fq/doc/usage.md

Basic usage

fq tries to behave the same way as jq as much as possible, so you can do:

fq . file
fq < file
cat file | fq
fq . < file
fq . *.png *.mp3
fq '.frames[0]' *.mp3

Common usages

# recursively display decode tree but truncate long arrays
fq d file
# same as
fq display file

# display all bytes for each value
fq 'd({display_bytes: 0})' file
# display 200 bytes for each value
fq 'd({display_bytes: 200})' file

# recursively display decode tree without truncating
fq da file

# recursively and verbosely display decode tree
fq dv file

# JSON repersenation for whole file
fq tovalue file

# recursively look for decode value roots for a format
fq '.. | select(format=="jpeg")' file
# can also use grep_by
fq 'grep_by(format=="jpeg")' file

# recursively look for first decode value root for a format
fq 'first(.. | select(format=="jpeg"))' file
fq 'first(grep_by(format=="jpeg"))' file

# recursively look for objects fullfilling condition
fq '.. | select(.type=="trak")?' file
fq 'grep_by(.type=="trak")' file

# grep whole tree
fq 'grep("^prefix")' file
fq 'grep(123)' file
fq 'grep_by(. >= 100 and . =< 100)' file

# decode file as mp4 and return a result even if there are some errors
fq -d mp4 file.mp4
# decode file as mp4 and also ignore validity assertions
fq -o force=true -d mp4 file.mp4

Display output

display or d is the main function for displying values and is also the function that will be used if no other output function is explicitly used. If its input is a decode value it will output a dump and tree structure or otherwise it will output as JSON.

Below demonstrates some usages:

First and second example does the same thing, inputs "hello" to display.

In the next few examples we select out the first "edit list" box in an mp4 file, it's a list of which part of media track to be included during playback, and displays it in various ways.

Default if not explicitly used display will only show the root level:

First row shows ruler with byte offset into the line and jq path for the value.

The columns are:

• Start address for the line. For example we see that type starts at 0xd60+0x09.
• Hex repersenation of input bits for value. Will show the whole byte even if the value only partially uses bits from it.
• ASCII representation of input bits for value. Will show the whole byte even if the value only partially uses bits from it.
• Tree structure of decoded value, symbolic value and description.

Notation:

• {} value is an object that might have nested values.
• [start:end] value is an array with index starting at start and ending at end (exclusive).

With display or d it will recursively show the whole tree:

Same but verbose dv:

In verbose mode bit ranges and array element names as shown.

Bit range uses bytes.bits notation. For example type start at byte 0xd69 bit 0 (left out if zero) and ends at 0xd6c bit 7 (inclusive) and have byte size of 4.

There are also some other display aliases:

• da same as display({array_truncate: 0}) which will not truncate long arrays.
• dd same as display({array_truncate: 0, display_bytes: 0}) which will not truncate long ranges.
• dv same as display({array_truncate: 0, verbose: true})
• ddv same as display({array_truncate: 0, display_bytes: 0 verbose: true}) which will not truncate long and also display verbosely.

Interactive REPL

The interactive REPL has auto completion and nested REPL support:

# start REPL with null input
$ fq -i
null>
# same as
$ fq -ni
null>

# in the REPL you will see a prompt indicating current input and you can type jq expression to evaluate.

# start REPL with one file as input
$ fq -i . doc/file.mp3
mp3>

$ fq -i . doc/file.mp3
# basic arithmetics and jq expressions
mp3> 1+1
2
mp3> 1, 2, 3 | . * 2
2
4
6
mp3> [1, 2, 3] | add
6
# "." is the identity function which just returns current input, the mp3 file.
mp3> .
# access the first frame in the mp3 file
mp3> .frames[0]
# start a new nested REPL with first frame as input
mp3> .frames[0] | repl
# prompt shows "path" to current input and that it's an mp3_frame.
# Ctrl-D to exit REPL or to shell if last REPL
> .frames[0] mp3_frame> ^D
# "jq" value of layer in first frame
mp3> .frames[0].header.layer | tovalue
3
mp3> .frames[0].header.layer * 2
6
# symbolic value, same as "jq" value
mp3> .frames[0].header.layer | tosym
3
# actual underlaying decoded value
mp3> .frames[0].header.layer | toactual
1
# description of value
mp3> .frames[0].header.layer | todescription
"MPEG Layer 3"
mp3> ^D
$

Use Ctrl-D to exit and Ctrl-C to interrupt current evaluation.

