syntax to reduce large inputs incrementally.

--stream-errors :

Like --stream , but invalid JSON inputs yield array values where the first element is the error and the second is a path. For example, ["a",n] produces ["Invalid literal at line 1, column 7",[1]] .

Implies --stream . Invalid JSON inputs produce no error values when --stream without --stream-errors .

--seq :

Use the application/json-seq MIME type scheme for separating JSON texts in jq's input and output. This means that an ASCII RS (record separator) character is printed before each value on output and an ASCII LF (line feed) is printed after every output. Input JSON texts that fail to parse are ignored (but warned about), discarding all subsequent input until the next RS. This mode also parses the output of jq without the --seq option.

-f filename / --from-file filename :

Read filter from the file rather than from a command line, like awk's -f option. You can also use '#' to make comments.

-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.

--arg name value :

This option passes a value to the jq program as a predefined variable. If you run jq with --arg foo bar , then $foo is available in the program and has the value "bar" . Note that value will be treated as a string, so --arg foo 123 will bind $foo to "123" .

Named arguments are also available to the jq program as $ARGS.named .

--argjson name JSON-text :

This option passes a JSON-encoded value to the jq program as a predefined variable. If you run jq with --argjson foo 123 , then $foo is available in the program and has the value 123 .

--slurpfile variable-name filename :

This option reads all the JSON texts in the named file and binds an array of the parsed JSON values to the given global variable. If you run jq with --slurpfile foo bar , then $foo is available in the program and has an array whose elements correspond to the texts in the file named bar .

--rawfile variable-name filename :

This option reads in the named file and binds its contents to the given global variable. If you run jq with --rawfile foo bar , then $foo is available in the program and has a string whose contents are to the texts in the file named bar .

--args :

Remaining arguments are positional string arguments. These are available to the jq program as $ARGS.positional[] .

--jsonargs :

Remaining arguments are positional JSON text arguments. These are available to the jq program as $ARGS.positional[] .

--exit-status / -e :

Sets the exit status of jq to 0 if the last output value was neither false nor null , 1 if the last output value was either false or null , or 4 if no valid result was ever produced. Normally jq exits with 2 if there was any usage problem or system error, 3 if there was a jq program compile error, or 0 if the jq program ran.

Another way to set the exit status is with the halt_error builtin function.

--binary / -b :

Windows users using WSL, MSYS2, or Cygwin, should use this option when using a native jq.exe, otherwise jq will turn newlines (LFs) into carriage-return-then-newline (CRLF).

--version / -V :

Output the jq version and exit with zero.

--build-configuration :

Output the build configuration of jq and exit with zero. This output has no supported format or structure and may change without notice in future releases.

--help / -h :

Output the jq help and exit with zero.

Terminates argument processing. Remaining arguments are positional, either strings, JSON texts, or input filenames, according to whether --args or --jsonargs were given.

--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.

Basic filters

Identity: .

The absolute simplest filter is . . This filter takes its input and produces the same value as output. That is, this is the identity operator.

Since jq by default pretty-prints all output, a trivial program consisting of nothing but . can be used to format JSON output from, say, curl .

Although the identity filter never modifies the value of its input, jq processing can sometimes make it appear as though it does. For example, using the current implementation of jq, we would see that the expression:

1E1234567890 | .
produces 1.7976931348623157e+308 on at least one platform.
This is because, in the process of parsing the number, this
particular version of jq has converted it to an IEEE754
double-precision representation, losing precision.
The way in which jq handles numbers has changed over time
and further changes are likely within the parameters set by
the relevant JSON standards.  The following remarks are
therefore offered with the understanding that they are
intended to be descriptive of the current version of jq and
should not be interpreted as being prescriptive:
(1) Any arithmetic operation on a number that has not
already been converted to an IEEE754 double precision
representation will trigger a conversion to the IEEE754
representation.
(2) jq will attempt to maintain the original decimal
precision of number literals, but in expressions such
1E1234567890, precision will be lost if the exponent is
too large.
(3) In jq programs, a leading minus sign will trigger the
conversion of the number to an IEEE754 representation.
(4) Comparisons are carried out using the untruncated
big decimal representation of numbers if available, as
illustrated in one of the following examples.
                Examples
            
