Out-of-the-box AWS Lambda custom runtime (implemented in bash!)

bootstrap

#!/bin/sh

set -euo pipefail

# Handler format: <script_name>.<function_name>
# The script file <script_name>.sh  must be located in
# the same directory as the bootstrap executable.
source $(dirname "$0")/"$(echo $_HANDLER | cut -d. -f1).sh"

while true
do
    # Request the next event from the Lambda Runtime
    HEADERS="$(mktemp)"
    EVENT_DATA=$(curl  -v -sS  -LD "$HEADERS"  -X GET  "http://${AWS_LAMBDA_RUNTIME_API}/2018-06-01/runtime/invocation/next")
    INVOCATION_ID=$(grep -Fi Lambda-Runtime-Aws-Request-Id "$HEADERS" | tr -d '[:space:]' | cut -d: -f2)

    # Execute the handler function from the script
    RESPONSE=$($(echo "$_HANDLER" | cut -d. -f2) "$EVENT_DATA")

    # Send the response to Lambda Runtime
    curl  -v  -sS  -X POST  "http://${AWS_LAMBDA_RUNTIME_API}/2018-06-01/runtime/invocation/$INVOCATION_ID/response"  -d "$RESPONSE"
done

hello.sh

function handler () {
    EVENT_DATA=$1

    RESPONSE="{\"statusCode\": 200, \"body\": \"Hello from Lambda!\"}"
    echo $RESPONSE
}

To create a zip bundle that you can deploy to Lambda:

zip lambda.zip bootstrap hello.sh

(and then use hello.handler as the Lambda handler)