Processing JSON using jq

jq is useful to slice, filter, map and transform structured json data.

Installing jq

On Mac OS

brew install jq

On AWS Linux

Not available as yum install on our current AMI. It should be on the latest AMI though: https://aws.amazon.com/amazon-linux-ami/2015.09-release-notes/

Installing from the source proved to be tricky.

Useful arguments

When running jq, the following arguments may become handy:

Argument	Description
--version	Output the jq version and exit with zero.
--sort-keys	Output the fields of each object with the keys in sorted order.

Basic concepts

The syntax for jq is pretty coherent:

Syntax	Description
,	Filters separated by a comma will produce multiple independent outputs
?	Will ignores error if the type is unexpected
[]	Array construction
{}	Object construction
+	Concatenate or Add
-	Difference of sets or Substract
length	Size of selected element
|	Pipes are used to chain commands in a similar fashion than bash

Dealing with json objects

Description	Command
Display all keys	jq 'keys'
Adds + 1 to all items	jq 'map_values(.+1)'
Delete a key	jq 'del(.foo)'
Convert an object to array	to_entries | map([.key, .value])

Dealing with fields

Description	Command
Concatenate two fields	fieldNew=.field1+' '+.field2

Dealing with json arrays

Slicing and Filtering

Description	Command
All	jq .[]
First	jq '.[0]'
Range	jq '.[2:4]'
First 3	jq '.[:3]'
Last 2	jq '.[-2:]'
Before Last	jq '.[-2]'
Select array of int by value	jq 'map(select(. >= 2))'
Select array of objects by value	** jq '.[] | select(.id == "second")'**
Select by type	** jq '.[] | numbers' ** with type been arrays, objects, iterables, booleans, numbers, normals, finites, strings, nulls, values, scalars

Mapping and Transforming

Description	Command
Add + 1 to all items	jq 'map(.+1)'
Delete 2 items	jq 'del(.[1, 2])'
Concatenate arrays	jq 'add'
Flatten an array	jq 'flatten'
Create a range of numbers	jq '[range(2;4)]'
Display the type of each item	jq 'map(type)'
Sort an array of basic type	jq 'sort'
Sort an array of objects	jq 'sort_by(.foo)'
Group by a key - opposite to flatten	jq 'group_by(.foo)'
Minimun value of an array	jq 'min' .See also min, max, min_by(path_exp), max_by(path_exp)
Remove duplicates	jq 'unique' or jq 'unique_by(.foo)' or jq 'unique_by(length)'
Reverse an array	jq 'reverse'