The simplest useful filter has the form .foo. When given a
JSON object (aka dictionary or hash) as input, .foo produces
the value at the key "foo" if the key is present, or null otherwise.
A filter of the form .foo.bar is equivalent to .foo | .bar.
The .foo syntax only works for simple, identifier-like keys, that
is, keys that are all made of alphanumeric characters and
underscore, and which do not start with a digit.
If the key contains special characters or starts with a digit,
you need to surround it with double quotes like this:
."foo$", or else .["foo$"].
For example .["foo::bar"] and .["foo.bar"] work while
.foo::bar does not.
                Examples
            
You can also look up fields of an object using syntax like
.["foo"] (.foo above is a shorthand version of this, but
only for identifier-like strings).
              Array Index: .[<number>]
            
When the index value is an integer, .[<number>] can index
arrays.  Arrays are zero-based, so .[2] returns the third
element.




    

Negative indices are allowed, with -1 referring to the last
element, -2 referring to the next to last element, and so on.
                Examples
            
The .[<number>:<number>] 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).
Indices are zero-based.
                Examples
            
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. A filter of the form .foo[] is equivalent to
.foo | .[].
You can also use this on an object, and it will return all
the values of the object.
Note that the iterator operator is a generator of values.
                Examples
            
Like .[], but no errors will be output if . is not an array
or object. A filter of the form .foo[]? is equivalent to
.foo | .[]?.
              Comma: ,
            
If two filters are separated by a comma, then the
same input will be fed into both and the two filters' output
value streams will be concatenated in order: 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 is one way to contruct generators.
                Examples
            
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
similar to 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.  This is a cartesian product,
which can be surprising.
Note that .a.b.c is the same as .a | .b | .c.
Note too that . is the input value at the particular stage
in a "pipeline", specifically: where the . expression appears.
Thus .a | . | .b is the same as .a.b, as the . in the
middle refers to whatever value .a produced.
                Example
          
Types and Values 
          
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 JSON. 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.
Numbers in jq are internally represented by their IEEE754 double
precision approximation. Any arithmetic operation with numbers,
whether they are literals or results of previous filters, will
produce a double precision floating point result.
However, when parsing a literal jq will store the original literal
string. If no mutation is applied to this value then it will make
to the output in its original form, even if conversion to double
would result in a loss.
              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, including a pipeline. 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
            
Like JSON, {} is for constructing objects (aka
dictionaries or hashes), as in: {"a": 42, "b": 17}.
If the keys are "identifier-like", then the quotes can be left
off, as in {a:42, b:17}.  Variable references as key
expressions use the value of the variable as the key.  Key
expressions other than constant literals, identifiers, or
variable references, need to be parenthesized, e.g.,
{("a"+"b"):59}.
The value can be any expression (although you may need to wrap
it in parentheses if, for example, it contains colons), 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} as its input. 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 is so common, there's a shortcut syntax for it:
{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"]}
Variable references as keys use the value of the variable as
the key.  Without a value then the variable's name becomes the
key and its value becomes the value,
"f o o" as $foo | "b a r" as $bar | {$foo, $bar:$foo}
produces
{"foo":"f o o","b a r":"f o o"}
                Examples
            
Recursively descends ., producing every value.  This is the
same as the zero-argument recurse builtin (see below).  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" ..
This is particularly useful in conjunction with path(EXP)
(also see below) and the ? operator.
                Example
          
Builtin operators and functions 
          
Some jq operators (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.
Please note that all numbers are converted to IEEE754 double precision
floating point representation. Arithmetic and logical operators are working
with these converted doubles. Results of all such operations are also limited
to the double precision.
The only exception to this behaviour of number is a snapshot of original number
literal. When a number which originally was provided as a literal is never
mutated until the end of the program then it is printed to the output in its
original literal form. This also includes cases when the original literal
would be truncated when converted to the IEEE754 double precision floating point
number.
              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.
                Examples
            
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
            
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 "".
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
            
The builtin function abs is defined naively as: if . < 0 then - . else . end.
For numeric input, this is the absolute value.  See the
section on the identity filter for the implications of this
definition for numeric input.
To compute the absolute value of a number as a floating point number, you may wish use fabs.
                Example
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 a number is its absolute value.
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.
It is an error to use length on a boolean.
            
