Environment Variables

Introduction

This lab will cover the basics of environment variables in Unix programs. Environment variables are used to convey configuration information to programs.

In this lab, you will need to set, get, and list environment variables.

Use os.Setenv to set a key/value pair.
Use os.Getenv to get a value for a key.
Use os.Environ to list all key/value pairs in the environment.
Use strings.SplitN to split the key and value.

## 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
...

There is the full code below:

// [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])
    }
}

Summary

In this lab, you learned how to set, get, and list environment variables in Unix programs. This is a fundamental concept that is used in many programs to convey configuration information.

Fundamentals of Unix Environment Variables

Introduction

Environment Variables

Summary