blob: 4ec88c28f10048c387ddf41d013ccaea399255d6 [file] [log] [blame]
headline: jq Manual
body: |
A jq program is a "filter": it takes an input, and produces an
output. There are a lot of builtin filters for extracting a
particular field of an object, or converting a number to a string,
or various other standard tasks.
Filters can be combined in various ways - you can pipe the output of
one filter into another filter, or collect the output of a filter
into an array.
Some filters produce multiple results, for instance there's one that
produces all the elements of its input array. Piping that filter
into a second runs the second filter for each element of the
array. Generally, things that would be done with loops and iteration
in other languages are just done by gluing filters together in jq.
It's important to remember that every filter has an input and an
output. Even literals like "hello" or 42 are filters - they take an
input but always produce the same literal as output. Operations that
combine two filters, like addition, generally feed the same input to
both and combine the results. So, you can implement an averaging
filter as `add / length` - feeding the input array both to the `add`
filter and the `length` filter and dividing the results.
But that's getting ahead of ourselves. :) Let's start with something
simpler:
sections:
- title: Invoking jq
body: |
jq filters run on a stream of JSON data. The input to jq is
parsed as a sequence of whitespace-separated JSON values which
are passed through the provided filter one at a time. The
output(s) of the filter are written to standard out, again as a
sequence of whitespace-separated JSON data.
You can affect how jq reads and writes its input and output
using some command-line options:
* `--slurp`/`-s`
Instead of running the filter for each JSON object in the
input, read the entire input stream into a large array and run
the filter just once.
* `--raw-input`/`-R`
Don't parse the input as JSON. Instead, each line of text is
passed to the filter as a string. If combined with `--slurp`,
then the entire input is passed to the filter as a single long
string.
* `--null-input`/`-n`
Don't read any input at all! Instead, the filter is run once
using `null` as the input. This is useful when using jq as a
simple calculator or to construct JSON data from scratch.
* `--compact-output` / `-c`
By default, jq pretty-prints JSON output. Using this option
will result in more compact output by instead putting each
JSON object on a single line.
* `--ascii-output` / `-a`
jq usually outputs non-ASCII Unicode codepoints as UTF-8, even
if the input specified them as escape sequences (like
"\u03bc"). Using this option, you can force jq to produce pure
ASCII output with every non-ASCII character replaced with the
equivalent escape sequence.
* `--raw-output` / `-r`
With this option, if the filter's result is a string then it
will be written directly to standard output rather than being
formatted as a JSON string with quotes. This can be useful for
making jq filters talk to non-JSON-based systems.
- title: Basic filters
entries:
- title: "`.`"
body: |
The absolute simplest (and least interesting) filter
is `.`. This is a filter that takes its input and
produces it unchanged as output.
Since jq by default pretty-prints all output, this trivial
program can be a useful way of formatting JSON output from,
say, `curl`.
examples:
- program: '.'
input: '"Hello, world!"'
output: ['"Hello, world!"']
- title: "`.foo`"
body: |
The simplest *useful* filter is .foo. When given a
JSON object (aka dictionary or hash) as input, it produces
the value at the key "foo", or null if there\'s none present.
examples:
- program: '.foo'
input: '{"foo": 42, "bar": "less interesting data"}'
output: [42]
- program: '.foo'
input: '{"notfoo": true, "alsonotfoo": false}'
output: ['null']
- title: "`.[foo]`"
body: |
You can also look up fields of an object using syntax like
`.["foo"]` (.foo above is a shorthand version of this). This
one works for arrays as well, if the key is an
integer. Arrays are zero-based (like javascript), so .[2]
returns the third element of the array.
examples:
- program: '.[0]'
input: '[{"name":"JSON", "good":true}, {"name":"XML", "good":false}]'
output: ['{"name":"JSON", "good":true}']
- program: '.[2]'
input: '[{"name":"JSON", "good":true}, {"name":"XML", "good":false}]'
output: ['null']
- title: "`.[]`"
body: |
If you use the `.[foo]` syntax, but omit the index
entirely, it will return *all* of the elements of an
array. Running `.[]` with the input `[1,2,3]` will produce the
numbers as three seperate results, rather than as a single
array.