Example usages

Second mp3 frame header as JSON

fq '.frames[1].header | tovalue' file.mp3

Byte start position for the first 10 mp3 frames in an array

fq '.frames[0:10] | map(tobytesrange.start)' file.mp3

Decode at range

# decode byte range 100 to end
fq -d raw 'tobytes[100:] | mp3_frame | d' file.mp3
# decode byte range 10 bytes into .somefield and preseve relative position in file
fq '.somefield | tobytesrange[10:] | mp3_frame | d' file.mp3

Show AVC SPS difference between two mp4 files

-n tells fq to not have an implicit input, f is function to select out some interesting value, call diff with two arguments, decoded value for a.mp4 and b.mp4 filtered thru f.

fq -n 'def f: .. | select(format=="avc_sps"); diff(input|f; input|f)' a.mp4 b.mp4

Extract first JPEG found in file

Recursively look for first value that is a jpeg decode value root. Use tobytes to get bytes for value. Redirect bytes to a file.

fq 'first(.. | select(format=="jpeg")) | tobytes' file > file.jpeg

Sample size histogram

Recursively look for a all sample size boxes "stsz" and use ? to ignore errors when doing .type on arrays etc. Save reference to box, count unique values, save the max, output the path to the box and output a historgram scaled to 0-100.

fq '.. | select(.type=="stsz")? as $stsz | .entries | count | max_by(.[1])[1] as $m | ($stsz | topath | path_to_expr), (.[] | "\(.[0]): \((100*.[1]/$m)*"=") \(.[1])") | println' file.mp4

Find TCP streams that looks like HTTP GET requests in a PCAP file

Use grep to recursively find strings matching a regexp.

fq '.tcp_connections | grep("GET /.* HTTP/1.?")' file.pcap

Use representation of a format

Some formats like msgpack, bson etc are used to represent some data structure. In those cases the torepr function can be used to get the representation.

# whole represented value
fq -d msgpack torepr file.msgpack
# value of the key "field" from the represented value
fq -d msgpack `torepr.field` file.msgpack
# query or transform represented value
fq -d msgpack 'torepr | ...' file.msgpack

Widest PNG in a directory

$ fq -rn '[inputs | [input_filename, first(.chunks[] | select(.type=="IHDR") | .width)]] | max_by(.[1]) | .[0]' *.png

What values include the byte at position 0x123

$ fq '.. | select(scalars and in_bytes_range(0x123))' file

Support formats

See formats

The jq language

fq is based on the jq language and for basic usage its syntax is similar to how object and array access looks in JavaScript or JSON path, .food[10] etc. but it can do much more and is a very expressive language.

To get the most out of fq it's recommended to learn more about jq, here are some good starting points:

• jq manual
• Peter Koppstein's A Stream oriented Introduction to jq
• jq wiki: Language Description
• jq wiki: page Cookbook
• jq wiki: Pitfalls
• FAQ

Common beginner gotcha are:

• jq's use of ; and ,. jq uses ; as argument separator and , as output separator. To call a function f with two arguments use f(1; 2). If you do f(1, 2) you pass a single argument 1, 2 (a lambda expression that output 1 and then output 2) to f.
• Expressions can return or "output" zero or more values. This is how loops, foreach etc is achieved.
• Expressions have one implicit input and output value. This how pipelines like 1 | . * 2 work.

Types specific to fq

fq has two additional types compared to jq, decode value and binary. In standard jq expressions they will in most case behave as some standard jq type.

Decode value

This type is returned by decoders and it used to represent parts of the decoed input. It can act as all standard jq types, object, array, number, string etc.

