Jeff Goldblum / jg is a command-line JSON processor which searches for structural patterns in json input and prints each json object that matches the pattern.

Read all about the reasoning behind the jg's creation here: https://gidi.io/2019/03/01/structured-greping-of-structured-logging-using-grep-for-json.html

SYNOPSIS

jg [−^cfimnqv] [−e pattern] [−f file] [−colour when] [pattern]

DESCRIPTION

The jg utility searches any given input files, selecting lines that correctly parse as valid JSON and match one or more selector patterns.

The following options are available:

−c, −−count

Only a count of selected lines is written to standard output.

−−color=_when_, −−colour=_when_

Surround the matched JSON input with escape sequences to display them in colour on the terminal. when is never, always, always-cycle, auto, or auto-cycle. The always and auto options use a single default colour to highlight matches, the difference being t hat auto only highlights content when output is to the terminal. The always-cycle and auto-cycle options will cycle through a list of colours using a different colour for each match.

−e pattern

Specify a selector pattern used during the search of the JSON input: an input line is selected if it parses as valid JSON and matches any of the specified selector patterns. This option is most useful when multiple −e options are used to specify multiple patterns.

−−help

Print a brief help message.

−i, −−ignore-case

Perform case insensitive matching. By default, jg is case sensitive.

−m num, −−max-count=_num_

Stop reading the file after num matches.

−n, −−line-number

Each output line is preceded by its relative line number in the file, starting at line 1.

−q, −−quiet, −−silent

Quiet mode: suppress normal output. jg will only search a file until a match has been found, making searches potentially less expensive. This is useful if you're trying to ensure a certain match is present in the file and can rely on the Exit Code to get the result. See Exit Codes section

−v, −−invert-match

Selected lines are those not matching any of the specified selector patterns.

−^, −−match-root

Only match objects if the selector matches the root of the JSON shape. This means that, for example, if your selector is '.name' then the JSON shape must have a name property on JSON object itself, and a not an inner object.

SELECTOR PATTERNS

Selector Patterns are a way of describing a JSON structure which jg can then use to try and match to the structure of JSON input from within a specified file or piped input. If jg finds that the input JSON contains the structure specified by the selector pattern then the input is considered matched and will be printed out.

For example, suppose you consume the Nasa API which returns a JSON dataset describing How Many People Are In Space Right Now...

$ curl 'http://api.open-notify.org/astros.json'

...you might get a response resembling this:

{
	"people": [{
		"name": "Oleg Kononenko",
		"craft": "ISS"
	}, {
		"name": "David Saint-Jacques",
		"craft": "ISS"
	}, {
		"name": "Anne McClain",
		"craft": "ISS"
	}],
	"number": 3
}

If you wish to use the cli to verify that there are people in space, you can use jg to ensure there is a people property on the object. For that we can use the property matcher which matches any JSON object who has the specified property on it.

$ curl 'http://api.open-notify.org/astros.json' | jg '.people'

If you do so you'll see that the cli prints out the JSON object, because it matches.

Suppose though you learn that Nasa always return the people property, but it will be an empty array. Luckily, patterns can be chained to describe complex structures deep within the provided input. Instead we'll ensure the people array contains an object.

$ curl 'http://api.open-notify.org/astros.json' | jg '.people[.]'

The JSON output is still getting matched, so this is pretty cool, but then you realise there's another issue which is that the identity matcher, which matches any JSON value, can also match Null.

To avoid mistaking Null for a real astronaut, lets ensure that the response contains a proper JSON object with a name:

$ curl 'http://api.open-notify.org/astros.json' | jg '.people[.name]'

There, that should do it.

If curl-ing the API doesn't return a match, we'll know that no people are in space. Lets hope that never happens.

Pattern Syntax

Identity: .

This is the most straight forward matcher, as it matches anything and everything. This is useful for printing out all valid JSON object in the input or matching against a non-empty sequence in a JSON array, such as .people[.].

Property: .prop_name

This matcher matches against any JSON object which has a property named as the specified property in the pattern. So, for example, in the above pattern, any object which has a property with the name "prop_name" will be matched against.

