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How to parse JSON with shell scripting in Linux?

Solutons:


The availability of parsers in nearly every programming language is one of the advantages of JSON as a data-interchange format.

Rather than trying to implement a JSON parser, you are likely better off using either a tool built for JSON parsing such as jq or a general purpose script language that has a JSON library.

For example, using jq, you could pull out the ImageID from the first item of the Instances array as follows:

jq '.Instances[0].ImageId' test.json

Alternatively, to get the same information using Ruby’s JSON library:

ruby -rjson -e 'j = JSON.parse(File.read("test.json")); puts j["Instances"][0]["ImageId"]'

I won’t answer all of your revised questions and comments but the following is hopefully enough to get you started.

Suppose that you had a Ruby script that could read a from STDIN and output the second line in your example output[0]. That script might look something like:

#!/usr/bin/env ruby
require 'json'

data = JSON.parse(ARGF.read)
instance_id = data["Instances"][0]["InstanceId"]
name = data["Instances"][0]["Tags"].find {|t| t["Key"] == "Name" }["Value"]
owner = data["Instances"][0]["Tags"].find {|t| t["Key"] == "Owner" }["Value"]
cost_center = data["Instances"][0]["SubnetId"].split("-")[1][0..3]
puts "#{instance_id}t#{name}t#{cost_center}t#{owner}"

How could you use such a script to accomplish your whole goal? Well, suppose you already had the following:

  • a command to list all your instances
  • a command to get the json above for any instance on your list and output it to STDOU

One way would be to use your shell to combine these tools:

echo -e "Instance idtNametcost centretOwner"
for instance in $(list-instances); do
    get-json-for-instance $instance | ./ugly-ruby-scriptrb
done

Now, maybe you have a single command that give you one json blob for all instances with more items in that “Instances” array. Well, if that is the case, you’ll just need to modify the script a bit to iterate through the array rather than simply using the first item.

In the end, the way to solve this problem, is the way to solve many problems in Unix. Break it down into easier problems. Find or write tools to solve the easier problem. Combine those tools with your shell or other operating system features.

[0] Note that I have no idea where you get cost-center from, so I just made it up.

You can use following python script to parse that data. Lets assume that you have JSON data from arrays in files like array1.json, array2.json and so on.

import json
import sys
from pprint import pprint

jdata = open(sys.argv[1])

data = json.load(jdata)

print "InstanceId", " - ", "Name", " - ", "Owner"
print data["Instances"][0]["InstanceId"], " - " ,data["Instances"][0]["Tags"][1]["Value"], " - " ,data["Instances"][0]["Tags"][2]["Value"] 

jdata.close()

And then just run:

$ for x in `ls *.json`; do python parse.py $x; done
InstanceId  -  Name  -  Owner
i-1234576  -  RDS_Machine (us-east-1c)  -  Jyoti Bhanot

I haven’t seen cost in your data, that’s why I didn’t include that.

According to discussion in comments, I have updated parse.py script:

import json
import sys
from pprint import pprint

jdata = sys.stdin.read()

data = json.loads(jdata)

print "InstanceId", " - ", "Name", " - ", "Owner"
print data["Instances"][0]["InstanceId"], " - " ,data["Instances"][0]["Tags"][1]["Value"], " - " ,data["Instances"][0]["Tags"][2]["Value"] 

You can try to run following command:

#ec2-describe-instance <instance> | python parse.py

Others have provided general answers for your question which demonstrate good ways of parsing json however I, like you, were looking for a way to extract an aws instance id using a core tool like awk or sed without depending on other packages. To accomplish this you can pass the “–output=text” argument to your aws command which will give you an awk parsable string. With that you can simply get the instance ID using something like the following…

aws ec2 run-instances --output text  | awk -F"t" '$1=="INSTANCES" {print $8}'

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