jq [options…] filter [files…]
Transforms JSON by selecting, iterating, reducing and otherwise mangling JSON documents. For
instance, running the command jq 'map(.price) | add' will take an array of JSON objects as input and return the sum
of their "price" fields.
Accepts text input as well, by default, jq reads a stream of JSON entities (including numbers and other literals) from stdin. Whitespace is only needed to separate entities such as 1 and 2, and true and false. One or more files may be specified, in which case jq will read input from those instead.
The options are described in the [INVOKING JQ] section; input and output formatting. The filter is written in the jq language and specifies how to transform the input file or document.
Filters can be combined ; 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 one 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.
Every filter has an input and an output. literals like "hello" or 42 are filters -
they take an input and 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.
An averaging filter as add / length - feeding the input array both to the add filter and the length filter and then performing the division.
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.
Mind the shell's quoting rules. As a general rule quote with apostrophes on Unix shells. the jq program, as 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.
Windows command shell (cmd.exe) use double quotes around jq program on the command-line (instead of the -f program-file option).
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.
o Unix shells: jq '.["foo"]'
o Powershell: jq '.[\"foo\"]'
o Windows command shell: jq ".[\"foo\"]"
jq allows user-defined functions, but every jq program must have a top-level expression.
reads and writes its input and output
--null-input |
Identity |
BUILTIN OPERATORS AND FUNCTIONS
Some operators (for instance, +) do different things depending on the type of their arguments (arrays, numbers,
etc.). does notperform implicit type conversions. Adding a string to an object produces an error message and no result.
all numbers are converted to IEEE754 double precision floating point . 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.
|
REGULAR EXPRESSIONS
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:
o STRING, REGEX, and FLAGS are jq strings and subject to jq string interpolation;
o REGEX, after string interpolation, should be a valid regular expression;
o 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:
o g - Global search (find all matches, not just the first)
o i - Case insensitive search
o m - Multi line mode (. will match newlines)
o n - Ignore empty matches
o p - Both s and m modes are enabled
o s - Single line mode (^ -> \A, $ -> \Z)
o l - Find longest possible matches
o 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.
jq 'test("foo")'
"foo"
=> true
jq '.[] | test("a b c # spaces are ignored"; "ix")'
["xabcd", "ABC"]
=> true, true
match(val), match(regex; flags)
match outputs an object for each match it finds. Matches have the following fields:
o offset - offset in UTF-8 codepoints from the beginning of the input
o length - length in UTF-8 codepoints of the match
o string - the string that it matched
o captures - an array of objects representing capturing groups.
Capturing group objects have the following fields:
o offset - offset in UTF-8 codepoints from the beginning of the input
o length - length in UTF-8 codepoints of this capturing group
o string - the string that was captured
o 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
jq 'match("(abc)+"; "g")'
"abc abc"
=> {"offset": 0, "length": 3, "string": "abc", "captures": [{"offset": 0, "length": 3, "string": "abc", "name": null}]}, {"offset": 4, "length": 3, "string": "abc", "captures": [{"offset": 4, "length": 3, "string": "abc", "name": null}]}
jq 'match("foo")'
"foo bar foo"
=> {"offset": 0, "length": 3, "string": "foo", "captures": []}
jq 'match(["foo", "ig"])'
"foo bar FOO"
=> {"offset": 0, "length": 3, "string": "foo", "captures": []}, {"offset": 8, "length": 3, "string": "FOO", "captures": []}
jq 'match("foo (?<bar123>bar)? foo"; "ig")'
"foo bar foo foo foo"
=> {"offset": 0, "length": 11, "string": "foo bar foo", "captures": [{"offset": 4, "length": 3, "string": "bar", "name": "bar123"}]}, {"offset": 12, "length": 8, "string": "foo foo", "captures": [{"offset": -1, "length": 0, "string": null, "name": "bar123"}]}
jq '[ match("."; "g")] | length'
"abc"
=> 3
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.
jq 'capture("(?<a>[a-z]+)-(?<n>[0-9]+)")'
"xyzzy-14"
=> { "a": "xyzzy", "n": "14" }
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) ].
jq 'scan("c")'
"abcdefabc"
=> "c", "c"
split(regex; flags)
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.
jq 'split(", *"; null)'
"ab,cd, ef"
=> ["ab","cd","ef"]
splits(regex), splits(regex; flags)
These provide the same results as their split counterparts, but as a stream instead of an array.
jq 'splits(", *")'
"ab,cd, ef, gh"
=> "ab", "cd", "ef", "gh"
sub(regex; tostring), sub(regex; tostring; 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 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)".
jq 'sub("[^a-z]*(?<x>[a-z]+)"; "Z\(.x)"; "g")'
"123abc456def"
=> "ZabcZdef"
jq '[sub("(?<a>.)"; "\(.a|ascii_upcase)", "\(.a|ascii_downcase)")]'
"aB"
=> ["AB","aB"]
gsub(regex; tostring), gsub(regex; tostring; flags)
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.
