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[Environment variables](http://en.wikipedia.org/wiki/Environment_variable) are a universal mechanism for [conveying configuration information to Unix programs](http://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"))	FOO: 1
	fmt.Println("BAR:", os.Getenv("BAR"))	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])	PERLBREW_SHELLRC_VERSION NVM_INC MANPATH XDG_SESSION_ID TRAVIS_ARCH TRAVIS_FILTERED rvm_bin_path MYSQL_UNIX_PORT NVM_CD_FLAGS GEM_HOME PYENV_ROOT HAS_JOSH_K_SEAL_OF_APPROVAL TRAVIS_STACK_JOB_BOARD_REGISTER TERM TRAVIS_TEST_RESULT TRAVIS_STACK_LANGUAGES SHELL HISTSIZE MAKEFLAGS SSH_CLIENT IRBRC TRAVIS_COMMIT TRAVIS_APT_PROXY TRAVIS_OS_NAME TRAVIS_JOB_NAME TRAVIS_UID TRAVIS_INTERNAL_RUBY_REGEX MY_RUBY_HOME TRAVIS_ROOT SSH_TTY TRAVIS_TIMER_ID LC_ALL NVM_DIR USER HISTFILESIZE TRAVIS_GO_VERSION _TRAVIS_RESOLVED_GIMME_GO_VERSION_INT ANSI_GREEN TRAVIS_INFRA TRAVIS_LANGUAGE ANSI_RESET rvm_path TRAVIS_DIST TRAVIS MAKELEVEL TRAVIS_REPO_SLUG ANSI_YELLOW PYTHON_CONFIGURE_OPTS HAS_ANTARES_THREE_LITTLE_FRONZIES_BADGE DKEY TRAVIS_PULL_REQUEST PAGER TRAVIS_COMMIT_MESSAGE RACK_ENV TRAVIS_BUILD_STAGE_NAME GO111MODULE MFLAGS PERLBREW_ROOT TRAVIS_CMD TRAVIS_STACK_TIMESTAMP _TRAVIS_RESOLVED_GIMME_GO_VERSION TRAVIS_CPU_ARCH rvm_prefix MAIL PATH PYTHON_CFLAGS TRAVIS_JOB_WEB_URL TRAVIS_OSX_IMAGE TRAVIS_PULL_REQUEST_SHA TRAVIS_TMPDIR TRAVIS_BUILD_WEB_URL PWD APT_KEY_DONT_WARN_ON_DANGEROUS_USAGE JAVA_HOME CONTINUOUS_INTEGRATION LANG TRAVIS_PRE_CHEF_BOOTSTRAP_TIME MERB_ENV TZ PERLBREW_HOME TRAVIS_ENABLE_INFRA_DETECTION TRAVIS_SUDO TRAVIS_TAG RBENV_SHELL TRAVIS_ALLOW_FAILURE GIMME_ENV TRAVIS_HOME HISTCONTROL GOMAXPROCS TRAVIS_INIT rvm_version PS4 TRAVIS_JOB_NUMBER SHLVL HOME PYENV_SHELL TRAVIS_EVENT_TYPE LANGUAGE GOROOT ANSI_CLEAR RAILS_ENV TRAVIS_TIMER_START_TIME CI GIMME_GO_VERSION TRAVIS_BUILD_ID LOGNAME TRAVIS_STACK_FEATURES COMPOSER_NO_INTERACTION TRAVIS_PULL_REQUEST_SLUG GEM_PATH SSH_CONNECTION XDG_DATA_DIRS LC_CTYPE TRAVIS_SECURE_ENV_VARS DEBIAN_FRONTEND TRAVIS_STACK_NODE_ATTRIBUTES NVM_BIN GOPATH TRAVIS_STACK_NAME TRAVIS_APP_HOST GIT_ASKPASS TRAVIS_GO_IMPORT_PATH TRAVIS_BRANCH XDG_RUNTIME_DIR JRUBY_OPTS TRAVIS_COMMIT_RANGE TRAVIS_PULL_REQUEST_BRANCH ANSI_RED TRAVIS_JOB_ID RUBY_VERSION TRAVIS_BUILD_NUMBER TRAVIS_BUILD_DIR _ FOO
	}
	}

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