NAME
    yq - Filter YAML through a command-line program

VERSION
    version 0.012

SYNOPSIS
        yq [-v] <script> [<file>...]

        yq -h|--help|--version

DESCRIPTION
    This program takes a stream of YAML documents (on STDIN or file
    arguments), applies a filter, then writes the results to STDOUT.

ARGUMENTS
  script
    The script to run. For the script syntax, see SYNTAX.

  <file>
    A YAML file to filter. The special file "-" refers to STDIN. If no files
    are specified, filter STDIN.

OPTIONS
  -v | --verbose
    Set verbose mode to print out some internal program messages on STDERR
    to help with debugging.

  -h | --help
    Show this help document.

  --version
    Print the current yq and Perl versions.

SYNTAX
  EXPRESSIONS
    An "EXPRESSION" is allowed to be either a FILTER, VALUE, or a
    COMPARISON.

  FILTERS
    Filters select a portion of the incoming documents. Filters can be
    combined to reach deep inside the documents you're working with.

    .   Returns the entire document, unfiltered. Useful in if/then
        statements.

            # INPUT
            foo: bar
            baz: fuzz

            $ yq .
            foo: bar
            baz: fuzz

    .key
        Extract a single item out of a hash.

            # INPUT
            foo:
                bar: baz
                fizz: fuzz

            $ yq .foo
            bar: baz
            fizz: fuzz

            $ yq .foo.fizz
            fuzz

    .[#]
        Extract a single item out of an array.

            # INPUT
            - foo: fuzz
            - bar: buzz
            - baz:
                good: bad
                new: old

            $ yq .[1]
            bar: buzz

            $ yq .[2]
            baz:
                good: bad
                new: old

            $ yq .[2].baz
            good: bad
            new: old

            $ yq .[2].baz.new
            old

    []  Use [] with no index to flatten an array.

            # INPUT
            - foo: fuzz
            - bar: buzz

            $ yq '.[]'
            foo: fuzz
            ---
            bar: buzz

  VALUES
    'STRING' "STRING"
        Both single- and double-quoted strings are allowed. Using \ will
        escape the string delimiter.

    { KEY: EXPRESSION, ... }
        The hash constructor. "KEY" may be any "FILTER" or a bare value.

            # INPUT
            foo: bar
            baz: fuzz
            ---
            foo: 1
            baz: 2

            $ yq '{ bar: .foo, .baz: foo }'
            bar: bar
            fuzz: foo
            ---
            2: foo
            bar: 1

    [ EXPRESSION, ... ]
        The array constructor.

            # INPUT
            foo: bar
            baz: fuzz
            ---
            foo: 1
            baz: 2

            $ yq '[ .foo, .baz ]'
            - bar
            - fuzz
            ---
            - 1
            - 2

    empty
        The special value empty suppresses printing of a document. Normally,
        an undefined document will show up in the output as "--- ~". If your
        filter instead yields empty, the document will not be printed at
        all.

        This is especially useful in conditionals:

            # INPUT
            foo: bar
            baz: fuzz

            $ yq 'if .foo eq bar then . else empty'
            foo: bar
            baz: fuzz

            $ yq 'if .foo eq buzz then . else empty'
            $

        ... though see the "grep()" function for a shorter way of writing
        this.

    Values
        Any bareword that is not recognized as a syntax element is treated
        as a value. These barewords may only contain letters, numbers, and
        underscore.

        NOTE: This may be subject to change to only allow quoted strings and
        bare numbers in a future version.

  COMPARISONS
    eq  String equals comparison. Returns true if both sides are equal to
        each other when treated as a string.

        The two sides may be FILTERS or VALUES.

            # INPUT
            foo: bar
            baz: fuzz
            buzz: fuzz

            $ yq '.foo eq bar'
            true

            $ yq '.baz eq .buzz'
            true

            $ yq '.baz eq bar'
            false

        YAML treats the string "true" as a true value, and the string
        "false" as a false value.

    ne  String not equals comparison. Returns true if one side is not equal
        to the other side when compared as a string.

        The two sides may be FILTERS or VALUES.

            # INPUT
            foo: bar
            baz: fuzz
            buzz: fuzz

            $ yq '.foo eq bar'
            true

            $ yq '.baz eq .buzz'
            true

            $ yq '.baz eq bar'
            false

        YAML treats the string "true" as a true value, and the string
        "false" as a false value.

    ==  Numeric equals comparison. Returns true if both sides are equal to
        each other when treated as numbers. If one of the items is not a
        number, will print a warning to STDERR but try to compare anyway.

        The two sides may be FILTERS or VALUES.

            # INPUT
            one: 1
            two: 2
            uno: 1

            $ yq '.one == 1'
            true

            $ yq '.one == 2'
            false

            $ yq '.one == .uno'
            true

    !=  Numeric not equals comparison. Returns true if both sides are equal
        to each other when treated as numbers. If one of the items is not a
        number, will print a warning to STDERR but try to compare anyway.

        The two sides may be FILTERS or VALUES.

            # INPUT
            one: 1
            two: 2
            uno: 1

            $ yq '.two != 1'
            true

            $ yq '.two != 2'
            false

            $ yq '.one != .uno'
            false

    > / >=
        Numeric greater-than (or equal-to) comparison. Returns true if the
        left side is greater than (or equal-to) the right side. If one of
        the items is not a number, will print a warning to STDERR but try to
        compare anyway.

