# Processing JSON using jq

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

## 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'` |