examples:
- program: '.[]'
input: '[{name":"JSON", "good":true}, {"name":"XML", "good":false}]'
output:
- '{"name":"JSON", "good":true}'
- '{"name":"XML", "good":false}'
- program: '.[]'
input: '[]'
output: []
- title: "`,`"
body: |
If two filters are separated by a comma, then the
input will be fed into both and there will be multiple
outputs: first, all of the outputs produced by the left
expression, and then all of the outputs produced by the
right. For instance, filter `.foo, .bar`, produces
both the "foo" fields and "bar" fields as separate outputs.
examples:
- program: '.foo, .bar'
input: '{"foo": 42, "bar": "something else", "baz": true}'
output: ['42', '"something else"']
- program: ".user, .projects[]"
input: '{"user":"stedolan", "projects": ["jq", "wikiflow"]}'
output: ['"stedolan"', '"jq"', '"wikiflow"']
- program: '.[4,2]'
input: '["a","b","c","d","e"]'
output: ['"d"', '"c"']
- title: "`|`"
body: |
The | operator combines two filters by feeding the output(s) of
the one on the left into the input of the one on the right. It\'s
pretty much the same as the Unix shell\'s pipe, if you\'re used to
that.
If the one on the left produces multiple results, the one on
the right will be run for each of those results. So, the
expression `.[] | .foo` retrieves the "foo" field of each
element of the input array.
examples:
- program: '.[] | .name'
input: '[{name":"JSON", "good":true}, {"name":"XML", "good":false}]'
output: ['"JSON"', '"XML"']
- title: Types and Values
body: |
jq supports the same set of datatypes as JSON - numbers,
strings, booleans, arrays, objects (which in JSON-speak are
hashes with only string keys), and "null".
Booleans, null, strings and numbers are written the same way as
in javascript. Just like everything else in jq, these simple
values take an input and produce an output - `42` is a valid jq
expression that takes an input, ignores it, and returns 42
instead.
entries:
- title: Array construction - `[]`
body: |
As in JSON, `[]` is used to construct arrays, as in
`[1,2,3]`. The elements of the arrays can be any jq
expression. All of the results produced by all of the
expressions are collected into one big array. You can use it
to construct an array out of a known quantity of values (as
in `[.foo, .bar, .baz]`) or to "collect" all the results of a
filter into an array (as in `[.items[].name]`)
Once you understand the "," operator, you can look at jq\'s array
syntax in a different light: the expression [1,2,3] is not using a
built-in syntax for comma-separated arrays, but is instead applying
the `[]` operator (collect results) to the expression 1,2,3 (which
produces three different results).
If you have a filter `X` that produces four results,
then the expression `[X]` will produce a single result, an
array of four elements.
examples:
- program: "[.user, .projects[]]"
input: '{"user":"stedolan", "projects": ["jq", "wikiflow"]}'
output: ['["stedolan", "jq", "wikiflow"]']
- title: Objects - `{}`
body: |
Like JSON, `{}` is for constructing objects (aka
dictionaries or hashes), as in: `{"a": 42, "b": 17}`.
If the keys are "sensible" (all alphabetic characters), then
the quotes can be left off. The value can be any expression
(although you may need to wrap it in parentheses if it\'s a
complicated one), which gets applied to the {} expression\'s
input (remember, all filters have an input and an
output).
{foo: .bar}
will produce the JSON object `{"foo": 42}` if given the JSON
object `{"bar":42, "baz":43}`. You can use this to select
particular fields of an object: if the input is an object
with "user", "title", "id", and "content" fields and you
just want "user" and "title", you can write
{user: .user, title: .title}
Because that\'s so common, there\'s a shortcut syntax: `{user, title}`.
