Environment Variables

Environment variables supply configuration to a running process without hard-coding a value in the source file.

os.Getenv returns the variable’s value or an empty string if absent, while os.LookupEnv returns the value and a boolean indicating presence. os.Setenv changes the current process’s environment, which can be useful for controlled tests.

Use environment variables for configuration such as a port or application mode, but treat secret values carefully: prevent accidental printing and avoid committing .env files containing credentials.

Follow this diagram before executing the example. Check where a value or decision changes what runs next.

This example is a complete program. Save it as main.go or paste it into the Go Playground.

Go code

package main

import (
	"fmt"
	"os"
)

func main() {
	const key = "GO_COURSE_MODE_DEMO"
	if err := os.Setenv(key, "development"); err != nil {
		panic(err)
	}
	mode, ok := os.LookupEnv(key)
	fmt.Println("Set:", ok)
	fmt.Println("Mode:", mode)
}

Output

Set: true
Mode: development

The example sets a known process variable to development, then LookupEnv retrieves it. This ensures deterministic output regardless of the parent shell configuration.

• Environment variable names and case handling can vary between systems.
• Avoid printing secrets or assuming environment variables are a secure encrypted vault.

Read an optional APP_PORT environment variable and default to 8080 when it is unset.

Tip: Edit the example above, predict the output, and run the modified code to test your reasoning.

You have practiced Environment Variables in a small, self-contained program. The important habit is to predict what runs, inspect the actual output and distinguish the language rule from the particular example.

Before continuing, make sure you can explain the program to someone who has not seen it, including the line that changes its result.