Generate parameter.json from a CloudFormation Template

Retrieve the parameter list from a template using aws cloudformation validate-template, then use jq to override default values with environment variables and write the result to parameters.json.

aws cloudformation validate-template \
  --template-body "file://path/to/template" \
  --query "Parameters[].{ParameterKey:ParameterKey,ParameterValue:DefaultValue}" \
  | jq 'sort_by(.ParameterKey)' \
  | jq 'map(
    .ParameterValue = (env[.ParameterKey] // .ParameterValue)
  )' > "parameters.json"

Each jq Step

Step	Processing
`sort_by(.ParameterKey)`	Sort alphabetically by key name (stabilizes diffs)
`env[.ParameterKey]`	Retrieve the environment variable with the same name. Returns `null` if undefined
`// .ParameterValue`	Fall back to the default value when `null`

Pattern: Fetch Existing Values from a Deployed Stack

To generate the file based on the values currently set in the stack rather than the template defaults, use describe-stacks.

aws cloudformation describe-stacks \
  --stack-name "your-stack-name" \
  --query "Stacks[0].Parameters[].{ParameterKey:ParameterKey,ParameterValue:ParameterValue}" \
  | jq 'sort_by(.ParameterKey)' \
  | jq 'map(
    .ParameterValue = (env[.ParameterKey] // .ParameterValue)
  )' > "parameters.json"

Exclude Parameters with a null DefaultValue

To exclude parameters that have no default value set in the template:

aws cloudformation validate-template \
  --template-body "file://path/to/template" \
  --query "Parameters[].{ParameterKey:ParameterKey,ParameterValue:DefaultValue}" \
  | jq '[.[] | select(.ParameterValue != null)]' \
  | jq 'sort_by(.ParameterKey)' \
  | jq 'map(
    .ParameterValue = (env[.ParameterKey] // .ParameterValue)
  )' > "parameters.json"

Shell Script Version

#!/usr/bin/env bash
set -euo pipefail

TEMPLATE="${1:?Usage: $0 <template-path> [output-path]}"
OUTPUT="${2:-parameters.json}"

aws cloudformation validate-template \
  --template-body "file://${TEMPLATE}" \
  --query "Parameters[].{ParameterKey:ParameterKey,ParameterValue:DefaultValue}" \
  | jq 'sort_by(.ParameterKey)' \
  | jq 'map(
    .ParameterValue = (env[.ParameterKey] // .ParameterValue)
  )' > "${OUTPUT}"

echo "Generated: ${OUTPUT}"

# Example usage
Env1=value1 Env2=value2 ./generate-params.sh path/to/template.yaml