The builtin function utf8bytelength outputs the number of
bytes used to encode a string in UTF-8.
                Example
            
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
            
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
            
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
            
For any filter f, map(f) and map_values(f) apply f
to each of the values in the input array or object, that is,
to the values of .[].
In the absence of errors, map(f) always outputs an array
whereas map_values(f) outputs an array if given an array,
or an object if given an object.
When the input to map_values(f) is an object, the output
object has the same keys as the input object except for
those keys whose values when piped to f produce no values
at all.
The key difference between map(f) and map_values(f) is
that the former simply forms an array from all the values of
($x|f) for each value, $x, in the input array or object,
but map_values(f) only uses first($x|f).
Specifically, for object inputs, map_value(f) constructs
the output object by examining in turn the value of
first(.[$k]|f) for each key, $k, of the input.  If this
expression produces no values, then the corresponding key
will be dropped; otherwise, the output object will have that
value at the key, $k.
Here are some examples to clarify the behavior of map and
map_values when applied to arrays. These examples assume the
input is [1] in all cases:
map(.+1)          #=>  [2]
map(., .)         #=>  [1,1]
map(empty)        #=>  []
map_values(.+1)   #=>  [2]
map_values(., .)  #=>  [1]
map_values(empty) #=>  []
map(f) is equivalent to [.[] | f] and
map_values(f) is equivalent to .[] |= f.
In fact, these are their implementations.
                Examples
            
Emit the projection of the input object or array defined by the
specified sequence of path expressions, such that if p is any
one of these specifications, then (. | p) will evaluate to the
same value as (. | pick(pathexps) | p). For arrays, negative
indices and .[m:n] specifications should not be used.
                Examples
            
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
            
The builtin function delpaths deletes the PATHS in ..
PATHS must be an array of paths, where each path is an array
of strings and numbers.
                Example
            
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(f)
is a shorthand for to_entries | map(f) | from_entries, useful for
doing some operation to all keys and values of an object.
from_entries accepts "key", "Key", "name", "Name",
"value", and "Value" as keys.
                Examples
            
The function select(f) produces its input unchanged if
f 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
              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.
                Example
            
empty returns no results. None at all. Not even null.
It's useful on occasion. You'll know if you need it :)
                Examples
            
Produces an error with the input value, or with the message
given as the argument. Errors can be caught with try/catch;
see below.
                Examples
            
Stops the jq program with no further outputs.  The input will
be printed on stderr as raw output (i.e., strings will not
have double quotes) with no decoration, not even a newline.
The given exit_code (defaulting to 5) will be jq's exit
status.
For example, "Error: something went wrong\n"|halt_error(1).
              $__loc__
            
Produces an object with a "file" key and a "line" key, with
the filename and line number where $__loc__ occurs, as
values.
                Example
            
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(type == "number") outputs the paths to all numeric
values.
                Examples
            
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
            
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
            
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
            
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
            
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
            
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.
                Example
            
The tostring function prints its input as a
string. Strings are left unchanged, and all other values are
JSON-encoded.
                Example
            
The type function returns the type of its argument as a
string, which is one of null, boolean, number, string, array
or object.
                Example
            
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
            
The sort functions sorts its input, which must be an
array. Values are sorted in the following order:
false
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(f) compares
two elements by comparing the result of f on each element.
When f produces multiple values, it firstly compares the
first values, and the second values if the first values are
equal, and so on.
                Examples
                    Output
                    [{"foo":2, "bar":1}, {"foo":3, "bar":10}, {"foo":3, "bar":20}, {"foo":4, "bar":10}]
            
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.
                Example
            
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
            
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
            
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
            
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
            
The filter inside(b) will produce true if the input is
completely contained within b. It is, essentially, an
inversed version of contains.
                Examples
            
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
            
Splits an input string on the separator argument.
split can also split on regex matches when called with
two arguments (see the regular expressions section below).
                Example
            
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.
Numbers and booleans in the input are converted to strings.
Null values are treated as empty strings. Arrays and objects
in the input are not supported.
                Examples
            