        The two sides may be FILTERS or VALUES.

            # INPUT
            one: 1
            two: 2
            uno: 1

            $ yq '.two >= 1'
            true

            $ yq '.two > 2'
            false

            $ yq '.one >= .uno'
            true

    < / <=
        Numeric less-than (or equal-to) comparison. Returns true if the left
        side is less than (or equal-to) the right side. If one of the items
        is not a number, will print a warning to STDERR but try to compare
        anyway.

        The two sides may be FILTERS or VALUES.

            # INPUT
            one: 1
            two: 2
            uno: 1

            $ yq '.two <= 1'
            false

            $ yq '.two < 2'
            false

            $ yq '.one <= .uno'
            true

  FUNCTIONS
    length( EXPRESSION )
        Returns the length of the thing returned by EXPRESSION. Depending on
        what type of thing that is:

            string/number   - Returns the number of characters
            array           - Returns the number of items
            hash            - Returns the number of pairs

        If EXPRESSION is missing, gives the length of the entire document
        (length(.)). Returns a number suitable for assignment.

        Although length() takes an expression, certain constructs are
        redundant:

            length( keys( EXPRESSION ) ) -> length( EXPRESSION )
            # length() works on hashes

        A future version may optimize these away, or warn you of their
        redundancy.

            # INPUT
            foo:
                one: 1
                two: onetwothreefourfive
                three: 3
            baz: [ 3, 2, 1 ],

            $ yq 'length(.)'
            2

            $ yq 'length'
            2

            $ yq 'length( .foo )'
            3

            $ yq 'length( .baz )'
            3

            $ yq 'length( .foo.two )'
            19

            $ yq '{ l: length( .foo.two ) }'
            l: 19

    keys( EXPRESSION )
        Return the keys of the hash or the indicies of the array returned by
        EXPRESSION. If EXPRESSION is missing, gives the keys of the entire
        document (keys(.)).

        Returns an array suitable for assignment.

            # INPUT
            foo:
                one: 1
                two: 2
                three: 3
            baz: [ 3, 2, 1 ]

            $ yq 'keys( .foo )'
            - one
            - two
            - three

            $ yq 'keys( .baz )'
            - 0
            - 1
            - 2

            $ yq 'keys( . )'
            - foo
            - baz

            $ yq 'keys'
            - foo
            - baz

            $ yq '{ k: keys( .foo ) }'
            k:
                - one
                - two
                - three

    grep( EXPRESSION )
        If "EXPRESSION" is true, return the current document. Otherwise,
        return "empty".

        This is exactly the same as:

            if EXPRESSION then . else empty

    select( EXPRESSION )
        Another name for "grep()" to match "jq"'s syntax.

    group_by( EXPRESSION )
        Group incoming documents based on the result of "EXPRESSION",
        yielding a single document containing a hash of arrays.

            # INPUT
            ---
            foo: 'bar'
            baz: 1
            ---
            foo: 'bar'
            baz: 2
            ---
            foo: 'baz'
            baz: 3

            $ yq 'group_by( .foo )'
            bar:
                - foo: bar
                  baz: 1
                - foo: bar
                  baz: 2
            baz:
                - foo: baz
                  baz: 3

        NOTE: If you are filtering a lot of documents, this will consume a
        lot of memory.

  CONDITIONALS
    if EXPRESSION then TRUE_FILTER else FALSE_FILTER
        If the "EXPRESSION" is true, return the result of "TRUE_FILTER",
        otherwise return the result of "FALSE_FILTER".

            # INPUT
            foo: bar
            baz: fuzz

            $ yq 'if .foo eq bar then .baz else .foo'
            fuzz

            $ yq 'if .foo eq buzz then .baz else .foo'
            bar

            $ yq 'if .foo then .baz'
            fuzz

            $ yq 'if .herp then .derp else .'
            foo: bar
            baz: fuzz

        The "else FALSE_FILTER" is optional and defaults to returning
        undefined.

  COMBINATORS
    Combinators combine multiple expressions to yield one or more documents
    in the output stream.

    ,   Multiple EXPRESSIONS may be separated by commas to yield multiple
        documents in the output.

            # INPUT
            foo: bar
            baz: fuzz

            $ yq '.foo, .baz'
            bar
            ---
            fuzz

    |   Multiple EXPRESSIONS may be separated by pipes to give the output of
        the left expression as the input of the right expression (much like
        how shell pipes work).

            # INPUT
            foo: bar
            baz: fuzz
            pop: more
            ---
            foo: buzz
            baz: fizz
            pop: jump

            $ yq '{ foo: .foo, val: .pop } | group_by( .foo )'
            bar:
                - foo: bar
                  val: more
            buzz:
                - foo: buzz
                  val: jump

        The above example can be useful to avoid "group_by" memory issues
        when dealing with very large streams: Reduce the size of the working
        document by keeping only the keys you want, then group those
        documents.

ENVIRONMENT
    YQ_VERBOSE
        Set the verbosity level. Useful when running the tests.

SEE ALSO
    jq  <http://stedolan.github.io/jq/> A filter for JSON documents. The
        inspiration for this project.

AUTHOR
    Doug Bell <preaction@cpan.org>

COPYRIGHT AND LICENSE
    This software is copyright (c) 2014 by Doug Bell.

    This is free software; you can redistribute it and/or modify it under
    the same terms as the Perl 5 programming language system itself.