Environment variables are "a set of dynamic named values that can affect the way running processes will behave on a computer".
We have seen environment variables on our projects when we type commands like RACK_ENV=test rake db:migrate.
Let's see if we can break down that definition and figure out what it means...
The main goals of this warmup are the ability to:
- access environment variables in a Ruby program
- write Ruby code that behaves differently depending on the value of a certain environment variable
- understand how
rake db:migrateis different thanRACK_ENV=test rake db:migrate
Open a Terminal and type env and you'll see something similar to this:
|ruby-2.1.1@gschool| Hunters-MacBook-Pro in ~/gschool/dev/exercises/environment-variables
± |master ✗| → env
GREP_COLOR=1;33
rvm_bin_path=/Users/lemonofpink/.rvm/bin
TERM_PROGRAM=iTerm.app
COMP_WORDBREAKS=
"'@><=;|&(
GEM_HOME=/Users/lemonofpink/.rvm/gems/ruby-2.1.1@gschool
NGINX_PATH=/opt/nginx
SHELL=/bin/bash
...
These are the environment variables that are set in your terminal process. Each variable has a name and a value. Some of these variables should be familiar, like PATH:
± |master ✗| → env | grep PATH
PATH=/Users/lemonofpink/.rvm/gems/ruby-2.1.1@gschool/bin:/Users/lemonofpink/.rvm/gems/ruby-2.1.1@global/bin:/Users/lemonofpink/.rvm/rubies/ruby-2.1.1/bin:/Users/lemonofpink/.rvm/bin:/usr/bin:/bin:/usr/sbin:/sbin:/usr/local/bin:/opt/X11/bin:/Users/lemonofpink/.gem/ruby/1.8/bin:/opt/nginx/sbin
GEM_PATH=/Users/lemonofpink/.rvm/gems/ruby-2.1.1@gschool:/Users/lemonofpink/.rvm/gems/ruby-2.1.1@global
The PATH environment variable, as you may remember, stores a list of directories that your computer should look in to find other programs.
You can look at the value of a single environment variable from the command line using echo $<VARIABLE_NAME>.
Check out the example below:
|ruby-2.1.1@gschool| Hunters-MacBook-Pro in ~/gschool/dev/exercises/environment-variables
± |master ✗| → echo $PATH
/Users/lemonofpink/.rvm/gems/ruby-2.1.1@gschool/bin:/Users/lemonofpink/.rvm/gems/ruby-2.1.1@global/bin:/Users/lemonofpink/.rvm/rubies/ruby-2.1.1/bin:/Users/lemonofpink/.rvm/bin:/usr/bin:/bin:/usr/sbin:/sbin:/usr/local/bin:/opt/X11/bin:/Users/lemonofpink/.gem/ruby/1.8/bin:/opt/nginx/sbin
Try to print out the value of the USER and SHELL environment variables. What is the SHELL environment variable?
Environment variables are dynamic in that their value can be changed, and new ones can be created. Try this:
You can use the export command to set the value of an environment variable. Here is
and example of setting a custom environment variable:
|ruby-2.1.1@gschool| Hunters-MacBook-Pro in ~/gschool/dev/exercises/sinatra-crud-reference
± |master ✓| → export FAVORITE_FOOD=lasagna
This sets the value of the FAVORITE_FOOD environment variable. You can look at the value like this:
|ruby-2.1.1@gschool| Hunters-MacBook-Pro in ~/gschool/dev/exercises/sinatra-crud-reference
± |master ✓| → echo $FAVORITE_FOOD
lasagna
Notice that "lasagna" was printed out to the terminal.
Try setting some custom environment variables and printing them out.
