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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.

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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
.env
FOO=hello
BAR=world
You can also set variables on the command line. 
FOO=helloworld bun run dev

# Using CMD
set FOO=helloworld && bun run dev

# Using PowerShell
$env:FOO="helloworld"; bun run dev
For a cross-platform solution, use Bun Shell, for example through bun exec.
bun exec 'FOO=helloworld bun run dev'
On Windows, package.json scripts called with bun run automatically use the Bun Shell, so the following is also cross-platform.
package.json
"scripts": {
  "dev": "NODE_ENV=development bun --watch app.ts",
},
Or set them programmatically by assigning a property to process.env. 
process.env.FOO = "hello";

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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

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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:
bunfig.toml
# Disable loading .env files
env = false
Files passed with --env-file still load even when default loading is disabled.

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Quotation marks

Bun supports double quotes, single quotes, and template literal backticks:
.env
FOO='hello'
FOO="hello"
FOO=`hello`

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Expansion

Bun automatically expands environment variables, so you can reference previously-defined variables.
.env
FOO=world
BAR=hello$FOO

process.env.BAR; // => "helloworld"
This is useful for constructing connection strings or other compound values.
.env
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.
.env
FOO=world
BAR=hello\$FOO

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

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dotenv

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

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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>

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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

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Configuring Bun

Bun reads these environment variables to configure aspects of its behavior.
NameDescription
NODE_TLS_REJECT_UNAUTHORIZEDNODE_TLS_REJECT_UNAUTHORIZED=0 disables SSL certificate validation. Useful for testing and debugging, but be very hesitant to use it in production. Node.js introduced this variable; Bun keeps the name for compatibility.
BUN_CONFIG_VERBOSE_FETCHIf BUN_CONFIG_VERBOSE_FETCH=curl, then fetch requests log the URL, method, request headers and response headers to the console. This also works with node:http. BUN_CONFIG_VERBOSE_FETCH=1 is equivalent to BUN_CONFIG_VERBOSE_FETCH=curl except without the curl output.
BUN_RUNTIME_TRANSPILER_CACHE_PATHThe runtime transpiler caches the transpiled output of source files larger than 4 KB, which makes CLIs using Bun load faster. If BUN_RUNTIME_TRANSPILER_CACHE_PATH is set, the cache is written to that directory. If it is set to an empty string or the string "0", caching is disabled. If it is unset, the cache is written 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 forced on, even if NO_COLOR is set.
BUN_CONFIG_MAX_HTTP_REQUESTSSets the maximum number of concurrent HTTP requests sent by fetch and bun install. Defaults to 256. Lower it if you run into rate limits or connection issues.
BUN_CONFIG_NO_CLEAR_TERMINAL_ON_RELOADIf BUN_CONFIG_NO_CLEAR_TERMINAL_ON_RELOAD=true, then bun --watch does not clear the console on reload
DO_NOT_TRACKDisable uploading crash reports to bun.report on crash. On macOS & Windows, crash report uploads are enabled by default. Other telemetry is not sent, though we plan to add some. If DO_NOT_TRACK=1, then auto-uploading crash reports and telemetry are both disabled.
BUN_OPTIONSPrepends command-line arguments to any Bun execution. For example, BUN_OPTIONS="--hot" makes bun run dev behave like bun --hot run dev.

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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.

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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

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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.