If one of the expressions produces multiple results,
multiple dictionaries will be produced. If the input\'s
{"user":"stedolan","titles":["JQ Primer", "More JQ"]}
then the expression
{user, title: .titles[]}
will produce two outputs:
{"user":"stedolan", "title": "JQ Primer"}
{"user":"stedolan", "title": "More JQ"}
Putting parentheses around the key means it will be evaluated as an
expression. With the same input as above,
{(.user): .titles}
produces
{"stedolan": ["JQ Primer", "More JQ"]}
examples:
- program: '{user, title: .titles[]}'
input: '{"user":"stedolan","titles":["JQ Primer", "More JQ"]}'
output:
- '{"user":"stedolan", "title": "JQ Primer"}'
- '{"user":"stedolan", "title": "More JQ"}'
- program: '{(.user): .titles}'
input: '{"user":"stedolan","titles":["JQ Primer", "More JQ"]}'
output: ['{"stedolan": ["JQ Primer", "More JQ"]}']
- title: Builtin operators and functions
body: |
Some jq operator (for instance, `+`) do different things
depending on the type of their arguments (arrays, numbers,
etc.). However, jq never does implicit type conversions. If you
try to add a string to an object you'll get an error message and
no result.
entries:
- title: Addition - `+`
body: |
The operator `+` takes two filters, applies them both
to the same input, and adds the results together. What
"adding" means depends on the types involved:
- **Numbers** are added by normal arithmetic.
- **Arrays** are added by being concatenated into a larger array.
- **Strings** are added by being joined into a larger string.
- **Objects** are added by merging, that is, inserting all
the key-value pairs from both objects into a single
combined object. If both objects contain a value for the
same key, the object on the right of the `+` wins.
examples:
- program: '.a + 1'
input: '{"a": 7}'
output: '{"a": 8}'
- program: '.a + .b'
input: '{"a": [1,2], "b": [3,4]}'
output: ['[1,2,3,4]']
- program: '{a: 1} + {b: 2} + {c: 3} + {a: 42}'
input: 'null'
output: ['{"a": 42, "b": 2, "c": 3}']
- title: Subtraction - `-`
body: |
As well as normal arithmetic subtraction on numbers, the `-`
operator can be used on arrays to remove all occurences of
the second array's elements from the first array.
examples:
- program: '4 - .a'
input: '{"a":3}'
output: ['1']
- program: . - ["xml", "yaml"]
input: '["xml", "yaml", "json"]'
output: ['["json"]']
- title: Multiplication, division - `*` and `/`
body: |
These operators only work on numbers, and do the expected.
examples:
- program: '10 / . * 3'
input: 5
output: [6]
- title: `length`
body: |
The builtin function `length` gets the length of various
different types of value:
- The length of a **string** is the number of Unicode
codepoints it contains (which will be the same as its
JSON-encoded length in bytes if it's pure ASCII).
- The length of an **array** is the number of elements.
- The length of an **object** is the number of key-value pairs.
- The length of **null** is zero.
examples:
- program: '.[] | length'
input: '[[1,2], "string", {"a":2}, null]'
output: [2, 6, 1, 0]
- title: `map(x)`
body: |
For any filter `x`, `map(x)` will run that filter for each
element of the input array, and produce the outputs a new
array. `map(.+1)` will increment each element of an array of numbers.
`map(x)` is equivalent to `[.[] | x]`. In fact, this is how
it's defined.
examples:
- program: 'map(.+1)'
input: '[1,2,3]'
output: ['[2,3,4]']
- title: `add`
body: |
The filter `add` takes as input an array, and produces as
output the elements of the array added together. This might
mean summed, concatenated or merged depending on the types
of the elements of the input array - the rules are the same
as those for the `+` operator (described above).