Each decode value has these properties:

• A bit range in the input
  • Can be accessed as a binary using tobits/tobytes. Use the start and size keys to access position and size.
  • .name as bytes .name | tobytes
  • Bit 4-8 of .name as bits .name | tobits[4:8]

Each non-compound decode value has these properties:

• An actual value:
  • This is the decoded representation of the bits, a number, string, bool etc.
  • Can be accessed using toactual.
• An optional symbolic value:
  • Is usually a mapping of the actual to symbolic value, ex: map number to a string value.
  • Can be accessed using tosym.
• An optional description:
  • Can be accessed using todescription
• parent is the parent decode value
• parents is the all parent decode values
• topath is the jq path for the decode value
• torepr convert decode value to its representation if possible

The value of a decode value is the symbolic value if available and otherwise the actual value. To explicitly access the value use tovalue. In most expression this is not needed as it will be done automactically.

Binary

Binaries are raw bits with a unit size, 1 (bits) or 8 (bytes), that can have a non-byte aligned size. Will act as byte padded strings in standard jq expressions.

Use tobits and tobytes to create them from a decode values, strings, numbers or binary arrays. tobytes will if needed zero pad most significant bits to be byte aligned.

There is also tobitsrange and tobytesrange which does the same thing but will preserve it's source range when displayed.

• "string" | tobytes produces a binary with UTF8 codepoint bytes.
• 1234 | tobits produces a binary with the unsigned big-endian integer 1234 with enough bits to represent the number. Use tobytes to get the same but with enough bytes to represent the number. This is different to how numbers works inside binary arrays where they are limited to 0-255.
• ["abc", 123, ...] | tobytes produce a binary from a binary array. See binary array below.
• .[index] access bit or byte at index index. Index is in units.
  • [0x12, 0x34, 0x56] | tobytes[1] is 0x35
  • [0x12, 0x34, 0x56] | tobits[3] is 1
• .[start:], .[start:end] or .[:end] is normal jq slice syntax and will slice the binary from start to end. start and end is in units.
  • [0x12, 0x34, 0x56] | tobytes[1:2] will be a binary with the byte 0x34
  • [0x12, 0x34, 0x56] | tobits[4:12] will be a binary with the byte 0x23
  • [0x12, 0x34, 0x56] | tobits[4:20] will be a binary with the byte 0x23, 0x45
  • [0x12, 0x34, 0x56] | tobits[4:20] | tobytes[1:] will be a binary with the byte 0x45,

Both .[index] and .[start:end] support negative indices to index from end.

TODO: tobytesrange, padding

Binary array

Is an array of numbers, strings, binaries or other nested binary arrays. When used as input to tobits/tobytes the following rules are used:

• Number is a byte with value be 0-255
• String it's UTF8 codepoint bytes
• Binary as is
• Binary array used recursively

Binary arrays are similar to and inspired by Erlang iolist.

Some examples:

[0, 123, 255] | tobytes will be binary with 3 bytes 0, 123 and 255

[0, [123, 255]] | tobytes same as above

[0, 1, 1, 0, 0, 1, 1, 0 | tobits] will be binary with 1 byte, 0x66 an "f"

[(.a | tobytes[-10:]), 255, (.b | tobits[:10])] | tobytes the concatenation of the last 10 bytes of .a, a byte with value 255 and the first 10 bits of .b.

