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NAME

       jq - Command-line JSON processor

SYNOPSIS

       jq [options...] filter [files...]

       jq  can  transform  JSON  in various ways, by selecting, iterating, reducing and otherwise
       mangling JSON documents. For instance, running the command jq  ´map(.price)  |  add´  will
       take an array of JSON objects as input and return the sum of their "price" fields.

       By  default,  jq  reads a stream of JSON objects (whitespace separated) from stdin. One or
       more files may be specified, in which case jq will read input from those instead.

       The options are described in the INVOKING JQ section, they mostly concern input and output
       formatting.  The  filter  is written in the jq language and specifies how to transform the
       input document.

FILTERS

       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:

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 out, again as a sequence of
       whitespace-separated JSON data.

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

       --slurp/-s:

              Instead  of  running  the filter for each JSON object in the input, read the entire
              input stream into a large array and run the filter just once.

       --raw-input/-R:

              Don´t parse the input as JSON. Instead, each line of text is passed to  the  filter
              as  a  string.  If  combined  with  --slurp, then the entire input is passed to the
              filter as a single long string.

       --null-input/-n:

              Don´t read any input at all! Instead, the filter is run  once  using  null  as  the
              input.  This  is  useful  when using jq as a simple calculator or to construct JSON
              data from scratch.

       --compact-output / -c:

              By default, jq pretty-prints JSON output. Using this option  will  result  in  more
              compact output by instead putting each JSON object on a single line.

       --colour-output / -C and --monochrome-output / -M:

              By  default,  jq outputs colored JSON if writing to a terminal. You can force it to
              produce color even if writing to a pipe or a file using -C, and disable color  with
              -M.

       --ascii-output / -a:

              jq  usually  outputs  non-ASCII  Unicode  codepoints  as  UTF-8,  even if the input
              specified them as escape sequences (like "\u03bc").  Using  this  option,  you  can
              force  jq to produce pure ASCII output with every non-ASCII character replaced with
              the equivalent escape sequence.

       --raw-output / -r:

              With this option, if the filter´s result is  a  string  then  it  will  be  written
              directly  to  standard  output  rather  than  being formatted as a JSON string with
              quotes. This can be useful for making jq filters talk to non-JSON-based systems.

       --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".

BASIC FILTERS

   .
       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.

           jq ´.´
              "Hello, world!"
           => "Hello, world!"

   .foo
       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.

           jq ´.foo´
              {"foo": 42, "bar": "less interesting data"}
           => 42

           jq ´.foo´
              {"notfoo": true, "alsonotfoo": false}
           => null

   .[foo], .[2], .[10:15]
       You  can  also  look  up  fields  of an object using syntax like .["foo"] (.foo above is a
       shorthand version of this). This one works for arrays as well, if the key is  an  integer.
       Arrays are zero-based (like javascript), so .[2] returns the third element of the array.

       The  .[10:15]  syntax  can be used to return a subarray of an array. 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).

           jq ´.[0]´
              [{"name":"JSON", "good":true}, {"name":"XML", "good":false}]
           => {"name":"JSON", "good":true}

           jq ´.[2]´
              [{"name":"JSON", "good":true}, {"name":"XML", "good":false}]
           => null

           jq ´.[2:4]´
              ["a","b","c","d","e"]
           => ["c", "d"]

           jq ´.[:3]´
              ["a","b","c","d","e"]
           => ["a", "b", "c"]

           jq ´.[-2:]´
              ["a","b","c","d","e"]
           => ["d", "e"]

   .[]
       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
       separate results, rather than as a single array.

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

           jq ´.[]´
              [{"name":"JSON", "good":true}, {"name":"XML", "good":false}]
           => {"name":"JSON", "good":true}, {"name":"XML", "good":false}

           jq ´.[]´
              []
           =>

           jq ´.[]´
              {"a": 1, "b": 1}
           => 1, 1

   ,
       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.

           jq ´.foo, .bar´
              {"foo": 42, "bar": "something else", "baz": true}
           => 42, "something else"

           jq ´.user, .projects[]´
              {"user":"stedolan", "projects": ["jq", "wikiflow"]}
           => "stedolan", "jq", "wikiflow"

           jq ´.[4,2]´
              ["a","b","c","d","e"]
           => "e", "c"

   |
       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.

           jq ´.[] | .name´
              [{"name":"JSON", "good":true}, {"name":"XML", "good":false}]
           => "JSON", "XML"

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 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.