If the input is an empty array, `add` returns `null`.
examples:
- program: add
input: '["a","b","c"]'
output: ["abc"]
- program: add
input: '[1, 2, 3]'
output: [6]
- program: add
input: '[]'
output: ["null"]
- title: `tonumber`
body: |
The `tonumber` function parses its input as a number. It
will convert correctly-formatted strings to their numeric
equivalent, leave numbers alone, and give an error on all other input.
examples:
- program: '.[] | tonumber'
input: '[1, "1"]'
output: [1,1]
- title: `tostring`
body: |
The `tostring` function prints its input as a
string. Strings are left unchanged, and all other values are
JSON-encoded.
examples:
- program: '.[] | tostring'
input: '[1, "1", [1]]'
output: ['"1"', '"1"', '"[1]"']
- title: "String interpolation - `@(text)`"
body: |
jq supports an alternative syntax for strings. Instead of
"foo", you can write `@(foo)`. When using this syntax,
`%(expression)` may be used to insert the value of
`expression` into the string (converted with `tostring`).
String interpolation does not occur for normal double-quoted
strings (like `"foo"`) in order to be fully compatible with
JSON.
All of the usual JSON escapes (`\n`, `\r` and the like) work
inside `@()`-quoted strings, as well as `\%` and `\)` if
those characters are needed literally.
examples:
- program: '@(The input was %(.), which is one less than %(.+1))'
input: '42'
output: ['"The input was 42, which is one less than 43"']
- title: Conditionals and Comparisons
entries:
- title: `==`, `!=`
body: |
The expression 'a == b' will produce 'true' if the result of a and b
are equal (that is, if they represent equivalent JSON documents) and
'false' otherwise. In particular, strings are never considered equal
to numbers. If you're coming from Javascript, jq's == is like
Javascript's === - considering values equal only when they have the
same type as well as the same value.
!= is "not equal", and 'a != b' returns the opposite value of 'a == b'
examples:
- program: '.[] == 1'
input: '[1, 1.0, "1", "banana"]'
output: ['[true, true, false, false]']
- title: if-then-else
body: |
`if A then B else C end` will act the same as `B` if `A`
produces a value other than false or null, but act the same
as `C` otherwise.
Checking for false or null is a simpler notion of
"truthiness" than is found in Javascript or Python, but it
means that you'll sometimes have to be more explicit about
the condition you want: you can't test whether, e.g. a
string is empty using `if .name then A else B end`, you'll
need something more like 'if (.name | count) > 0 then A else
B end' instead.
If the condition A produces multiple results, it is
considered "true" if any of those results is not false or
null. If it produces zero results, it's considered false.
More cases can be added to an if using `elif A then B` syntax.
examples:
- program: |-
if . == 0 then
"zero"
elif . == 1 then
"one"
else
"many"
end
input: 2
output: ['"many"']
- title: and/or/not
body: |
jq supports the normal Boolean operators and/or/not. They have the
same standard of truth as if expressions - false and null are
considered "false values", and anything else is a "true value".
If an operand of one of these operators produces multiple
results, the operator itself will produce a result for each input.
`not` is in fact a builtin function rather than an operator,
so it is called as a filter to which things can be piped
rather than with special syntax, as in `.foo and .bar |
not`.
These three only produce the values "true" and "false", and
so are only useful for genuine Boolean operations, rather
than the common Perl/Python/Ruby idiom of
"value_that_may_be_null or default". If you want to use this
form of "or", picking between two values rather than
evaluating a condition, see the "//" operator below.
examples:
- program: '42 and "a string"'
input: 'null'
output: ['true']
- program: '(true, false) or false'
input: 'null'
output: ['true', 'false']
- program: '(true, false) and (true, false)'
input: 'null'
output: ['true', 'false', 'false', 'false']
- program: '[true, false | not]'
input: 'null'
output: ['[false, true]']
- title: Alternative operator - `//`
body: |
A filter of the form `a // b` produces the same
results as `a`, if `a` produces results other than `false`
and `null`. Otherwise, `a // b` produces the same results as `b`.
