Environment variables

Bun reads your .env files automatically and provides idiomatic ways to read and write your environment variables programmatically. Plus, some aspects of Bun's runtime behavior can be configured with Bun-specific environment variables.

Setting environment variables

Bun reads the following files automatically (listed in order of increasing precedence).

  • .env
  • .env.production, .env.development, .env.test (depending on value of NODE_ENV)
  • .env.local

Variables can also be set via the command line.

FOO=helloworld bun run dev

Or programmatically by assigning a property to process.env.

process.env.FOO = "hello";

Manually specifying .env files

Bun supports --env-file to override which specific .env file to load. You can use --env-file when running scripts in bun's runtime, or when running package.json scripts.

bun --env-file=.env.1 src/index.ts

bun --env-file=.env.abc --env-file=.env.def run build

Quotation marks

Bun supports double quotes, single quotes, and template literal backticks:



Environment variables are automatically expanded. This means you can reference previously-defined variables in your environment variables.

process.env.BAR; // => "helloworld"

This is useful for constructing connection strings or other compound values.


This can be disabled by escaping the $ with a backslash.

process.env.BAR; // => "hello$FOO"


Generally speaking, you won't need dotenv or dotenv-expand anymore, because Bun reads .env files automatically.

Reading environment variables

The current environment variables can be accessed via process.env.

process.env.API_TOKEN; // => "secret"

Bun also exposes these variables via Bun.env and import.meta.env, which is a simple alias of process.env.

Bun.env.API_TOKEN; // => "secret"
import.meta.env.API_TOKEN; // => "secret"

To print all currently-set environment variables to the command line, run bun --print process.env. This is useful for debugging.

bun --print process.env
<lots more lines>


In TypeScript, all properties of process.env are typed as string | undefined.

// string | undefined

To get autocompletion and tell TypeScript to treat a variable as a non-optional string, we'll use interface merging.

declare module "bun" {
  interface Env {
    AWESOME: string;

Add this line to any file in your project. It will globally add the AWESOME property to process.env and Bun.env.

process.env.AWESOME; // => string

Configuring Bun

These environment variables are read by Bun and configure aspects of its behavior.

BUN_RUNTIME_TRANSPILER_CACHE_PATHThe runtime transpiler caches the transpiled output of source files larger than 50 kb. This makes CLIs using Bun load faster. If BUN_RUNTIME_TRANSPILER_CACHE_PATH is set, then the runtime transpiler will cache transpiled output to the specified directory. If BUN_RUNTIME_TRANSPILER_CACHE_PATH is set to an empty string or the string "0", then the runtime transpiler will not cache transpiled output. If BUN_RUNTIME_TRANSPILER_CACHE_PATH is unset, then the runtime transpiler will cache transpiled output to the platform-specific cache directory.
TMPDIRBun occasionally requires a directory to store intermediate assets during bundling or other operations. If unset, defaults to the platform-specific temporary directory: /tmp on Linux, /private/tmp on macOS.
NO_COLORIf NO_COLOR=1, then ANSI color output is disabled.
FORCE_COLORIf FORCE_COLOR=1, then ANSI color output is force enabled, even if NO_COLOR is set.
BUN_CONFIG_MAX_HTTP_REQUESTSControl the maximum number of concurrent HTTP requests sent by fetch and bun install. Defaults to 256. If you are running into rate limits or connection issues, you can reduce this number.
BUN_CONFIG_NO_CLEAR_TERMINAL_ON_RELOADIf BUN_CONFIG_NO_CLEAR_TERMINAL_ON_RELOAD=1, then bun --watch will not clear the console on reload
DO_NOT_TRACKTelemetry is not sent yet as of November 28th, 2023, but we are planning to add telemetry in the coming months. If DO_NOT_TRACK=1, then analytics are disabled. Bun records bundle timings (so we can answer with data, "is Bun getting faster?") and feature usage (e.g., "are people actually using macros?"). The request body size is about 60 bytes, so it's not a lot of data. Equivalent of telemetry=false in bunfig.

Runtime transpiler caching

For files larger than 50 KB, Bun caches transpiled output into $BUN_RUNTIME_TRANSPILER_CACHE_PATH or the platform-specific cache directory. This makes CLIs using Bun load faster.

This transpiler cache is global and shared across all projects. It is safe to delete the cache at any time. It is a content-addressable cache, so it will never contain duplicate entries. It is also safe to delete the cache while a Bun process is running.

It is recommended to disable this cache when using ephemeral filesystems like Docker. Bun's Docker images automatically disable this cache.

Disable the runtime transpiler cache

To disable the runtime transpiler cache, set BUN_RUNTIME_TRANSPILER_CACHE_PATH to an empty string or the string "0".


What does it cache?

It caches:

  • The transpiled output of source files larger than 50 KB.
  • The sourcemap for the transpiled output of the file

The file extension .pile is used for these cached files.