If you have required to set Environment Variable in Linux can set in a single command, for example Google webdirver for chrome you can not find in the environment variable,

ChromeDriver - WebDriver for Chrome

Download and extract the chrome driver file

$ wget http://chromedriver.storage.googleapis.com/2.20/chromedriver_linux32.zip

$ unzip  chromedriver_linux32.zip

Set Environment Varilable 
 
$ export WEBDRIVER_CHROME_DIRVER="/usr/local/chromedriver"

$ echo $WEBDRIVER_CHROME_DIRVER
/usr/local/chromedriver


Do you want to set the path permanently, open the .bash_profile or then use .bashrc and append this in last line,
 
$  vim ~/.bash_profile
export WEBDRIVER_CHROME_DIRVER="/usr/local/chromedriver"