This is useful for providing defaults: `.foo // 1` will
evaluate to `1` if there's no `.foo` element in the
input. It's similar to how `or` is sometimes used in Python
(jq's `or` operator is reserved for strictly Boolean
operations).
examples:
- program: '.foo // 42'
input: '{"foo": 19}'
output: [19]
- program: '.foo // 42'
input: '{}'
output: [42]
- title: Variables and Functions
body: |
Variables are an absolute necessity in most programming languages, but
they're relegated to an "advanced feature" in jq.
In most languages, variables are the only means of passing around
data. If you calculate a value, and you want to use it more than once,
you'll need to store it in a variable. To pass a value to another part
of the program, you'll need that part of the program to define a
variable (as a function parameter, object member, or whatever) in
which to place the data.
It is also possible to define functions in jq, although this is
is a feature whose biggest use is defining jq's standard library
(many jq functions such as `map` and `find` are in fact written
in jq).
entries:
- title: Variables
body: |
In jq, all filters have an input and an output, so manual
plumbing is not necessary to pass a value from one part of a program
to the next. Many expressions, for instance `a + b`, pass their input
to two distinct subexpressions (here `a` and `b` are both passed the
same input), so variables aren't usually necessary in order to use a
value twice.
For instance, calculating the average value of an array of numbers
requires a few variables in most languages - at least one to hold the
array, perhaps one for each element or for a loop counter. In jq, it's
simply `add / length` - the `sum` expression is given the array and
produces its sum, and the `count` expression is given the array and
produces its length.
So, there's generally a cleaner way to solve most problems in jq that
defining variables. Still, sometimes they do make things easier, so jq
lets you define variables using `expression as $variable`. All
variable names start with `$`. Here's a slightly uglier version of the
array-averaging example:
length as $array_length | add / $array_length
We'll need a more complicated problem to find a situation where using
variables actually makes our lives easier.
Suppose we have an array of blog posts, with "author" and "title"
fields, and another object which is used to map author usernames to
real names. Our input looks like:
{"posts": [{"title": "Frist psot", "author": "anon"},
{"title": "A well-written article", "author": "person1"}],
"realnames": {"anon": "Anonymous Coward",
"person1": "Person McPherson"}}
We want to produce the posts with the author field containing a real
name, as in:
{"title": "Frist psot", "author": "Anonymous Coward"}
{"title": "A well-written article", "author": "Person McPherson"}
We use a variable, $names, to store the realnames object, so that we
can refer to it later when looking up author usernames:
.realnames as $names | .posts[] | {title, author: $names[.author]}
The expression "foo as $x" runs foo, puts the result in $x,
and returns the original input. Apart from the side-effect
of binding the variable, it has the same effect as ".".
Variables are scoped over the rest of the expression that defines
them, so
.realnames as $names | (.posts[] | {title, author: $names[.author]})
will work, but
(.realnames as $names | .posts[]) | {title, author: $names[.author]}
won't.
examples:
- program: '.bar as $x | .foo | . + $x'
input: '{"foo":10, "bar":200}'
output: ['210']
- title: 'Defining Functions'
body: |
You can give a filter a name using "def" syntax:
def increment: . + 1;
From then on, `increment` is usable as a filter just like a
builtin function (in fact, this is how some of the builtins
are defined). A function may take arguments:
def map(f): [.[] | f];
Arguments are passed as filters, not as values. The
same argument may be referenced multiple times with
different inputs (here `f` is run for each element of the
input array). Arguments to a function work more like
callbacks than like value arguments.
