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

Interactive REPL

fq has an interactive REPL with auto completion and nested REPL support:

# start REPL with null input
fq -i
# same as
fq -ni
# start REPL with one file as input
fq -i . file.mp3

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

# basic arithmetics
mp3> 1+1
2
# "." is the identity function, 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.
# do Ctrl-D to exit REPL
> .frames[0] mp3_frame> ^D
# do Ctrl-D to exit to shell
mp3> ^D
$

Use Ctrl-D to exits, Ctrl-C to interrupt current evaluation.

The jq langauge

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.

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

• jq manual
• jq wiki pages jq Language Description, jq wiki page Cookbook, FAQ and Pitfalls

The most common beginner gotcha is probably 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.

Support formats

See formats

Arguments

TODO: examples, stdin/stdout

$ fq -h 
fq - jq for files
Tool, language and format decoders for exploring binary data.
For more information see https://github.com/wader/fq

Usage: fq [OPTIONS] [--] [EXPR] [FILE...]
--arg NAME VALUE         Set variable $NAME to string VALUE
--argjson NAME JSON      Set variable $NAME to JSON
--color-output,-C        Force color output
--compact-output,-c      Compact output
--decode,-d NAME         Decode format (probe)
--decode-file NAME PATH  Set variable $NAME to decode of file
--formats                Show supported formats
--from-file,-f PATH      Read EXPR from file
--help,-h                Show help
--include-path,-L PATH   Include search path
--join-output,-j         No newline between outputs
--monochrome-output,-M   Force monochrome output
--null-input,-n          Null input (use input/0 and inputs/0 to read input)
--null-output,-0         Null byte between outputs
--option,-o KEY=VALUE    Set option, eg: color=true
--options                Show all options
--raw-input,-R           Read raw input strings (don't decode)
--raw-output,-r          Raw string output (without quotes)
--rawfile NAME PATH      Set variable $NAME to string content of file
--repl,-i                Interactive REPL
--slurp,-s               Read (slurp) all inputs into an array
--version,-v             Show version

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.
• Has bitwise operations, band, bor, bxor, bsl, bsr, bnot.
• 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 []. Used for, _format, _bytes etc.
• Some values are readonly and can't be updated.

Functions

• All standard library functions from jq
• Adds a few new general functions:
  • streaks/0, streaks_by/1 like group but groups streaks based on condition.
  • count, count_by/1 like group but counts groups lengths.
  • debug/1 like debug/0 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/2 produce diff object between two values.
  • delta, delta_by/1, array with difference between all consecutive pairs.
  • chunk/1, split array or string into even chunks
• Adds some decode value specific functions:
  • root/0 return tree root for value
  • buffer_root/0 return root value of buffer for value
  • format_root/0 return root value of format for value
  • parent/0 return parent value
  • parents/0 output parents of value
• open open file for reading
• probe or decode probe format and decode
• mp3, matroska, ..., <name>, decode([name]) force decode as format
• d/display display value and truncate long arrays
• f/full display value and don't truncate arrays
• v/verbose display value verbosely and don't truncate array
• p/preview show preview of field tree
• hd/hexdump hexdump value
• repl nested REPL, must be last in a pipeline. 1 | repl, can "slurp" multiple outputs 1, 2, 3 | repl.

Decoded values (TODO: better name?)

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:

• _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

Binary and IO lists

• TODO: similar to erlang io lists, [], binary, string (utf8) and numbers

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 (TODO: not tested)

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 file.mp3 .unknown0 mp3_frame
# skip first 10 bytes then decode as `mp3_frame`
$ fq file.mp3 .unknown0._bytes[10:] mp3_frame

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.