Environment variables

Context

Environment variables are key‑value pairs that affect the behaviour of running processes. In Go, you can read them with os.Getenv, set them with os.Setenv, and delete them with os.Unsetenv.

Important functions

Function	Description
`os.Getenv(key)`	Returns the value as a string (empty string if not set).
`os.LookupEnv(key)`	Returns `(value, true)` if set, otherwise `("", false)`.
`os.Setenv(key, value)`	Sets an environment variable.
`os.Unsetenv(key)`	Deletes an environment variable.
`os.Environ()`	Returns a slice of `"key=value"` strings.

Example

Read the HOME directory (or USERPROFILE on Windows).

Code example

package main

import (
    "fmt"
    "os"
)

func main() {
    home := os.Getenv("HOME")
    if home == "" {
        home = os.Getenv("USERPROFILE") // Windows
    }
    fmt.Println("Home directory:", home)

    // Safer way
    if value, ok := os.LookupEnv("PATH"); ok {
        fmt.Println("PATH is set to:", value[:50]) // print first 50 chars
    }
}

Output (example)

Home directory: /home/alice
PATH is set to: /usr/local/bin:/usr/bin:/bin

Use case

Environment variables are ideal for configuration (e.g., database URLs, API keys) because they can be changed without recompiling the program.