jq 'gsub("(?<x>.)[^a]*"; "+\(.x)-")'
"Abcabc"
=> "+A-+a-"
jq '[gsub("p"; "a", "b")]'
"p"
=> ["a","b"]
|
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.
jq '.bar as $x | .foo | . + $x'
{"foo":10, "bar":200}
=> 210
jq '. as $i|[(.*2|. as $i| $i), $i]'
5
=> [10,5]
jq '. as [$a, $b, {c: $c}] | $a + $b + $c'
[2, 3, {"c": 4, "d": 5}]
=> 9
jq '.[] as [$a, $b] | {a: $a, b: $b}'
[[0], [0, 1], [2, 1, 0]]
=> {"a":0,"b":null}, {"a":0,"b":1}, {"a":2,"b":1}
Destructuring Alternative Operator: ?//
The destructuring alternative operator provides a concise mechanism for destructuring an input that can take one of
several forms.
Suppose we have an API that returns a list of resources and events associated with them, and we want to get the
user_id and timestamp of the first event for each resource. The API (having been clumsily converted from XML) will
only wrap the events in an array if the resource has multiple events:
{"resources": [{"id": 1, "kind": "widget", "events": {"action": "create", "user_id": 1, "ts": 13}},
{"id": 2, "kind": "widget", "events": [{"action": "create", "user_id": 1, "ts": 14}, {"action": "destroy", "user_id": 1, "ts": 15}]}]}
We can use the destructuring alternative operator to handle this structural change simply:
.resources[] as {$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,$b} end
jq '.[] as {$a, $b, c: {$d, $e}} ?// {$a, $b, c: [{$d, $e}]} | {$a, $b, $d, $e}'
[{"a": 1, "b": 2, "c": {"d": 3, "e": 4}}, {"a": 1, "b": 2, "c": [{"d": 3, "e": 4}]}]
=> {"a":1,"b":2,"d":3,"e":4}, {"a":1,"b":2,"d":3,"e":4}
jq '.[] as {$a, $b, c: {$d}} ?// {$a, $b, c: [{$e}]} | {$a, $b, $d, $e}'
[{"a": 1, "b": 2, "c": {"d": 3, "e": 4}}, {"a": 1, "b": 2, "c": [{"d": 3, "e": 4}]}]
=> {"a":1,"b":2,"d":3,"e":null}, {"a":1,"b":2,"d":null,"e":4}
jq '.[] as [$a] ?// [$b] | if $a != null then error("err: \($a)") else {$a,$b} end'
[[3]]
=> {"a":null,"b":3}
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 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.
jq 'def addvalue(f): . + [f]; map(addvalue(.[0]))'
[[1,2],[10,20]]
=> [[1,2,1], [10,20,10]]
jq 'def addvalue(f): f as $x | map(. + $x); addvalue(.[0])'
[[1,2],[10,20]]
=> [[1,2,1,2], [10,20,1,2]]
|
Scoping
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.
jq 'isempty(empty)'
null
=> true
jq 'isempty(.[])'
[]
=> true
jq 'isempty(.[])'
[1,2,3]
=> false
limit(n; exp)
The limit function extracts up to n outputs from exp.
jq '[limit(3;.[])]'
[0,1,2,3,4,5,6,7,8,9]
=> [0,1,2]
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. Note that nth(n; expr) doesn't support negative
values of n.
jq '[first(range(.)), last(range(.)), nth(./2; range(.))]'
10
=> [0,9,5]
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 ..
jq '[range(.)]|[first, last, nth(5)]'
10
=> [0,9,5]
reduce
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
jq 'reduce .[] as $item (0; . + $item)'
[1,2,3,4,5]
=> 15
jq 'reduce .[] as [$i,$j] (0; . + $i * $j)'
[[1,2],[3,4],[5,6]]
=> 44
jq 'reduce .[] as {$x,$y} (null; .x += $x | .y += [$y])'
[{"x":"a","y":1},{"x":"b","y":2},{"x":"c","y":3}]
=> {"x":"abc","y":[1,2,3]}
foreach
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.
jq 'foreach .[] as $item (0; . + $item)'
[1,2,3,4,5]
=> 1, 3, 6, 10, 15
jq 'foreach .[] as $item (0; . + $item; [$item, . * 2])'
[1,2,3,4,5]
=> [1,2], [2,6], [3,12], [4,20], [5,30]
jq 'foreach .[] as $item (0; . + 1; {index: ., $item})'
["foo", "bar", "baz"]
=> {"index":1,"item":"foo"}, {"index":2,"item":"bar"}, {"index":3,"item":"baz"}
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 the values generated 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 construct new generators
using only recursion and the comma operator. If recursive calls 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.
jq 'def range(init; upto; by): def _range: if (by > 0 and . < upto) or (by < 0 and . > upto) then ., ((.+by)|_range) else . end; if by == 0 then init else init|_range end | select((by > 0 and . < upto) or (by < 0 and . > upto)); range(0; 10; 3)'
null
=> 0, 3, 6, 9
jq 'def while(cond; update): def _while: if cond then ., (update | _while) else empty end; _while; [while(.<100; .*2)]'
1
=> [1,2,4,8,16,32,64]
MATH
jq currently only has IEEE754 double-precision (64-bit) floating point number support.
