Environment Variables

Bun reads your .env files automatically and provides idiomatic ways to read and write your environment variables programmatically. You can also configure parts of Bun's runtime behavior 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 the value of NODE_ENV)
• .env.local

FOO=hello
BAR=world

You can also set variables on the command line.

$ FOO=helloworld bun run dev

Or set them programmatically by assigning a property to process.env.

process.env.FOO = "hello";

Manually specifying .env files

The --env-file flag overrides which .env files Bun loads. It works both when running a file with bun and when running package.json scripts.

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

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

Disabling automatic .env loading

Use --no-env-file to disable Bun's automatic .env file loading, for example in production or CI/CD pipelines where you want to rely solely on system environment variables.

$ bun run --no-env-file index.ts

You can also disable it in bunfig.toml:

# Disable loading .env files
env = false

Files passed with --env-file still load even when default loading is disabled.

When Bun is invoked as node (for example via bun --bun, bunx --bun, or a node symlink pointing at Bun), automatic .env loading is disabled to match Node.js. This lets tools with their own mode-aware .env resolution, such as Vite's loadEnv, pick the correct .env.{mode} file instead of seeing Bun's pre-populated values as shell-set overrides. Explicit --env-file arguments are still honored.

Quotation marks

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

FOO='hello'
FOO="hello"
FOO=`hello`

Expansion

Bun automatically expands environment variables, so you can reference previously-defined variables.

FOO=world
BAR=hello$FOO

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

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

DB_USER=postgres
DB_PASSWORD=secret
DB_HOST=localhost
DB_PORT=5432
DB_URL=postgres://$DB_USER:$DB_PASSWORD@$DB_HOST:$DB_PORT/$DB_NAME

To disable expansion, escape the $ with a backslash.

FOO=world
BAR=hello\$FOO

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

dotenv

Bun reads .env files automatically, so dotenv and dotenv-expand are unnecessary.

Reading environment variables

Read the current environment variables from process.env.

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

Bun also exposes these variables as Bun.env and import.meta.env, both aliases of process.env.

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

To print all currently-set environment variables, run bun --print process.env.

$ bun --print process.env
BAZ=stuff
FOOBAR=aaaaaa
<lots more lines>

TypeScript

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

Bun.env.whatever;
// string | undefined

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

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

Add this declaration to any file in your project. It globally adds the AWESOME property to process.env and Bun.env.

process.env.AWESOME; // => string

Configuring Bun

Bun reads these environment variables to configure aspects of its behavior.

Runtime transpiler caching

For files larger than 4 KB, Bun caches transpiled output into $BUN_RUNTIME_TRANSPILER_CACHE_PATH or the platform-specific cache directory. This makes CLIs using Bun load faster.

The cache is global and shared across all projects, and it is content-addressable, so it never contains duplicate entries. It is safe to delete at any time, even while a Bun process is running.

Disable this cache when using ephemeral filesystems like Docker. Bun's Docker images disable it automatically.

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".

$ BUN_RUNTIME_TRANSPILER_CACHE_PATH=0 bun run dev

What does it cache?

It caches:

• The transpiled output of source files larger than 4 KB.
• The sourcemap for the transpiled output of the file

Cached files use the .pile extension.