   Array construction - []
       As in JSON, [] is used to construct arrays, as in [1,2,3]. The elements of the arrays  can
       be  any jq expression. All of the results produced by all of the expressions are collected
       into one big array. You can use it to construct an array out of a known quantity of values
       (as  in  [.foo, .bar, .baz]) or to "collect" all the results of a filter into an array (as
       in [.items[].name])

       Once you understand the "," operator, you can look at jq´s array  syntax  in  a  different
       light:  the  expression [1,2,3] is not using a built-in syntax for comma-separated arrays,
       but is instead applying the [] operator (collect results) to the expression  1,2,3  (which
       produces three different results).

       If  you have a filter X that produces four results, then the expression [X] will produce a
       single result, an array of four elements.

           jq ´[.user, .projects[]]´
              {"user":"stedolan", "projects": ["jq", "wikiflow"]}
           => ["stedolan", "jq", "wikiflow"]

   Objects - {}
       Like JSON, {} is for constructing objects (aka dictionaries or hashes), as in:  {"a":  42,
       "b": 17}.

       If  the  keys are "sensible" (all alphabetic characters), then the quotes can be left off.
       The value can be any expression (although you may need to wrap it in parentheses if it´s a
       complicated  one),  which gets applied to the {} expression´s input (remember, all filters
       have an input and an output).

           {foo: .bar}

       will produce the JSON object {"foo": 42} if given the JSON  object  {"bar":42,  "baz":43}.
       You  can use this to select particular fields of an object: if the input is an object with
       "user", "title", "id", and "content" fields and you just want "user" and "title", you  can
       write

           {user: .user, title: .title}

       Because that´s so common, there´s a shortcut syntax: {user, title}.

       If  one  of  the  expressions  produces  multiple  results,  multiple dictionaries will be
       produced. If the input´s

           {"user":"stedolan","titles":["JQ Primer", "More JQ"]}

       then the expression

           {user, title: .titles[]}

       will produce two outputs:

           {"user":"stedolan", "title": "JQ Primer"}
           {"user":"stedolan", "title": "More JQ"}

       Putting parentheses around the key means it will be evaluated as an expression.  With  the
       same input as above,

           {(.user): .titles}

       produces

           {"stedolan": ["JQ Primer", "More JQ"]}

           jq ´{user, title: .titles[]}´
              {"user":"stedolan","titles":["JQ Primer", "More JQ"]}
           => {"user":"stedolan", "title": "JQ Primer"}, {"user":"stedolan", "title": "More JQ"}

           jq ´{(.user): .titles}´
              {"user":"stedolan","titles":["JQ Primer", "More JQ"]}
           => {"stedolan": ["JQ Primer", "More JQ"]}

BUILTIN OPERATORS AND FUNCTIONS

       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.

   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.

       null can be added to any value, and returns the other value unchanged.

           jq ´.a + 1´
              {"a": 7}
           => 8

           jq ´.a + .b´
              {"a": [1,2], "b": [3,4]}
           => [1,2,3,4]

           jq ´.a + null´
              {"a": 1}
           => 1

           jq ´.a + 1´
              {}
           => 1

           jq ´{a: 1} + {b: 2} + {c: 3} + {a: 42}´
              null
           => {"a": 42, "b": 2, "c": 3}

   Subtraction - -
       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.

           jq ´4 - .a´
              {"a":3}
           => 1

           jq ´. - ["xml", "yaml"]´
              ["xml", "yaml", "json"]
           => ["json"]

   Multiplication, division - * and /
       These operators only work on numbers, and do the expected.

           jq ´10 / . * 3´
              5
           => 6

   length
       The builtin function length gets the length of various different types of value:

       ·   The  length of a string is the number of Unicode codepoints it contains (which will be
           the same as its JSON-encoded length in bytes if it´s pure ASCII).

       ·   The length of an array is the number of elements.

       ·   The length of an object is the number of key-value pairs.