Besides simple arithmetic operators such as +, jq also has most standard math functions from the C math library. C
math functions that take a single input argument (e.g., sin()) are available as zero-argument jq functions. C math
functions that take two input arguments (e.g., pow()) are available as two-argument jq functions that ignore .. C
math functions that take three input arguments are available as three-argument jq functions that ignore ..
Availability of standard math functions depends on the availability of the corresponding math functions in your
operating system and C math library. Unavailable math functions will be defined but will raise an error.
One-input C math functions: acos acosh asin asinh atan atanh cbrt ceil cos cosh erf erfc exp exp10 exp2 expm1 fabs
floor gamma j0 j1 lgamma log log10 log1p log2 logb nearbyint pow10 rint round significand sin sinh sqrt tan tanh
tgamma trunc y0 y1.
Two-input C math functions: atan2 copysign drem fdim fmax fmin fmod frexp hypot jn ldexp modf nextafter nexttoward
pow remainder scalb scalbln yn.
Three-input C math functions: fma.
See your system's manual for more information on each of these.
I/O
At this time jq has minimal support for I/O, mostly in the form of control over when inputs are read. Two builtins
functions are provided for this, input and inputs, that read from the same sources (e.g., stdin, files named on the
command-line) as jq itself. These two builtins, and jq's own reading actions, can be interleaved with each other.
They are commonly used in combination with the null input option -n to prevent one input from being read implicitly.
Two builtins provide minimal output capabilities, debug, and stderr. (Recall that a jq program's output values are
always output as JSON texts on stdout.) The debug builtin can have application-specific behavior, such as for
executables that use the libjq C API but aren't the jq executable itself. The stderr builtin outputs its input in
raw mode to stder with no additional decoration, not even a newline.
Most jq builtins are referentially transparent, and yield constant and repeatable value streams when applied to
constant inputs. This is not true of I/O builtins.
input
Outputs one new input.
Note that when using input it is generally be necessary to invoke jq with the -n command-line option, otherwise the
first entity will be lost.
echo 1 2 3 4 | jq '[., input]' # [1,2] [3,4]
inputs
Outputs all remaining inputs, one by one.
This is primarily useful for reductions over a program's inputs. Note that when using inputs it is generally
necessary to invoke jq with the -n command-line option, otherwise the first entity will be lost.
echo 1 2 3 | jq -n 'reduce inputs as $i (0; . + $i)' # 6
debug, debug(msgs)
These two filters are like . but have as a side-effect the production of one or more messages on stderr.
The message produced by the debug filter has the form
["DEBUG:",<input-value>]
where <input-value> is a compact rendition of the input value. This format may change in the future.
The debug(msgs) filter is defined as (msgs | debug | empty), . thus allowing great flexibility in the content of the
message, while also allowing multi-line debugging statements to be created.
For example, the expression:
1 as $x | 2 | debug("Entering function foo with $x == \($x)", .) | (.+1)
would produce the value 3 but with the following two lines being written to stderr:
["DEBUG:","Entering function foo with $x == 1"]
["DEBUG:",2]
stderr
Prints its input in raw and compact mode to stderr with no additional decoration, not even a newline.
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.
STREAMING
With the --stream option jq can parse input texts in a streaming fashion, allowing jq programs to start processing
large JSON texts immediately rather than after the parse completes. If you have a single JSON text that is 1GB in
size, streaming it will allow you to process it much more quickly.
However, streaming isn't easy to deal with as the jq program will have [<path>, <leaf-value>] (and a few other
forms) as inputs.
Several builtins are provided to make handling streams easier.
The examples below use the streamed form of [0,[1]], which is [[0],0],[[1,0],1],[[1,0]],[[1]].
Streaming forms include [<path>, <leaf-value>] (to indicate any scalar value, empty array, or empty object), and
[<path>] (to indicate the end of an array or object). Future versions of jq run with --stream and --seq may output
additional forms such as ["error message"] when an input text fails to parse.
