Environment Variables #

// [Environment variables](https://en.wikipedia.org/wiki/Environment_variable)
// are a universal mechanism for [conveying configuration
// information to Unix programs](https://www.12factor.net/config).
// 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"))

	// 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.SplitN` them to
	// get the key and value. Here we print all the keys.
	fmt.Println()
	for _, e := range os.Environ() {
		pair := strings.SplitN(e, "=", 2)
		fmt.Println(pair[0])
	}
}

# Running the program shows that we pick up the value
# for `FOO` that we set in the program, but that
# `BAR` is empty.
$ go run environment-variables.go
FOO: 1
BAR:

# The list of keys in the environment will depend on your
# particular machine.
TERM_PROGRAM
PATH
SHELL
...
FOO

# If we set `BAR` in the environment first, the running
# program picks that value up.
$ BAR=2 go run environment-variables.go
FOO: 1
BAR: 2
...