       ·   The length of null is zero.

           jq ´.[] | length´

            [[1,2], "string", {"a":2}, null]

       => 2, 6, 1, 0

   keys
       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.

           jq ´keys´
              {"abc": 1, "abcd": 2, "Foo": 3}
           => ["Foo", "abc", "abcd"]

           jq ´keys´
              [42,3,35]
           => [0,1,2]

   has
       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.

           jq ´map(has("foo"))´
              [{"foo": 42}, {}]
           => [true, false]

           jq ´map(has(2))´
              [[0,1], ["a","b","c"]]
           => [false, true]

   to_entries, from_entries, with_entries
       These  functions  convert between an object and an array of key-value pairs. If to_entries
       is passed an object, then for each k: v entry in the  input,  the  output  array  includes
       {"key": k, "value": v}.

       from_entries  does  the  opposite  conversion,  and  with_entries(foo)  is a shorthand for
       to_entries | map(foo) | from_entries, useful for doing some  operation  to  all  keys  and
       values of an object.

           jq ´to_entries´
              {"a": 1, "b": 2}
           => [{"key":"a", "value":1}, {"key":"b", "value":2}]

           jq ´from_entries´
              [{"key":"a", "value":1}, {"key":"b", "value":2}]
           => {"a": 1, "b": 2}

           jq ´with_entries(.key |= "KEY_" + .)´
              {"a": 1, "b": 2}
           => {"KEY_a": 1, "KEY_b": 2}

   select
       The  function select(foo) produces its input unchanged if foo returns true for that input,
       and produces no output otherwise.

       It´s useful for filtering lists: [1,2,3] | map(select(. >= 2)) will give you [3].

           jq ´map(select(. >= 2))´
              [1,5,3,0,7]
           => [5,3,7]

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

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

           jq ´1, empty, 2´
              null
           => 1, 2

           jq ´[1,2,empty,3]´
              null
           => [1,2,3]

   map(x)
       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.

           jq ´map(.+1)´
              [1,2,3]
           => [2,3,4]

   add
       The filter add takes as input an array, and produces as output the elements of  the  array
       added  together.  This might mean summed, concatenated or merged depending on the types of
       the elements of the input array - the rules are the same  as  those  for  the  +  operator
       (described above).

       If the input is an empty array, add returns null.

           jq ´add´
              ["a","b","c"]
           => "abc"

           jq ´add´
              [1, 2, 3]
           => 6

           jq ´add´
              []
           => null

   range
       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.

           jq ´range(2;4)´
              null
           => 2, 3

           jq ´[range(2;4)]´
              null
           => [2,3]

   tonumber
       The  tonumber  function  parses its input as a number. It will convert correctly-formatted
       strings to their numeric equivalent, leave numbers alone, and give an error on  all  other
       input.

           jq ´.[] | tonumber´
              [1, "1"]
           => 1, 1

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

           jq ´.[] | tostring´
              [1, "1", [1]]
           => "1", "1", "[1]"

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

           jq ´map(type)´
              [0, false, [], {}, null, "hello"]
           => ["number", "boolean", "array", "object", "null", "string"]

   sort, sort_by
       The  sort  functions  sorts  its  input,  which must be an array. Values are sorted in the
       following order:

       ·   null

       ·   false

       ·   true

       ·   numbers

       ·   strings, in alphabetical order (by unicode codepoint value)

       ·   arrays, in lexical order

       ·   objects

       The ordering for objects is a little complex: first they´re compared  by  comparing  their
       sets  of keys (as arrays in sorted order), and if their keys are equal then the values are
       compared key by key.

       sort_by may be used to sort by a particular field of an object,  or  by  applying  any  jq
       filter. sort_by(foo) compares two elements by comparing the result of foo on each element.

           jq ´sort´
              [8,3,null,6]
           => [null,3,6,8]

           jq ´sort_by(.foo)´
              [{"foo":4, "bar":10}, {"foo":3, "bar":100}, {"foo":2, "bar":1}]
           => [{"foo":2, "bar":1}, {"foo":3, "bar":100}, {"foo":4, "bar":10}]

   group_by
       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.

           jq ´group_by(.foo)´
              [{"foo":1, "bar":10}, {"foo":3, "bar":100}, {"foo":1, "bar":1}]
           => [[{"foo":1, "bar":10}, {"foo":1, "bar":1}], [{"foo":3, "bar":100}]]

   min, max, min_by, max_by
       Find the minimum or maximum element of the input array. The  _by  versions  allow  you  to
       specify a particular field or property to examine, e.g. min_by(.foo) finds the object with
       the smallest foo field.