Property & Value: {"prop_name":"prop_value"}

This matcher is the same as the property pattern, except it also allows us to specify the expected value. So, for example, in the above pattern, any object which has a property with the name "prop_name" whose value is the string prop_value will be matched against.

The value of a property can be any valid JSON primitive, which means it can be a String, Number, Boolean or Null.

String Value Matchers:

There are several ways to match against the value of a string and these are loosely based on the CSS Attribute Selector syntax.

Property Exact Value: {"prop_name":"prop_value"}

Matches JSON objects whose specified property has the exact specified value. For example: {"prop_name":"prop_value"}

Property Contains Exact Value: {"prop_name"~:"prop_value"}

Matches JSON objects whose specified property contains the specified value as a word. A word is a single part pf a whitespace-separated list of words. For example: {"prop_name":"This value will surely contain prop_value in it."}

Property Prefixed Value: {"prop_name"^:"prop_value"}

Matches JSON objects whose specified property begins with the specified value. For example: {"prop_name":"prop_value is what I'm all about."}

Property Suffixed Value: {"prop_name"$:"prop_value"}

Matches JSON objects whose specified property ends with the specified value. For example: {"prop_name":"Know what I'm all about? It's prop_value"}

Property Contains Value: {"prop_name"*:"prop_value"}

Matches JSON objects whose specified property contains the specified value. For example: {"prop_name":"Wildcard search for a the 'prop_value' is awesome"}

Array Index: [2]

This matcher matches against an array by verifying that it contains a value at the specified index.

For example: ["some different value","member_value","some other value"]

Array & Value: [="member_value"]

This matcher matches against an array by verifying that it contains the exact string "member_value" inside it.

Just like the property matcher, the value of an array member can be any valid JSON primitive, which means it can be a String, Number, Boolean or Null.

Array Value Matchers:

There are several ways to match against the value in an array and these are similar to the property selector's string value matchers but slightly different due to differences between String and Array.

Array Exact Value: [="member_value"]

Matches JSON array whose contains the exact specified value and that is the only single value in the array. This can be used with all primitives. For example: ["member_value"]

Array Contains Exact Value: [~="member_value"]

Matches JSON array whose contains the exact specified value. This can be used with all primitives. For example: ["some different value","member_value","some other value"]

Array Prefixed Value: [^="member_value"]

Matches JSON array which contains a value which begins with the specified value. This can only be used with strings. For example: ["some different value","member_value is cool","some other value"]

Array Suffixed Value: [$="member_value"]

Matches JSON array which contains a value which ends with the specified value. This can only be used with strings. For example: ["some different value","Know what's cool? member_value","some other value"]

Array Contains Value: [*="_value"]

Matches JSON array whose contains the specified value as a substring of a value in the array. For example: ["Know what's cool? wildcard search of a member_value value","some other value"]

EXIT CODES

In line with grep, the jg exit codes returns the exit status 0 if a selector match is found in the file and 1 if no selector is matched.

INSTALLATION

While we iron out the kinks we're avoiding publishing jg into public package managers. That said, under the releases tab you will find both an OSX and Linux release.

If you're on OSX and use Homebrew you can use the following script to install the latest OSX release:

brew install https://raw.githubusercontent.com/gmmorris/jg/master/packaging/homebrew.rb

If you're on linux we've created several binaries, including RPMs for supported distribution, which are available on the releases page.

If you wish to install using an RPM you can use the following command: Note that the bellow command is just an example, you're better getting the latest URL from the releases page

yum install -y https://github.com/gmmorris/jg/releases/download/0.1.4/jg-0.1.4-x86_64-unknown-linux-gnu.rpm

EXAMPLES

To find all JSON input with an object with property name on it:

$ jg '.name' -f myfile

To find all JSON input with an object with property fellowship, whose value is an array and in that array there's a JSON object with the property name on it whose value is Aragorn:

$ jg '.fellowship[{"name":"Aragorn"}]' -f myfile

To find all JSON input with whose root object has the property name on it:

$ jg -^ '.name' -f myfile

To find all JSON input which does not have the property name anywhere in its structure:

$ grep -v '.name' -f myfile