Get values for a given key and its parent with jq

As I understand it you're looking for the value of "Foo". This is really easy to do with the shell command-line tool jq. It is something like sed in that it implements its own kind of parser language. Given your example:

json='
{
    "Reservations" :
     {  
            "OwnerId" : "1345345",
            "Groups" :  [],
            "SecurityGroups" : [
               {
                  "Foo" : "yes",
                  "Bar" : "no"
               }
             ]
     }
}'

jq can get yes as simply as:

printf %s "$json" |
jq '.[].SecurityGroups[0].Foo?'                                                

OUTPUT

"yes"

You can walk through an object hash or dictionary list using the .dot notation, and indexed arrays can be indexed more simply, with, as you have probably guessed, numeric, square-bracketed indices. In the command above I use the empty index form to indicate that I want all of that level's iterable items expanded. That may be easier to understand in this way:

printf %s "$json" | jq '.[][]'

... which breaks out all values for the second level items in the hash and gets me...

"1345345"
[]
[
  {
    "Foo": "yes",
    "Bar": "no"
  }
]

This barely scratches the surface with regards to jq's capabilities. It is an immensely powerful tool for serializing data in the shell, it compiles to a single executable binary in the classic Unix-style, it is very likely available via package-manager for your distribution, and it is very well documented. Please visit its git-page and see for yourself.

By the way, another way to tackle layered-data in json - at least to get an idea of what you're working with - might be to go the other way and use the .dot notation to break out all values at all levels like:

printf %s "$json" | jq '..'

{
  "Reservations": {
    "OwnerId": "1345345",
    "Groups": [],
    "SecurityGroups": [
      {
        "Foo": "yes",
        "Bar": "no"
      }
    ]
  }
}
{
  "OwnerId": "1345345",
  "Groups": [],
  "SecurityGroups": [
    {
      "Foo": "yes",
      "Bar": "no"
    }
  ]
}
"1345345"
[]
[
  {
    "Foo": "yes",
    "Bar": "no"
  }
]
{
  "Foo": "yes",
  "Bar": "no"
}
"yes"
"no"

But far better, probably, would be just to use one of the many discovery or search methods that jq offers for the various types of nodes.

There is a raw flag for this

    -r      output raw strings, not JSON texts;

jq -rc .stuff stuff.json

Output

{"date":"2018-01-08"}