The difference between tobits and tobytes is

TODO: padding and alignment

Functions

• All standard library functions from jq
• Adds a few new general functions:
  • print, println, printerr, printerrln prints to stdout and stderr.
  • streaks, streaks_by(f) like group but groups streaks based on condition.
  • count, count_by(f) like group but counts groups lengths.
  • debug(f) like debug but uses arg to produce debug message. {a: 123} | debug({a}) | ....
  • path_to_expr from ["key", 1] to ".key[1]".
  • expr_to_path from ".key[1]" to ["key", 1].
  • diff($a; $b) produce diff object between two values.
  • delta, delta_by(f), array with difference between all consecutive pairs.
  • chunk(f), split array or string into even chunks
• Bitwise functions band, bor, bxor, bsl, bsr and bnot. Works the same as jq math functions, unary uses input and if more than one argument all as arguments ignoring the input. Ex: 1 | bnot bsl(1; 3)
• Adds some decode value specific functions:
  • root tree root for value
  • buffer_root root value of buffer for value
  • format_root root value of format for value
  • parent parent value
  • parents output parents of value
  • topath path of value. Use path_to_expr to get a string representation.
  • tovalue, tovalue($opts) symbolic value if available otherwise actual value
  • toactual actual value (decoded etc)
  • tosym symbolic value (mapped etc)
  • todescription description of value
  • torepr convert decode value into what it reptresents. For example convert msgpack decode value into a value representing its JSON representation.
  • All regexp functions work with binary as input and pattern argument with these differences compared to when using string input:
    • All offset and length will be in bytes.
    • For capture the .string value is a binary.
    • If pattern is a binary it will be matched literally and not as a regexp.
    • If pattern is a binary or flags include "b" each input byte will be read as separate code points
  • String function are not overloaded to support binary for now as some of them are bahaviours that might be confusing.
  • explode is overloaded to work with binary. Will explode into array of the unit of the binary. end of binary. instead of possibly multi-byte UTF-8 codepoints. This allows to match raw bytes. Ex: match("\u00ff"; "b") will match the byte 0xff and not the UTF-8 encoded codepoint for 255, match("[^\u00ff]"; "b") will match all non-0xff bytes.
  • grep functions take 1 or 2 arguments. First is a scalar to match, where a string is treated as a regexp. A binary will be matches exact bytes. Second argument are regexp flags with addition that "b" will treat each byte in the input binary as a code point, this makes it possible to match exact bytes.
    • grep($v), grep($v; $flags) recursively match value and binary
    • vgrep($v), vgrep($v; $flags) recursively match value
    • bgrep($v), bgrep($v; $flags) recursively match binary
    • fgrep($v), fgrep($v; $flags) recursively match field name
  • grep_by(f) recursively match using a filter. Ex: grep_by(. > 180 and . < 200), first(grep_by(format == "id3v2")).
  • Binary:
    • tobits - Transform input to binary with bit as unit, does not preserving source range, will start at zero.
    • tobitsrange - Transform input to binary with bit as unit, preserves source range if possible.
    • tobytes - Transform input to binary with byte as unit, does not preserving source range, will start at zero.
    • tobytesrange - Transform input binary with byte as unit, preserves source range if possible.
    • .[start:end], .[:end], .[start:] - Slice binary from start to end preserving source range.
• open open file for reading
• All decode function takes a optional option argument. The only option currently is force to ignore decoder asserts. For example to decode as mp3 and ignore assets do mp3({force: true}) or decode("mp3"; {force: true}), from command line you currently have to do fq -d raw 'mp3({force: true})' file.
• decode, decode($format), decode($format; $opts) decode format
• probe, probe($opts) probe and decode format
• mp3, mp3($opts), ..., <name>, <name>($opts) same as decode(<name>)($opts), decode($format; $opts) decode as format
• Display shows hexdump/ASCII/tree for decode values and jq value for other types.
  • d/d($opts) display value and truncate long arrays and binaries
  • da/da($opts) display value and don't truncate arrays
  • dd/dd($opts) display value and don't truncate arrays or binaries
  • dv/dv($opts) verbosely display value and don't truncate arrays but truncate binaries
  • ddv/ddv($opts) verbosely display value and don't truncate arrays or binaries
• p/preview show preview of field tree
• hd/hexdump hexdump value
• repl/repl($opts) nested REPL, must be last in a pipeline. 1 | repl, can "slurp" outputs. Ex: 1, 2, 3 | repl, [1,2,3] | repl({compact: true}).
• slurp($name) slurp outputs and save them to $name, must be last in pipeline. Will be available as global array $name. Ex 1,2,3 | slurp("a"), $a[] same as spew("a").
• spew/spew($name) output previously slurped values. spew outputs all slurps as an object, spew($name) outouts one slurp. Ex: spew("a").
• paste read string from stdin until ^D. Useful for pasting text.
  • Ex: paste | frompem | asn1_ber | repl read from stdin then decode and start a new sub-REPL with result.

