How to retrieve single value with grep from Json? - shell

Question

How to extract a single value from a given json?
{
"Vpc": {
"InstanceTenancy": "default",
"State": "pending",
"VpcId": "vpc-123",
"CidrBlock": "10.0.0.0/16",
"DhcpOptionsId": "dopt-123"
}
}
Tried this but with no luck:
grep -e '(?<="VpcId": ")[^"]*'

Answer 1

You probably wanted -Po, which works with your regex:
$ grep -oP '(?<="VpcId": ")[^"]*' infile
vpc-123
If GNU grep with its -P option isn't available, we can't use look-arounds and have to resort to for example using grep twice:
$ grep -o '"VpcId": "[^"]*' infile | grep -o '[^"]*$'
vpc-123
The first one extracts up to and excluding the closing quotes, the second one searches from the end of the line for non-quotes.
But, as mentioned, you'd be better off properly parsing your JSON. Apart from jq mentioned in another answer, I know of
Jshon
JSON.sh
A jq solution would be as simple as this:
$ jq '.Vpc.VpcId' infile
"vpc-123"
Or, to get raw output instead of JSON:
$ jq -r '.Vpc.VpcId' infile
vpc-123

Answer 2

Something like
grep '^ *"VpcId":' json.file \
| awk '{ print $2 }' \
| sed -e 's/,$//' -e 's/^"//' -e 's/"$//'

Answer 3

you can do:
sed -r -n -e '/^[[:space:]]*"VpcId":/s/^[^:]*: *"(.*)", *$/\1/p'
but really, using any shell tools to run regexes over JSON content is a bad idea. you should consider a much saner language like python.
python -c 'import json, sys; print(json.loads(sys.stdin.read())["Vpc"]["VpcId"]);'

Answer 4

Try this regex pattern:
\"VpcId\":\s?(\"\S+\")

Answer 5

If you can install a tool I would suggest using jq jq. It allows very simple grep, with great support for piping too.

Answer 6

The OP asks for solutions using grep. In case he means using terminal, the node cli is an alternative, since support for JSON is total. One alternative could be the command node --eval "script"
echo '{"key": 42}' \
| node -e 'console.log(JSON.parse(require("fs").readFileSync(0).toString()).key)' //prints 42