In Ruby, you have access to the environment variables that are set with the ENV hash.
|ruby-2.1.1@gschool| Hunters-MacBook-Pro in ~/gschool/dev/exercises/environment-variables
± |master ✗| → ruby -e "puts ENV.inspect"
{"GREP_COLOR"=>"1;33", "rvm_bin_path"=>"/Users/lemonofpink/.rvm/bin", "TERM_PROGRAM"=>"iTerm.app", "COMP_WORDBREAKS"=>" \t\n\"'@><=;|&(", "GEM_HOME"=>"/Users/lemonofpink/.rvm/gems/ruby-2.1.1@gschool", "NGINX_PATH"=>"/opt/nginx", "SHELL"=>"/bin/bash", "TERM"=>"xterm", "HISTSIZE"=>"5000", "IRBRC"=>"/Users/lemonofpink/.rvm/rubies/ruby-2.1.1/.irbrc", "TMPDIR"=>"/var/folders/t3/k1dmmdjs4mgblcx2ksjwhxwr0000gn/T/", "Apple_PubSub_Socket_Render"=>"/tmp/launch-vUke3I/Render", "GIT_HOSTING"=>"[email protected]", "MY_RUBY_HOME"=>"/Users/lemonofpink/.rvm/rubies/ruby-2.1.1", "GIT_EDITOR"=>"vim", "USER"=>"lemonofpink", "_system_type"=>"Darwin", "BASH_IT_THEME"=>"bobby", "rvm_path"=>"/Users/lemonofpink/.rvm", "SSH_AUTH_SOCK"=>"/tmp/launch-ySDG9j/Listeners", "__CF_USER_TEXT_ENCODING"=>"0x1F5:0:0", "AUTOFEATURE"=>"true", "BASH_IT"=>"/Users/lemonofpink/.bash_it", "LSCOLORS"=>"Gxfxcxdxdxegedabagacad", "rvm_prefix"=>"/Users/lemonofpink", "__CHECKFIX1436934"=>"1", "PATH"=>"/Users/lemonofpink/.rvm/gems/ruby-2.1.1@gschool/bin:/Users/lemonofpink/.rvm/gems/ruby-2.1.1@global/bin:/Users/lemonofpink/.rvm/rubies/ruby-2.1.1/bin:/Users/lemonofpink/.rvm/bin:/usr/bin:/bin:/usr/sbin:/sbin:/usr/local/bin:/opt/X11/bin:/Users/lemonofpink/.gem/ruby/1.8/bin:/opt/nginx/sbin", "PWD"=>"/Users/lemonofpink/gschool/dev/exercises/environment-variables", "EDITOR"=>"vim", "LANG"=>"en_US.UTF-8", "ITERM_PROFILE"=>"Default", "_system_arch"=>"x86_64", "_system_version"=>"10.9", "HISTCONTROL"=>"erasedups", "rvm_version"=>"1.25.26 (master)", "SHLVL"=>"1", "COLORFGBG"=>"7;0", "HOME"=>"/Users/lemonofpink", "GREP_OPTIONS"=>"--color=auto", "rvm_ruby_string"=>"ruby-2.1.1", "ITERM_SESSION_ID"=>"w0t1p1", "LOGNAME"=>"lemonofpink", "GEM_PATH"=>"/Users/lemonofpink/.rvm/gems/ruby-2.1.1@gschool:/Users/lemonofpink/.rvm/gems/ruby-2.1.1@global", "TODO"=>"t", "IRC_CLIENT"=>"irssi", "rvm_delete_flag"=>"0", "DISPLAY"=>"/tmp/launch-HCwwUw/org.macosforge.xquartz:0", "RUBY_VERSION"=>"ruby-2.1.1", "_system_name"=>"OSX", "FAVORITE_FOOD"=>"lasagna", "_"=>"/Users/lemonofpink/.rvm/rubies/ruby-2.1.1/bin/ruby", "OLDPWD"=>"/Users/lemonofpink/gschool/dev/exercises/sinatra-crud-reference"}
We see the same values as when we typed env, but in Ruby we have access to the environment
variables through the ENV hash. (-e let's you pass the ruby command a string of Ruby
code to be executed instead a file name).
To see which environment variables are available to us, we could print out the keys of the ENV hash:
|ruby-2.1.1@gschool| Hunters-MacBook-Pro in ~/gschool/dev/exercises/environment-variables
± |master ✗| → ruby -e "puts ENV.keys.sort"
AUTOFEATURE
Apple_PubSub_Socket_Render
BASH_IT
BASH_IT_THEME
COLORFGBG
COMP_WORDBREAKS
DISPLAY
EDITOR
FAVORITE_FOOD
GEM_HOME
GEM_PATH
...