Emit a copy of the input string with its alphabetic characters (a-z and A-Z)
converted to the specified case.
                Example
            
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.
                Example
            
The repeat(exp) function allows you to repeatedly
apply expression exp to . until an error is raised.
Note that repeat(exp) is internally defined as a
recursive jq function.  Recursive calls within repeat will
not consume additional memory if exp produces at most one
output for each input.  See advanced topics below.
                Example
            
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.
                Example
            
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; true) 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).
The recursive calls in recurse will not consume additional
memory whenever f produces at most a single output for each
input.
                Examples
            
The walk(f) function applies f recursively to every
component of the input entity.  When an array is
encountered, f is first applied to its elements and then to
the array itself; when an object is encountered, f is first
applied to all the values and then to the object.  In
practice, f will usually test the type of its input, as
illustrated in the following examples.  The first example
highlights the usefulness of processing the elements of an
array of arrays before processing the array itself.  The second
example shows how all the keys of all the objects within the
input can be considered for alteration.
                Examples
                    Command
                    jq 'walk( if type == "object" then with_entries( .key |= sub( "^_+"; "") ) else . end )'
                    Input
                    [ { "_a": { "__b": 2 } } ]
                    Output
                    [{"a":{"b":2}}]
            
This builtin binding shows the jq executable's build
configuration.  Its value has no particular format, but
it can be expected to be at least the ./configure
command-line arguments, and may be enriched in the
future to include the version strings for the build
tooling used.




    

Note that this can be overriden in the command-line
with --arg and related options.
              $ENV, env
            
$ENV is an object representing the environment variables as
set when the jq program started.
env outputs an object representing jq's current environment.
At the moment there is no builtin for setting environment
variables.
                Examples
            
Transpose a possibly jagged matrix (an array of arrays).
Rows are padded with nulls so the result is always rectangular.
                Example
            
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
            
Inside a string, you can put an expression inside parens
after a backslash. Whatever the expression returns will be
interpolated into the string.
                Example
            
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
            
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 &lt;, &gt;,
  &amp;, &apos;, &quot;.
@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.
@base64d:
The inverse of @base64, input is decoded as specified by RFC 4648.
  Note\: If the decoded string is not UTF-8, the results are undefined.
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
            
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, strflocaltime,
mktime, gmtime, and localtime.  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
Greenwich Mean Time as an array of numbers representing
(in this order): the year, the month (zero-based), the day of
the month (one-based), 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
day of the week number may be wrong on some systems for dates
before March 1st 1900, or after December 31 2099.
The localtime builtin works like the gmtime builtin, but
using the local timezone setting.
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 (GMT) with the
given format.  The strflocaltime does the same, but using
the local timezone setting.
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. In particular, the %u and %j specifiers for
strptime(fmt) are not supported on macOS.
                Examples
INDEX(stream; index_expression):
This builtin produces an object whose keys are computed by
  the given index expression applied to each value from the
  given stream.
JOIN($idx; stream; idx_expr; join_expr):
This builtin joins the values from the given stream to the
  given index.  The index's keys are computed by applying the
  given index expression to each value from the given stream.
  An array of the value in the stream and the corresponding
  value from the index is fed to the given join expression to
  produce each result.
JOIN($idx; stream; idx_expr):
Same as JOIN($idx; stream; idx_expr; .).
JOIN($idx; idx_expr):
This builtin joins the input . to the given index, applying
  the given index expression to . to compute the index key.
  The join operation is as described above.
IN(s):
This builtin outputs true if . appears in the given
  stream, otherwise it outputs false.
IN(source; s):
This builtin outputs true if any value in the source stream
  appears in the second stream, otherwise it outputs false.
              builtins
            
Returns a list of all builtin functions in the format name/arity.
Since functions with the same name but different arities are considered
separate functions, all/0, all/1, and all/2 would all be present
in the list.
          
