Iterating through JSON array in Shell script

问题

I have a JSON data as follows in data.json file

[
  {"original_name":"pdf_convert","changed_name":"pdf_convert_1"},
  {"original_name":"video_encode","changed_name":"video_encode_1"},
  {"original_name":"video_transcode","changed_name":"video_transcode_1"}
]

I want to iterate through the array and extract the value for each element in a loop. I saw jq. I find it difficult to use it to iterate. How can I do that?

回答1:

Just use a filter that would return each item in the array. Then loop over the results, just make sure you use the compact output option (-c) so each result is put on a single line and is treated as one item in the loop.

jq -c '.[]' input.json | while read i; do
    # do stuff with $i
done

回答2:

jq has a shell formatting option: @sh.

You can use the following to format your json data as shell parameters:

cat data.json | jq '. | map([.original_name, .changed_name])' | jq @sh

The output will look like:

"'pdf_convert' 'pdf_convert_1'"
"'video_encode' 'video_encode_1'",
"'video_transcode' 'video_transcode_1'"

To process each row, we need to do a couple of things:

Set the bash for-loop to read the entire row, rather than stopping at the first space (default behavior).
Strip the enclosing double-quotes off of each row, so each value can be passed as a parameter to the function which processes each row.

To read the entire row on each iteration of the bash for-loop, set the IFS variable, as described in this answer.

To strip off the double-quotes, we'll run it through the bash shell interpreter using xargs:

stripped=$(echo $original | xargs echo)

Putting it all together, we have:

#!/bin/bash

function processRow() {
  original_name=$1
  changed_name=$2

  # TODO
}

IFS=$'\n' # Each iteration of the for loop should read until we find an end-of-line
for row in $(cat data.json | jq '. | map([.original_name, .changed_name])' | jq @sh)
do
  # Run the row through the shell interpreter to remove enclosing double-quotes
  stripped=$(echo $row | xargs echo)

  # Call our function to process the row
  # eval must be used to interpret the spaces in $stripped as separating arguments
  eval processRow $stripped
done
unset IFS # Return IFS to its original value

回答3:

Try Build it around this example. (Source: Original Site)

Example:

jq '[foreach .[] as $item ([[],[]]; if $item == null then [[],.[0]]     else [(.[0] + [$item]),[]] end; if $item == null then .[1] else empty end)]'

Input [1,2,3,4,null,"a","b",null]

Output [[1,2,3,4],["a","b"]]

回答4:

An earlier answer in this thread suggested using jq's foreach, but that may be much more complicated than needed, especially given the stated task. Specifically, foreach (and reduce) are intended for certain cases where you need to accumulate results.

In many cases (including some cases where eventually a reduction step is necessary), it's better to use .[] or map(_). The latter is just another way of writing [.[] | _] so if you are going to use jq, it's really useful to understand that .[] simply creates a stream of values. For example, [1,2,3] | .[] produces a stream of the three values.

To take a simple map-reduce example, suppose you want to find the maximum length of an array of strings. One solution would be [ .[] | length] | max.

来源：https://stackoverflow.com/questions/33950596/iterating-through-json-array-in-shell-script

标签

json

bash