We can access specific environment variables just like we would with a hash:
|ruby-2.1.1@gschool| Hunters-MacBook-Pro in ~/gschool/dev/exercises/environment-variables
± |master ✗| → ruby -e "puts ENV['GEM_HOME']"
/Users/lemonofpink/.rvm/gems/ruby-2.1.1@gschool
The environment variables that are accessible to a Ruby program are inherited from the environment of the process that started the Ruby program.
Typing env at the command line shows you what environment variables are set
in the terminal process. When you run ruby environment_variables.rb from the
command line, the ENV hash has the same set of environment variables that the
terminal had.
You can also change environment variables with Ruby:
Edit environment_variables.rb:
puts "Name is: #{ENV["NAME"]}"
ENV["NAME"] = "Jeff"
puts "Now name is: #{ENV["NAME"]}"
Ruby this program and pass it some other value as NAME:
|ruby-2.1.1@gschool| Hunters-MacBook-Pro in ~/gschool/dev/exercises/environment-variables
± |master ✗| → NAME=hunter ruby environment_variables.rb
Name is: hunter
Now name is: Jeff
As you can see, the second time ENV['NAME'] is printed, we see that value that was set by the Ruby code.
We can write Ruby programs that behave differently depending on the value of environment variables.
Edit environment_variables.rb so that it looks like this:
puts ENV["WOULD_RISK_HIS_NECK_FOR_HIS_BROTHER_MAN"]
Run the file:
|ruby-2.1.1@gschool| Hunters-MacBook-Pro in ~/gschool/dev/exercises/environment-variables
± |master ✗| → ruby environment_variables.rb
We don't see anything because we are trying to access an environment variable that isn't set.
Trying running the same Ruby program, but setting the WOULD_RISK_HIS_NECK_FOR_HIS_BROTHER_MAN environment variable:
|ruby-2.1.1@gschool| Hunters-MacBook-Pro in ~/gschool/dev/exercises/environment-variables
± |master ✗| → WOULD_RISK_HIS_NECK_FOR_HIS_BROTHER_MAN=shaft ruby environment_variables.rb
shaft
Notice that WOULD_RISK_HIS_NECK_FOR_HIS_BROTHER_MAN=shaft causes the program to have access to the environment variable, so when the program prints it, we see "shaft".
This is basically the same thing that happens when you type RACK_ENV=test rake db:migrate.
Some Ruby code runs, checks ENV["RACK_ENV"], and decides to either migrate the development,
test, or production database.
Edit environment_variables.rb to do the following:
-
Write a method that prints out "Migrate the [environment] database", where [environment] is replaced by the value of the RACK_ENV environment variable.
-
Write a method that prints out "FEED ME" if the HUNGRY environment variable is true and prints out "NOT NOW" otherwise.
The last thing to understand about environment variables is that they are defined per process.
Edit environment_variables.rb to look the following:
puts "Favorite color in parent starts as: #{ENV['FAVORITE_COLOR']}"
fork do
puts "Favorite color in child starts as: #{ENV['FAVORITE_COLOR']}"
ENV["FAVORITE_COLOR"] = 'red'
puts "Favorite color in child ends as: #{ENV['FAVORITE_COLOR']}"
end
puts "Favorite color in parent ends as: #{ENV['FAVORITE_COLOR']}"
The fork method creates a copy of the current process. More here.
The original method is called the parent process and newly created process is called the child.
Here is what it looks like when we run that code:
|ruby-2.1.1@gschool| Hunters-MacBook-Pro in ~/gschool/dev/exercises/environment-variables
± |master ✗| → FAVORITE_COLOR=orange ruby environment_variables.rb
Favorite color in parent starts as: orange
Favorite color in parent ends as: orange
Favorite color in child starts as: orange
Favorite color in child ends as: red
Why does FAVORITE_COLOR start out equal to "orange" in the child process. Why doesn't changing the value of FAVORITE_COLOR in the child process affect the it in the parent process?
So that is what "a set of dynamic named values that can affect the way running processes will behave on a computer" means :)
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