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.
jq 'truncate_stream([[0],1],[[1,0],2],[[1,0]],[[1]])'
1
=> [[0],2], [[0]]
fromstream(stream_expression)
Outputs values corresponding to the stream expression's outputs.
jq 'fromstream(1|truncate_stream([[0],1],[[1,0],2],[[1,0]],[[1]]))'
null
=> [2]
tostream
The tostream builtin outputs the streamed form of its input.
jq '. as $dot|fromstream($dot|tostream)|.==$dot'
[0,[1,{"a":1},{"b":2}]]
=> true
ASSIGNMENT
Assignment works a little differently in jq than in most programming languages. jq doesn't distinguish between
references to and copies of something - two objects or arrays are either equal or not equal, without any further
notion of being "the same object" or "not the same object".
If an object has two fields which are arrays, .foo and .bar, and you append something to .foo, then .bar will not
get bigger, even if you've previously set .bar = .foo. If you're used to programming in languages like Python, Java,
Ruby, JavaScript, etc. then you can think of it as though jq does a full deep copy of every object before it does
the assignment (for performance it doesn't actually do that, but that's the general idea).
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.
All the assignment operators in jq have path expressions on the left-hand side (LHS). The right-hand side (RHS)
provides values to set to the paths named by the LHS path expressions.
Values in jq are always immutable. Internally, assignment works by using a reduction to compute new, replacement
values for . that have had all the desired assignments applied to ., then outputting the modified value. This might
be made clear by this example: {a:{b:{c:1}}} | (.a.b|=3), .. This will output {"a":{"b":3}} and {"a":{"b":{"c":1}}}
because the last sub-expression, ., sees the original value, not the modified value.
Most users will want to use modification assignment operators, such as |= or +=, rather than =.
Note that the LHS of assignment operators 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.
Note too that .a,.b=0 does not set .a and .b, but (.a,.b)=0 sets both.
Update-assignment: |=
This is the "update" operator |=. It 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, .bar) |= .+1
will build an object with the foo field set to the input's foo plus 1, and the bar field set to the input's bar plus
1.
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 no values (i.e., empty), then the left-hand side path will be deleted, as with
del(path).
If the right-hand side outputs multiple values, only the first one will be used (COMPATIBILITY NOTE: in jq 1.5 and
earlier releases, it used to be that only the last one was used).
jq '(..|select(type=="boolean")) |= if . then 1 else 0 end'
[true,false,[5,true,[true,[false]],false]]
=> [1,0,[5,1,[1,[0]],0]]
Arithmetic update-assignment: +=, -=, *=, /=, %=, //=
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, being the same as |= . + 1.
jq '.foo += 1'
{"foo": 42}
=> {"foo": 43}
Plain assignment: =
This is the plain assignment operator. Unlike the others, the input to the right-hand side (RHS) is the same as the
input to the left-hand side (LHS) rather than the value at the LHS path, and all values output by the RHS will be
used (as shown below).
If the RHS 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 above) do not do this.
This example should show the difference between = and |=:
Provide input {"a": {"b": 10}, "b": 20} to the programs
.a = .b
and
.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, "b": 20}.
The latter will set the a field of the input to the a field's b field, producing {"a": 10, "b": 20}.
jq '.a = .b'
{"a": {"b": 10}, "b": 20}
=> {"a":20,"b":20}
jq '.a |= .b'
{"a": {"b": 10}, "b": 20}
=> {"a":10,"b":20}
jq '(.a, .b) = range(3)'
null
=> {"a":0,"b":0}, {"a":1,"b":1}, {"a":2,"b":2}
jq '(.a, .b) |= range(3)'
null
=> {"a":0,"b":0}
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."]
MODULES
jq has a library/module system. Modules are files whose names end in .jq.
Modules imported by a program are searched for in a default search path (see below). The import and include
directives allow the importer to alter this path.
Paths in the a search path are subject to various substitutions.
For paths starting with ~/, the user's home directory is substituted for ~.
For paths starting with $ORIGIN/, the directory where the jq executable is located is substituted for $ORIGIN.
For paths starting with ./ or paths that are ., the path of the including file is substituted for .. For top-level
programs given on the command-line, the current directory is used.
Import directives can optionally specify a search path to which the default is appended.
The default search path is the search path given to the -L command-line option, else ["~/.jq", "$ORIGIN/../lib/jq",
"$ORIGIN/../lib"].
Null and empty string path elements terminate search path processing.
A dependency with relative path foo/bar would be searched for in foo/bar.jq and foo/bar/bar.jq in the given search
path. This is intended to allow modules to be placed in a directory along with, for example, version control files,
README files, and so on, but also to allow for single-file modules.
Consecutive components with the same name are not allowed to avoid ambiguities (e.g., foo/foo).
For example, with -L$HOME/.jq a module foo can be found in $HOME/.jq/foo.jq and $HOME/.jq/foo/foo.jq.
If $HOME/.jq is a file, it is sourced into the main program.
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 and the module's defined functions as an array value for the defs 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.
|