           jq ´min´
              [5,4,2,7]
           => 2

           jq ´max_by(.foo)´
              [{"foo":1, "bar":14}, {"foo":2, "bar":3}]
           => {"foo":2, "bar":3}

   unique
       The unique function takes as input an array and produces an array of the same elements, in
       sorted order, with duplicates removed.

           jq ´unique´
              [1,2,5,3,5,3,1,3]
           => [1,2,3,5]

   reverse
       This function reverses an array.

           jq ´reverse´
              [1,2,3,4]
           => [4,3,2,1]

   contains
       The  filter contains(b) will produce true if b is completely contained within the input. A
       string B is contained in a string A if B is a substring of A. An array B is  contained  in
       an  array  A  is  all  elements  in  B  are  contained in any element in A. An object B is
       contained in object A if all of the values in B are contained in the value in A  with  the
       same key. All other types are assumed to be contained in each other if they are equal.

           jq ´contains("bar")´
              "foobar"
           => true

           jq ´contains(["baz", "bar"])´
              ["foobar", "foobaz", "blarp"]
           => true

           jq ´contains(["bazzzzz", "bar"])´
              ["foobar", "foobaz", "blarp"]
           => false

           jq ´contains({foo: 12, bar: [{barp: 12}]})´
              {"foo": 12, "bar":[1,2,{"barp":12, "blip":13}]}
           => true

           jq ´contains({foo: 12, bar: [{barp: 15}]})´
              {"foo": 12, "bar":[1,2,{"barp":12, "blip":13}]}
           => false

   recurse
       The  recurse  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

           jq ´recurse(.foo[])´
              {"foo":[{"foo": []}, {"foo":[{"foo":[]}]}]}
           => {"foo":[{"foo":[]},{"foo":[{"foo":[]}]}]}, {"foo":[]}, {"foo":[{"foo":[]}]}, {"foo":[]}

   String interpolation - \(foo)
       Inside  a  string, you can put an expression inside parens after a backslash. Whatever the
       expression returns will be interpolated into the string.

           jq ´"The input was \(.), which is one less than \(.+1)"´
              42
           => "The input was 42, which is one less than 43"

   Format strings and escaping
       The @foo syntax is used to format and escape strings, which is useful for  building  URLs,
       documents  in  a  language like HTML or XML, and so forth. @foo can be used as a filter on
       its own, the possible escapings are:

       @text:

              Calls tostring, see that function for details.

       @json:

              Serialises 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.

       @sh:

              The input is escaped suitable for use in a command-line for a POSIX shell.  If  the
              input is an array, the output will be a series of space-separated strings.

       @base64:

              The input is converted to base64 as specified by RFC 4648.

       This  syntax  can  be combined with string interpolation in a useful way. You can follow a
       @foo token with a string literal. The contents of the string literal will not be  escaped.
       However, all interpolations made inside that string literal will be escaped. For instance,

           @uri "http://www.google.com/search?q=\(.search)"

       will produce the following output for the input {"search":"jq!"}:

           http://www.google.com/search?q=jq%21

       Note  that  the slashes, question mark, etc. in the URL are not escaped, as they were part
       of the string literal.

           jq ´@html´
              "This works if x < y"
           => "This works if x &lt; y"

           jq ´@sh "echo \(.)"´
              "O´Hara´s Ale"
           => "echo ´O´\\´´Hara´\\´´s Ale´"

CONDITIONALS AND COMPARISONS

   ==, !=
       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´

           jq ´.[] == 1´
              [1, 1.0, "1", "banana"]
           => true, true, false, false

   if-then-else
       if A then B else C end will act the same as B if A produces a value other  than  false  or
       null, but act the same as C otherwise.