Encodings, serializations and hashes

In an addition to binary formats fq also support reading to and from encodings and serialization formats.

At the moment fq does not have any dedicated argument for serialization formats but raw string input -R slurp -s and raw string output -r can make things easier. The combination -Rs will read all inputs into one string (same as jq).

Note that from* functions output jq values and to* takes jq values as input so in some cases not all information will properly preserved, for example for XML element and attribute order might change and text and comment nodes might move and will be merged.

Some example usages:

# read yml and do some query
$ fq -Rs 'fromyaml | ...' file.yml

# convert YAML to JSON
# note -r for raw string output, without a JSON string with JSON would outputted
$ fq -Rsr 'fromyaml | tojson({indent:2})' file.yml

# add token to URL
$ echo -n "https://host.org" | fq -Rsr 'fromurl | .user.username="token" | tourl'
https://token@host.org

$ curl -s https://www.discogs.com/ | fq -Rsr 'fromhtml | .. | select(."-id" == "hot-releases")? | .div[].a."-aria-label"'
Arcade Fire - We
Bell Biv Devoe - Poison
Jazz Sabbath - Vol. 2
Jonathan Richman And The Modern Lovers* - Modern Lovers 88
Kylie* - Infinite Disco
Vince Guaraldi Trio - Baseball Theme

# shows how serialization functions can be used on any string, how to transform values and output som other format
# read decode zip file and start an interactive REPL
$ fq  -i . <(curl -sL https://github.com/stefangabos/world_countries/archive/master.zip)
# select from interesting xml file
zip> .local_files[] | select(.file_name == "world_countries-master/data/countries/en/world.xml").uncompressed | repl
# convert xml into jq values
> .local_files[95].uncompressed string> fromxml | repl
# sort countries by and select the first one
>> object> .countries.country | sort_by(."-name") | first |
# see what current input is
>>> object> .
{
  "-alpha2": "af",
  "-alpha3": "afg",
  "-id": "4",
  "-name": "Afghanistan"
}
# remove "-" prefix from keys and convert to YAML and print it
>>> object> with_entries(.key |= .[1:]) | toyaml | print
alpha2: af
alpha3: afg
id: "4"
name: Afghanistan
# exit all REPLs back to shell
>>> object> ^D
>> object> ^D
> .local_files[95].uncompressed string> ^D
zip> ^D

• fromxml/fromxml($opts) Parse XML into jq values.
  {seq: true} preserve element ordering if more than one sibling.
  {array: true} use nested arrays to represent elements.
  

• fromhtml/fromhtml($opts) Parse HTML into jq values.
  Same as fromxml but less strict and follows html5 parsing rules. Will always have a html root with head and body elements.
  {array: true} use nested arrays to represent elements.
  {seq: true} preserve element ordering if more than one sibling.
  

• toxml/toxml($opts}) Serialize jq value into XML.
  {indent: number} indent child elements.
  Assumes object representation if input is an object, and nested arrays if input is an array.
  Will automatically add a root doc element if jq value has more then one root element.
  If a #seq is found on at least one element all siblings will be sort by sequence number. Attributes are always sorted.
  

  XML elements can be represented as jq values in two ways, as objects (inspired by mxj and xml.com's Converting Between XML and JSON ) or nested arrays. Both representations are lossy and might lose ordering of elements, text nodes and comments. In object representation fromxml, fromhtml and toxml support {seq:true} option to parse/serialize {"#seq"=<number>} attributes to preserve element sibling ordering.

  The object version is denser and convenient to query, the nested arrays version is probably easier to use when generating XML.