If you want the value-argument behaviour for defining simple
functions, you can just use a variable:
def addvalue(f): f as $value | map(, + $value);
With that definition, `addvalue(.foo)` will add the current
input's `.foo` field to each element of the array.
examples:
- program: 'def addvalue(f): map(. + [$value]); addvalue(.[0])'
input: '[[1,2],[10,20]]'
output: ['[[1,2,1], [10,20,10]]']
- program: 'def addvalue(f): f as $x | map (. + $x); addvalue(.[0])'
input: '[[1,2],[10,20]]'
output: ['[[1,2,[1,2]], [10,20,[1,2]]]']
- title: Assignment
body: |
Assignment works a little differently in jq than in most
programming languages. jq doesn't distinguish between references
to and copies of something - two objects or arrays are either
equal or not equal, without any further notion of being "the
same object" or "not the same object".
If an object has two fields which are arrays, `.foo` and `.bar`,
and you append something to `.foo`, then `.bar` will not get
bigger. Even if you've just set `.bar = .foo`. If you're used to
programming in languages like Python, Java, Ruby, Javascript,
etc. then you can think of it as though jq does a full deep copy
of every object before it does the assignment (for performance,
it doesn't actually do that, but that's the general idea).
entries:
- title: "`=`"
body: |
The filter `.foo = 1` will take as input an object
and produce as output an object with the "foo" field set to
1. There is no notion of "modifying" or "changing" something
in jq - all jq values are immutable. For instance,
.foo = .bar | .foo.baz = 1
will not have the side-effect of setting .bar.baz to be set
to 1, as the similar-looking program in Javascript, Python,
Ruby or other languages would. Unlike these languages (but
like Haskell and some other functional languages), there is
no notion of two arrays or objects being "the same array" or
"the same object". They can be equal, or not equal, but if
we change one of them in no circumstances will the other
change behind our backs.
This means that it's impossible to build circular values in
jq (such as an array whose first element is itself). This is
quite intentional, and ensures that anything a jq program
can produce can be represented in JSON.
- title: "`|=`"
body: |
As well as the assignment operator '=', jq provides the "update"
operator '|=', which takes a filter on the right-hand side and
works out the new value for the property being assigned to by running
the old value through this expression. For instance, .foo |= .+1 will
build an object with the "foo" field set to the input's "foo" plus 1.
This example should show the difference between '=' and '|=':
Provide input '{"a": {"b": 10}, "b": 20}' to the programs:
.a = .b
.a |= .b
The former will set the "a" field of the input to the "b" field of the
input, and produce the output {"a": 20}. The latter will set the "a"
field of the input to the "a" field's "b" field, producing {"a": 10}.
- title: "`+=`, `-=`, `*=`, `/=`, `//=`"
body: |
jq has a few operators of the form `a op= b`, which are all
equivalent to `a |= . op b`. So, `+= 1` can be used to increment values.
examples:
- program: .foo += 1
input: '{"foo": 42}'
output: ['{"foo": 43}']
- title: Complex assignments
body: |
Lots more things are allowed on the left-hand side of a jq assignment
than in most langauges. We've already seen simple field accesses on
the left hand side, and it's no surprise that array accesses work just
as well:
.posts[0].title = "JQ Manual"
What may come as a surprise is that the expression on the left may
produce multiple results, referring to different points in the input
document:
.posts[].comments |= . + ["this is great"]
That example appends the string "this is great" to the "comments"
array of each post in the input (where the input is an object with a
field "posts" which is an array of posts).
When jq encounters an assignment like 'a = b', it records the "path"
taken to select a part of the input document while executing a. This
path is then used to find which part of the input to change while
executing the assignment. Any filter may be used on the
left-hand side of an equals - whichever paths it selects from the
input will be where the assignment is performed.
This is a very powerful operation. Suppose we wanted to add a comment
to blog posts, using the same "blog" input above. This time, we only
want to comment on the posts written by "stedolan". We can find those
posts using the "select" function described earlier:
.posts[] | select(.author == "stedolan")
The paths provided by this operation point to each of the posts that
"stedolan" wrote, and we can comment on each of them in the same way
that we did before:
(.posts[] | select(.author == "stedolan") | .comments) |= . + ["terrible."]