Environment Variables

Environment variables are a universal mechanism for conveying configuration information to Unix programs. Let's look at how to set, get, and list environment variables.

package main

import (
	"fmt"
	"os"
	"strings"
)

func main() {

To set a key/value pair, use os.Setenv. To get a value for a key, use os.Getenv. This will return an empty string if the key isn't present in the environment.

	os.Setenv("FOO", "1")
	fmt.Println("FOO:", os.Getenv("FOO"))
	fmt.Println("BAR:", os.Getenv("BAR"))
	fmt.Println("BAZ:", os.Getenv("BAZ"))

LookupEnv tells an empty value apart from a missing one.

	if _, ok := os.LookupEnv("BAZ"); !ok {
		fmt.Println("BAZ is not set")
	}

ExpandEnv replaces $VAR and ${VAR} in a string.

	fmt.Println(os.ExpandEnv("foo is $FOO, bar is ${BAR}"))

Use os.Environ to list all key/value pairs in the environment. This returns a slice of strings in the form KEY=value. You can strings.Cut them to get the key and value. Here we look for our two keys.

	fmt.Println()
	for _, e := range os.Environ() {
		key, _, _ := strings.Cut(e, "=")
		if key == "FOO" || key == "BAR" {
			fmt.Println(e)
		}
	}

Unsetenv removes a variable.

	os.Unsetenv("FOO")
	fmt.Println("FOO after unset:", os.Getenv("FOO") == "")
}

$ BAR=2 go run ./80_environment_variables
FOO: 1
BAR: 2
BAZ: 
BAZ is not set
foo is 1, bar is 2

BAR=2
FOO=1
FOO after unset: true

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