  Let's assume $xml is this XML document as a string:

  <doc>
    <child attr="1"></child>
    <child attr="2">text</child>
    <other>text</other>
  </doc>
  

  With object representation an element is represented as:

  • Attributes as dash prefixed -<key> keys.
  • Text nodes as #text.
  • Comment nodes as #comment keys.
  • For explicit sibling ordering #seq keys with a number, can be negative, assumed zero if missing.
  • Child element with only text as <name> key with text as value.
  • Child element with more than just text as <name> key with value an object.
  • Multiple child element sibling with same name as name key with value as array with strings and objects.

  > $xml | fromxml
  {
    "doc": {
      "child": [
        {
          "-attr": "1"
        },
        {
          "#text": "text",
          "-attr": "2"
        }
      ],
      "other": "text"
    }
  }
  

  With nested array representation, an array with these values ["<name>", {attributes...}, [children...]]

  • Index zero is element name.
  • Optional first object attributes (including #text and #comment keys).
  • Optional first array are child elements.

  > $xml | fromxml({array: true})
  [
    "doc",
    [
      [
        "child",
        {
          "attr": "1"
        }
      ],
      [
        "child",
        {
          "#text": "text",
          "attr": "2"
        }
      ],
      [
        "other",
        {
          "#text": "text"
        }
      ]
    ]
  ]
  

  Parse and include #seq attributes if needed:

  > $xml | fromxml({seq:true})
  {
    "doc": {
      "child": [
        {
          "#seq": 0,
          "-attr": "1"
        },
        {
          "#seq": 1,
          "#text": "text",
          "-attr": "2"
        }
      ],
      "other": {
        "#seq": 2,
        "#text": "text"
      }
    }
  }
  

  Select values in <doc>, remove <child>, add a <new> element, serialize to xml with 2 space indent and print the string

  > $xml | fromxml.doc | del(.child) | .new = "abc" | {root: .} | toxml({indent: 2}) | println
  <root>
    <new>abc</new>
    <other>text</other>
  </root>
  

• fromjson Parse JSON into jq values.

• tojson/tojson($opt) Serialize jq value into JSON.
  {indent: number} indent array/object values.
  

• fromjq Parse jq-flavoured JSON into jq values.

• tojq/tojq($opt) Serialize jq value into jq-flavoured JSON
  {indent: number} indent array/object values.
  jq-flavoured JSON has optional key quotes, # comments and can have trailing comma in arrays.

• fromyaml Parse YAML into jq values.

• toyaml Serialize jq value into YAML.

• fromtoml Parse TOML into jq values.

• totoml Serialize jq value into TOML.

• fromcsv/fromcvs($opts) Parse CSV into jq values.
  {comma: string} field separator, default ",".
  {comment: string} comment line character, default "#".
  

• tocsv/tocsv($opts) Serialize jq value into CSV.
  {comma: string} field separator, default ",".
  

• fromxmlentities Decode XML entities.

• toxmlentities Encode XML entities.

• fromurlpath Decode URL path component.

• tourlpath Encode URL path component.

• fromurlencode Decode URL query encoding.

• tourlencode Encode URL to query encoding.

• fromurlquery Decode URL query into object. For duplicates keys value will be an array.

• tourlquery Encode objet into query string.

• fromurl Decode URL into object.

  > "schema://user:pass@host/path?key=value#fragement" | fromurl
  {
    "fragment": "fragement",
    "host": "host",
    "path": "/path",
    "query": {
      "key": "value"
    },
    "rawquery": "key=value",
    "scheme": "schema",
    "user": {
      "password": "pass",
      "username": "user"
    }
  }
  

• tourl Encode object into URL string.

• fromhex Decode hexstring to binary.

• tohex Encode binay into hexstring.

• frombase64/frombase64($opts) Decode base64 encodings into binary.
  {encoding:string} encoding variant: std (default), url, rawstd or rawurl

• tobase64/tobase64($opts) Encode binary into base64 encodings.
  {encoding:string} encoding variant: std (default), url, rawstd or rawurl

• tomd4 Hash binary using md4.

• tomd5 Hash binary using md5.

• tosha1 Hash binary using sha1.

