Interpolation

env.zig supports ${VAR} / $VAR with shell-like fallbacks and OS env fallback on Windows/Linux/macOS.

Enable Interpolation

var env = envMod.Env.init(allocator, .{
    .interpolate = true, // default true
});

Syntax

Form	Meaning
${VAR} / $VAR	Value of VAR (Env first, then OS env)
${VAR:-default}	default if VAR unset or empty
${VAR-default}	default if VAR unset (empty passthrough)
${VAR:+alt}	alt if VAR set and non-empty, else empty
${VAR+alt}	alt if VAR set (even if empty), else empty
${VAR:?msg} / ${VAR?msg}	msg if VAR missing/empty (error message)
${VAR:=default} / ${VAR=default}	Like :- / - (assign not persisted)

Nested defaults are expanded: ${MISSING:-${FALLBACK}}.

Reference another variable:

DATABASE_HOST=localhost
DATABASE_PORT=5432
DATABASE_URL=postgres://${DATABASE_HOST}:${DATABASE_PORT}/mydb

# OS fallback — uses $HOME from process env if not in .env
MY_HOME=${HOME}
MY_PATH=$PATH:/custom/bin

# Defaults & alts
PORT=${PORT:-3000}
DEBUG_MSG=${DEBUG:+enabled}
REQUIRED=${API_KEY:?API_KEY is required}

OS fallback walks: Env entries → runtime.get (POSIX getenv / Windows GetEnvironmentVariableW).

Circular Detection

env.zig detects circular references and prevents infinite loops:

// This will error with a circular reference detected
try env.parseString(
    \\A=${B}
    \\B=${A}
    \\
);

Examples

Basic Interpolation

APP_NAME=myapp
GREETING=hello
MESSAGE=${GREETING} from ${APP_NAME}

try stdout.print("MESSAGE = {s}\n", .{env.get("MESSAGE").?});
// Output: MESSAGE = hello from myapp

Database URL Construction

DB_HOST=localhost
DB_PORT=5432
DB_NAME=mydb
DATABASE_URL=postgres://${DB_HOST}:${DB_PORT}/${DB_NAME}

Nested Interpolation

BASE_URL=https://api.example.com
VERSION=v1
API_ENDPOINT=${BASE_URL}/${VERSION}/users

See Also

• Getting Started for configuration options
• Configuration for all config options
• API Reference for the full API