       Checking for false or null is a simpler notion of "truthiness" than is found in Javascript
       or Python, but it means that you´ll sometimes have to be more explicit about the condition
       you want: you can´t test whether, e.g. a string is empty using if .name then A else B end,
       you´ll need something more like if (.name | 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.

           jq ´if . == 0 then

       "zero" elif . == 1 then "one" else "many" end´

              2
           => "many"

   >, >=, <=, <
       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.

           jq ´. < 5´
              2
           => true

   and/or/not
       jq  supports the normal Boolean operators and/or/not. They have the same standard of truth
       as if expressions - false and null are considered "false values", and anything else  is  a
       "true value".

       If  an  operand  of  one of these operators produces multiple results, the operator itself
       will produce a result for each input.

       not is in fact a builtin function rather than an operator, so it is called as a filter  to
       which things can be piped rather than with special syntax, as in .foo and .bar | not.

       These three only produce the values "true" and "false", and so are only useful for genuine
       Boolean   operations,   rather    than    the    common    Perl/Python/Ruby    idiom    of
       "value_that_may_be_null or default". If you want to use this form of "or", picking between
       two values rather than evaluating a condition, see the "//" operator below.

           jq ´42 and "a string"´
              null
           => true

           jq ´(true, false) or false´
              null
           => true, false

           jq ´(true, true) and (true, false)´
              null
           => true, false, true, false

           jq ´[true, false | not]´
              null
           => [false, true]

   Alternative operator - //
       A filter of the form a // b produces the same results as a, if a  produces  results  other
       than false and null. Otherwise, a // b produces the same results as b.

       This  is  useful  for  providing defaults: .foo // 1 will evaluate to 1 if there´s no .foo
       element in the input. It´s similar to how or is sometimes used in Python (jq´s or operator
       is reserved for strictly Boolean operations).

           jq ´.foo // 42´
              {"foo": 19}
           => 19

           jq ´.foo // 42´
              {}
           => 42

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 find are in fact
       written in jq).

       Finally, jq has a reduce operation, which is very powerful but a bit tricky.  Again,  it´s
       mostly used internally, to define some useful bits of jq´s standard library.

   Variables
       In  jq,  all  filters  have an input and an output, so manual plumbing is not necessary to
       pass a value from one part of a program to the next. Many expressions, for instance a + b,
       pass  their  input  to  two distinct subexpressions (here a and b are both passed the same
       input), so variables aren´t usually necessary in order to use a value twice.

       For instance, calculating the average  value  of  an  array  of  numbers  requires  a  few
       variables in most languages - at least one to hold the array, perhaps one for each element
       or for a loop counter. In jq, it´s simply add / length - the add expression is  given  the
       array  and produces its sum, and the length expression is given the array and produces its
       length.

       So, there´s generally a cleaner way to solve most problems in jq 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 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.

       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.

           jq ´.bar as $x | .foo | . + $x´
              {"foo":10, "bar":200}
           => 210

   Defining Functions
       You can give a filter a name using "def" syntax:

           def increment: . + 1;

       From then on, increment is usable as a filter just like a builtin function (in fact,  this
       is how some of the builtins are defined). A function may take arguments:

           def map(f): [.[] | f];

       Arguments  are  passed  as  filters,  not  as  values. The same argument may be referenced
       multiple times with different inputs (here f is run for each element of the input  array).
       Arguments to a function work more like callbacks than like value arguments.

       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.

           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]]

   Reduce
       The  reduce  syntax  in  jq  allows  you to combine all of the results of an expression by
       accumulating them into a single  answer.  As  an  example,  we´ll  pass  [3,2,1]  to  this
       expression:

           reduce .[] as $item (0; . + $item)

       For  each  result  that  .[]  produces,  .  +  $item is run to accumulate a running total,
       starting from 0. In this example, .[] produces the results 3, 2, and 1, so the  effect  is
       similar to running something like this:

           0 | (3 as $item | . + $item) |
               (2 as $item | . + $item) |
               (1 as $item | . + $item)

           jq ´reduce .[] as $item (0; . + $item)´
              [10,2,5,3]
           => 20

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 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).

   =
       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.

   |=
       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}.

   +=, -=, *=, /=, //=
       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.

           jq ´.foo += 1´
              {"foo": 42}
           => {"foo": 43}

   Complex assignments
       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."]

BUGS

       Presumably. Report them or discuss them at:

           https://github.com/stedolan/jq/issues

AUTHOR

       Stephen Dolan <mu@netsoc.tcd.ie>

                                            April 2014                                      JQ(1)