os.Args / os.Getenv()

// Slice of command-line arguments (os.Args[0] is the program name)
os.Args

// Get an environment variable (returns an empty string if not set)
value := os.Getenv("VARIABLE_NAME")

// Set an environment variable
err := os.Setenv("VARIABLE_NAME", "value")

// Check whether an environment variable exists
value, ok := os.LookupEnv("VARIABLE_NAME")

package main

import (
	"fmt"
	"os"
)

func main() {
	// Print command-line argument info
	fmt.Println("Program name:", os.Args[0])
	fmt.Println("Number of arguments:", len(os.Args)-1)

	// Print the first argument if one was provided
	if len(os.Args) > 1 {
		fmt.Println("First argument:", os.Args[1])
	} else {
		fmt.Println("No arguments provided")
	}

	// Set and then retrieve an environment variable
	os.Setenv("APP_ENV", "development")
	env := os.Getenv("APP_ENV")
	fmt.Println("APP_ENV:", env)

	// Use LookupEnv to check existence before reading
	if port, ok := os.LookupEnv("PORT"); ok {
		fmt.Println("Port:", port)
	} else {
		fmt.Println("PORT is not set. Default: 8080")
	}
}

When building CLI tools, consider using the standard library's flag package for argument parsing — it handles flag-style input (such as --port=8080) with ease. os.Args gives you the raw string slice, which is well-suited for simple, positional arguments.

Environment variables are commonly used in place of configuration files, especially in container and cloud environments. Because os.Getenv() returns an empty string when a variable is not set, use os.LookupEnv() with the ok flag to safely verify that a required variable is actually present.