Conditionals and Comparisons 
              ==, !=
            
The expression 'a == b' will produce 'true' if the results of evaluating
a and b are equal (that is, if they represent equivalent JSON values) and
'false' otherwise. In particular, strings are never considered equal
to numbers.  In checking for the equality of JSON objects, the ordering of keys
is irrelevant.  If you're coming from JavaScript, please note that jq's == is like
JavaScript's ===, the "strict equality" operator.
!= is "not equal", and 'a != b' returns the opposite value of 'a == b'
                Examples
            
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.
if A then B end is the same as if A then B else .  end.
That is, the else branch is optional, and if absent is the
same as .. This also applies to elif with absent ending else branch.
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 like if .name == "" 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.
                Example
            
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.
                Example
            
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
            
The // operator produces all the values of its left-hand
side that are neither false nor null, or, if the
left-hand side produces no values other than false or
null, then // produces all the values of its right-hand
side.
A filter of the form a // b produces all the results of
a that are not false or null.  If a produces no
results, or no results other than false or null, then a
// b produces the results of 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).
Note: some_generator // defaults_here is not the same
as some_generator | . // defaults_here.  The latter will
produce default values for all non-false, non-null
values of the left-hand side, while the former will not.
Precedence rules can make this confusing.  For example, in
false, 1 // 2 the left-hand side of // is 1, not
false, 1 -- false, 1 // 2 parses the same way as false,
(1 // 2).  In (false, null, 1) | . // 42 the left-hand
side of // is ., which always produces just one value,
while in (false, null, 1) // 42 the left-hand side is a
generator of three values, and since it produces a
value other false and null, the default 42 is not
produced.
                Examples
            
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
            
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 if .=="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.
              Error Suppression / Optional Operator: ?
            
The ? operator, used as EXP?, is shorthand for try EXP.
                Examples
          
jq uses the
Oniguruma regular expression library,
as do PHP, TextMate, Sublime Text, etc, so the
description here will focus on jq specifics.
Oniguruma supports several flavors of regular expression, so it is important to know
that jq uses the "Perl NG" (Perl with named groups) flavor.
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 regular expression;
FILTER is one of test, match, or capture, as described below.
Since REGEX must evaluate to a JSON string, some characters that are needed
to form a regular expression must be escaped. For example, the regular expression
\s signifying a whitespace character would be written as "\\s".




    

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 a whitespace with the x flag, use \s, e.g.
jq -n '"a b" | 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.
              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.
                Examples
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
                    {"offset": 4, "length": 3, "string": "abc", "captures": [{"offset": 4, "length": 3, "string": "abc", "name": null}]}
                    {"offset": 12, "length": 8, "string": "foo  foo", "captures": [{"offset": -1, "length": 0, "string": null, "name": "bar123"}]}
            
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.
                Example
            
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
            
Splits an input string on each regex match.
For backwards compatibility, when called with a single argument,
split splits on a string, not a regex.
                Example
            
These provide the same results as their split counterparts,
but as a stream instead of an array.
                Example
            
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 or a stream
of such strings, each of which 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)".
                Examples
            
gsub is like sub but all the non-overlapping occurrences of the regex are
replaced by tostring, after interpolation. If the second argument is a stream
of jq strings, then gsub will produce a corresponding stream of JSON strings.
                Examples
          
Advanced features 
          
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 select 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.
              Variable / Symbolic Binding Operator: ... as $identifier | ...
            
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": "First post", "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": "First post", "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
            
