To use environment variables in Go, you can utilize the os package, which provides a portable way of using operating system functionality. Here's a simple guide on how to do it:

Import the os Package: First, import the os package into your Go program.
Read Environment Variables: Use the os.Getenv function to read an environment variable. It returns the value of the environment variable as a string. If the variable is not set, it returns an empty string.

Here's a basic example to demonstrate how to use environment variables in Go:

package main

import (
    "fmt"
    "os"
)

func main() {
    // Reading a single environment variable
    myEnvVar := os.Getenv("MY_ENV_VAR")
    fmt.Println("MY_ENV_VAR:", myEnvVar)

    // Setting a default value if the environment variable is not set
    if myEnvVar == "" {
        myEnvVar = "default value"
    }
    fmt.Println("MY_ENV_VAR (after setting default):", myEnvVar)

    // Reading all environment variables
    for _, env := range os.Environ() {
        fmt.Println(env)
    }
}

The example above presumes the environment variables are already accessible in your operating system. Follow these steps if you're trying to read from a .env* file.

Install the godotenv Package: First, you'll want to install the godotenv package. You can do this by running:

go get github.com/joho/godotenv

Create a .env file: Create a file named .env in the root of your project and add your environment variables in the format KEY=value. For example:

MY_ENV_VAR=some_value
DATABASE_URL=example_database_url

Load .env File in Your Go Application: Use the godotenv package to load your .env file at the beginning of your program.

Here's an example of how to use godotenv in your Go application:

package main

import (
    "fmt"
    "log"
    "os"

    "github.com/joho/godotenv"
)

func main() {
    // Load the .env file in the current directory
    err := godotenv.Load()
    if err != nil {
        log.Fatal("Error loading .env file")
    }

    // Reading an environment variable
    myEnvVar := os.Getenv("MY_ENV_VAR")
    fmt.Println("MY_ENV_VAR:", myEnvVar)
    
	// Reading environment variables
    dbUser := os.Getenv("DB_USER")
    dbPass := os.Getenv("DB_PASS")

    // Concatenating environment variables with additional text
    // to form a database connection string
    dbConnectionString := fmt.Sprintf("user=%s password=%s dbname=mydatabase sslmode=disable", dbUser, dbPass)

    fmt.Println("Database Connection String:", dbConnectionString)
}

Remember: Environment variables generally contain sensitive information; be sure to avoid publishing these values.

A Step-by-Step Guide to Using Environment Variables in Go