• tosha256 Hash binary using sha256.

• tosha512 Hash binary using sha512.

• tosha3_224 Hash binary using sha3 224.

• tosha3_256 Hash binary using sha3 256.

• tosha3_384 Hash binary using sha3 384.

• tosha3_512 Hash binary using sha3 512.

• toiso8859_1 Decode binary as ISO8859-1 into string.

• fromiso8859_1 Encode string as ISO8859-1 into binary.

• toutf8 Encode string as UTF8 into binary.

• fromutf8 Decode binary as UTF8 into string.

• toutf16 Encode string as UTF16 into binary.

• fromutf16 Decode binary as UTF16 into string.

• toutf16le Encode string as UTF16 little-endian into binary.

• fromutf16le Decode binary as UTF16 little-endian into string.

• toutf16be Encode string as UTF16 big-endian into binary.

• fromutf16be Decode binary as UTF16 big-endian into string.

Color and unicode output

fq by default tries to use colors if possible, this can be disabled with -M. You can also enable useage of unicode characters for improved output by setting the environment variable CLIUNICODE.

Configuration

To add own functions you can use init.fq that will be read from

• $HOME/Library/Application Support/fq/init.jq on macOS
• $HOME/.config/fq/init.jq on Linux, BSD etc
• %AppData%\fq\init.jq on Windows

Use as script interpreter

fq can be used as a scrip interpreter:

mp3_duration.jq:

#!/usr/bin/env fq -d mp3 -rf
[.frames[].header | .sample_count / .sample_rate] | add

Differences to jq

• gojq's differences to jq, notable is support for arbitrary-precision integers.
• Supports hexdecimal 0xab, octal 0o77 and binary 0b101 integer literals.
• Try include include "file?"; that don't fail if file is missing.
• Some values can act as a object with keys even when it's an array, number etc.
• There can be keys hidden from keys and [].
• Some values are readonly and can't be updated.

Decoded values

When you decode something you will get a decode value. A decode values work like normal jq values but has special abilities and is used to represent a tree structure of the decoded binary data. Each value always has a name, type and a bit range.

A value has these special keys (TODO: remove, are internal)

• _name name of value

• _value jq value of value

• _start bit range start

• _stop bit range stop

• _len bit range length (TODO: rename)

• _bits bits in range as a binary

• _bytes bits in range as binary using byte units

• _path jq path to value

• _unknown value is un-decoded gap

• _symbol symbolic string representation of value (optional)

• _description longer description of value (optional)

• _format name of decoded format (optional)

• _error error message (optional)

• TODO: unknown gaps

Own decoders and use as library

TODO

Known issues and useful tricks

Run interactive mode with no input

fq -i
null>

select fails with expected an ... but got: ...

Try add select(...)? to catch and ignore type errors in the select expression.

Manual decode

Sometimes fq fails to decode or you know there is valid data buried inside some binary or maybe you know the format of some unknown value. Then you can decode manually.

# try decode a `mp3_frame` that failed to decode
$ fq -d mp3 '.unknown0 | mp3_frame' file.mp3
# skip first 10 bytes then decode as `mp3_frame`
$ fq -d raw 'tobytes[10:] | mp3_frame' file.mp3

Use . as input and in a positional argument

The expression .a | f(.b) might not work as expected. . is .a when evaluating the arguments so the positional argument will end up being .a.b. Instead do . as $c | .a | f($c.b).

Building array is slow

Try to use map or foreach to avoid rebuilding the whole array for each append.

Use print and println to produce more friendly compact output

> [[0,"a"],[1,"b"]]
[
  [
    0,
    "a"
  ],
  [
    1,
    "b"
  ]
]
> [[0,"a"],[1,"b"]] | .[] | "\(.[0]): \(.[1])" | println
0: a
1: b

repl argument using function or variable causes variable not defined

true as $verbose | repl({verbose: $verbose}) will currently fail as repl is implemented by rewriting the query to map(true as $verbose | .) | repl({verbose: $verbose}).

error produces no output

null | error behaves as empty.