The destructuring alternative operator provides a concise mechanism
for destructuring an input that can take one of several forms.
Suppose we have an API that returns a list of resources and events
associated with them, and we want to get the user_id and timestamp of
the first event for each resource. The API (having been clumsily
converted from XML) will only wrap the events in an array if the resource
has multiple events:
{"resources": [{"id": 1, "kind": "widget", "events": {"action": "create", "user_id": 1, "ts": 13}},
               {"id": 2, "kind": "widget", "events": [{"action": "create", "user_id": 1, "ts": 14}, {"action": "destroy", "user_id": 1, "ts": 15}]}]}
We can use the destructuring alternative operator to handle this structural change simply:
.resources[] as {$id, $kind, events: {$user_id, $ts}} ?// {$id, $kind, events: [{$user_id, $ts}]} | {$user_id, $kind, $id, $ts}
Or, if we aren't sure if the input is an array of values or an object:
.[] as [$id, $kind, $user_id, $ts] ?// {$id, $kind, $user_id, $ts} | ...
Each alternative need not define all of the same variables, but all named
variables will be available to the subsequent expression. Variables not
matched in the alternative that succeeded will be null:
.resources[] as {$id, $kind, events: {$user_id, $ts}} ?// {$id, $kind, events: [{$first_user_id, $first_ts}]} | {$user_id, $first_user_id, $kind, $id, $ts, $first_ts}
Additionally, if the subsequent expression returns an error, the
alternative operator will attempt to try the next binding. Errors
that occur during the final alternative are passed through.
[[3]] | .[] as [$a] ?// [$b] | if $a != null then error("err: \($a)") else {$a,$b} end
                Examples
                    Input
                    [{"a": 1, "b": 2, "c": {"d": 3, "e": 4}}, {"a": 1, "b": 2, "c": [{"d": 3, "e": 4}]}]
                    Output
                    {"a":1,"b":2,"d":3,"e":4}
                    {"a":1,"b":2,"d":3,"e":4}
                    Input
                    [{"a": 1, "b": 2, "c": {"d": 3, "e": 4}}, {"a": 1, "b": 2, "c": [{"d": 3, "e": 4}]}]
                    Output
                    {"a":1,"b":2,"d":3,"e":null}
                    {"a":1,"b":2,"d":null,"e":4}
                    Command
                    jq '.[] as [$a] ?// [$b] | if $a != null then error("err: \($a)") else {$a,$b} end'
                    Input
                    [[3]]
                    Output
                    {"a":null,"b":3}
From then on, increment is usable as a filter just like a
builtin function (in fact, this is how many of the builtins
are defined). A function may take arguments:
def map(f): [.[] | f];
Arguments are passed as filters (functions with no
arguments), 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.  Do note
that calling addvalue(.[]) will cause the map(. + $f) part
to be evaluated once per value in the value of . at the call
site.
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.
See also the section below on scoping.
                Examples
            
There are two types of symbols in jq: value bindings (a.k.a.,
"variables"), and functions.  Both are scoped lexically,
with expressions being able to refer only to symbols that
have been defined "to the left" of them.  The only exception
to this rule is that functions can refer to themselves so as
to be able to create recursive functions.
For example, in the following expression there is a binding
which is visible "to the right" of it, ... | .*3 as
$times_three | [. + $times_three] | ..., but not "to the
left".  Consider this expression now, ... | (.*3 as
$times_three | [. + $times_three]) | ...: here the binding
$times_three is not visible past the closing parenthesis.
              isempty(exp)
            
Returns true if exp produces no outputs, false otherwise.
                Examples
            
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.
Note that nth(n; expr) doesn't support negative values of n.
                Example
            
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 ..
                Example
            
The reduce syntax allows you to combine all of the results of
an expression by accumulating them into a single answer.
The form is reduce EXP as $var (INIT; UPDATE).
As an example, we'll pass [1,2,3] to this expression:
reduce .[] as $item (0; . + $item)
For each result that .[] produces, . + $item is run to
accumulate a running total, starting from 0 as the input value.
In this example, .[] produces the results 1, 2, and 3,
so the effect is similar to running something like this:
0 | 1 as $item | . + $item |
    2 as $item | . + $item |
    3 as $item | . + $item
                Examples
            
The foreach syntax is similar to reduce, but intended to
allow the construction of limit and reducers that produce
intermediate results.
The form is foreach EXP as $var (INIT; UPDATE; EXTRACT).
As an example, we'll pass [1,2,3] to this expression:
foreach .[] as $item (0; . + $item; [$item, . * 2])
Like the reduce syntax, . + $item is run for each result
that .[] produces, but [$item, . * 2] is run for each
intermediate values. In this example, since the intermediate
values are 1, 3, and 6, the foreach expression produces
[1,2], [2,6], and [3,12]. So the effect is similar
to running something like this:
0 | 1 as $item | . + $item | [$item, . * 2],
    2 as $item | . + $item | [$item, . * 2],
    3 as $item | . + $item | [$item, . * 2]
When EXTRACT is omitted, the identity filter is used.
That is, it outputs the intermediate values as they are.
                Examples
            
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;
            
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,