Guide to default AWS Lambda environment variables

AWS Lambda runtimes set a number of default environment variables during initialization of every serverless function.

The following variables are reserved and cannot be set in your function configuration:

• _HANDLER - The location of a functions's handler.
• _X_AMZN_TRACE_ID - X-Ray tracing header
• AWS_REGION - The region the function is running in (we all know it's us-east-1).
• AWS_EXECUTION_ENV - The AWS Lambda runtime identifier. Examples: AWS_Lambda_nodejs14.x, AWS_Lambda_java8.
• AWS_LAMBDA_FUNCTION_NAME - The name of the function (which is either "hello world" or IaC randomly generated one, there's no between).
• AWS_LAMBDA_FUNCTION_MEMORY_SIZE - The amount of memory allocated to the function (e.g. 1024MB).
• AWS_LAMBDA_FUNCTION_VERSION - The version of the function (e.g. $LATEST).
• AWS_LAMBDA_INITIALIZATION_TYPE - The initialization type of the function, can be either on-demand or provisioned-concurrency.
• AWS_LAMBDA_LOG_GROUP_NAME - The name of the CloudWatch log group.
• AWS_LAMBDA_LOG_STREAM_NAME - The name of the CloudWatch stream.
• AWS_ACCESS_KEY, AWS_ACCESS_KEY_ID, AWS_SECRET_ACCESS_KEY, AWS_SESSION_TOKEN - The access keys obtained from the function's execution role. Be careful with those, read SAS-7: Insecure Application Secrets Storage for guidance. This post on Security for AWS Lambda Serverless Applications is also a good place to start.
• AWS_LAMBDA_RUNTIME_API - The host and port of the AWS Lambda runtime API, example: 127.0.0.1:9001
• LAMBDA_TASK_ROOT - The path to your Lambda function code
• LAMBDA_RUNTIME_DIR - The path to runtime libraries
• TZ - The timezone information, e.g. :UTC'

In addition to that, AWS Lambda runtime sets additional variables that are not reserved and can be extended in your function configuration:

• LANG - The language of your function's runtime, e.g. en_US.UTF-8
• PATH - The execution path, e.g. /var/lang/bin:/usr/local/bin:/usr/bin/:/bin:/opt/bin
• LD_LIBRARY_PATH - The system library path, e.g. /var/lang/lib:/lib64: /usr/lib64:/var/runtime: /var/runtime/lib:/var/task:/var/task/lib:/opt/lib
• AWS_XRAY_CONTEXT_MISSING - Quoting the docs: "For X-Ray tracing, Lambda sets this to LOG_ERROR to avoid throwing runtime errors from the X-Ray SDK."
• AWS_XRAY_DAEMON_ADDRESS - The IP address and port of the X-Ray daemon.

There are also a few runtime-specific environment variables:

• NODE_PATH - A node.js env variable, containing node.js library path, e.g. /opt/nodejs/node14/node_modules: /opt/nodejs/node_modules: /var/runtime/node_modules:/var/runtime:/var/task
• PYTHONPATH - The Python library path
• GEM_PATH - The Ruby library path
• AWS_LAMBDA_DOTNET_PREJIT - Quoting the docs: "For the .NET 3.1 runtime, set this variable to enable or disable .NET 3.1 specific runtime optimizations. Values include always, never, and provisioned-concurrency."