jq displaying filtered EC2 instances, only selected fields

filter_instances.sh

# Loops through regions
# Gets all data for instances in the inner array
# Outputs instance id, AZ, Launch Time, and the Name tag

for REGION in us-east-1 us-west-1; do
  aws --region $REGION ec2 describe-instances | \
  jq '.Reservations[].Instances[] | select(.InstanceId as $id | ["id-1234", "id-56789"] | index($id)) | [.InstanceId, .Placement.AvailabilityZone, .LaunchTime, [.Tags[] | select(.Key == "Name") | .Value][] ]'
done

# I want to comment on and break down the last part of the jq command:
#   [.InstanceId, .PlacementAvailabilityZone, .Launchtime, [.Tags[] | select(.Key == "Name) | .Value][] ]
# That makes it print out something like:
# [
#   "id-1234",
#   "us-west-1a",
#   "2020-02-24T18:03:22.000Z",
#   "ServerFoo"
# ]
#
# In particular I wanted to break down the odd construction of the last field:
#   [.Tags[] | select(.Key == "Name") | .Value][]
#
# The data coming in to this is an array of tag maps, so the ".Tags[]" flattens that array.  That's normal usage.
# We use "select()" to grab a specific tag, the "Name" tag, and print its value.  That's normal usage too.
# All of that is in array brackets, which will output these as an array.
# But then it's followed by another set of empty brackets.  The basic pattern is:
#   [ something | filter | output ][]
#
# Why the extra set of brackets?  It's the same reason we use ".Tags[]" earlier.
# That "output" is an array that looks like this:
#   [
#     "ServerFoo"
#   ]
#
# Since I only want the string, not an array with a single string, I also flatten it with an extra set of "[]" at the end.
#
# The concept I want to stress is that I can flatten an array with "[]" that *I* am creating as part of the output.
# When I first tried to do it, I thought there was no way it would work, but it did, and made my life simple.
# I use it all the time now, just never documented it anywhere.  fin

# jq is so so so cool

jq is just a marvel at its power and capability.