Objectives:

By the end of this chapter, you should be able to:

• Describe what a terminal environment is
• Create and modify terminal environment variables, including the PATH
• Save environment variables to a configuration file

What is your terminal's environment?

A terminal's environment is a list of settings that can be referenced by programs. To see what your terminal's environment looks like right now, try typing env in your terminal. You should see output similar to the following:

rvm_bin_path=/Users/tim/.rvm/bin
TERM_PROGRAM=Apple_Terminal
GEM_HOME=/Users/tim/.rvm/gems/ruby-2.3.1
TERM=xterm-256color
SHELL=/bin/bash
CLICOLOR=1
IRBRC=/Users/tim/.rvm/rubies/ruby-2.3.1/.irbrc
TMPDIR=/var/folders/5s/zstwqxy52pl_lq_lr3b5v7nc0000gn/T/
Apple_PubSub_Socket_Render=/private/tmp/com.apple.launchd.Q7TcyOvK4P/Render
TERM_PROGRAM_VERSION=361.1
OLDPWD=/Users/tim
TERM_SESSION_ID=281162A1-5C58-4285-9A35-AC9306923C34
USER=tim
__CF_USER_TEXT_ENCODING=0x1F5:0x0:0x0
LSCOLORS=GxFxCxDxBxegedabagaced
PATH=/usr/local/bin:/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin:/Users/tim/.rvm/bin
PWD=/Users/tim/Projects/Inf-Paces/prework
.
.
.

Each word on the left side of the equal sign is called an environment variable. The value on the right side is the value of the variable.

Using environment variables

In the terminal, you can see what value an environment variable has by using echo. When referencing an environment variable, you must use the $ as a prefix. In other words, to print the value for the environment variable PWD, the command would be:

echo $PWD

Try that in your terminal. You should get the same output as the output for the command pwd.

Creating environment variables

Now let's make our own environment variable. Go to your home directory and create a folder called Projects. Now let's make an environment variable called PROJDIR (environment variables are usually all caps) that keeps track of the path to your projects directory. To create an environment variable, you can use the export command. On a Mac or Linux machine, the command would be:

export PROJDIR=/Users/tim/Projects

Notice that the $ isn't being used in this case. When you define an environment variable, you do not use the $. Only use the $ when you want to reference the value of the variable.

Now that you've created the environment variable, let's use it:

cd ~
cd $PROJDIR

You should now be in your project directory.

So now we have a great way of saving a useful variable in our terminal's environment, but we have a problem. Every time you close your terminal window, the environmet variables get reset, so the PROJDIR environment variable will be lost! How do we fix that?

Saving environment variables

Now that we know how to create environment variables, we need to learn how to save them so that every time we open a new terminal window, we have those environment variables set. To save environment variables, you need to modify the shell configuration file in your home directory. This file is different depending on what your default shell is. If you are using oh-my-zsh, then your configuration file will be called .zshrc; if you are using bash, then your configuration file will be called .bash_profile.

Open the configuration file for your shell, either .zshrc or .bash_profile. Next, add the following line to your file:

export PROJDIR=/Users/$USER/Projects

Save the file, quit out of all terminal windows, and then open terminal again. Try executing echo $PROJDIR. You should see the path to your projects directory.

We did one other interesting thing here. Rather than using a hard coded path to the projects directory, we used another environment variable to figure out the correct user name. Try typing echo $USER in your terminal. You should see your user name. The important takeaway here is that an environment variable can be defined using other environment variables. For example, if we had a python folder inside of the Projects directory, we may want a variable for that as well. We could definite it like the following way:

export PYTHON_PROJ=/Users/$USER/Projects/python

Or we could use the PROJDIR environment variable that we already have set:

export PYTHON_DIR=$PROJDIR/python

The only catch is that the line for exporting PROJDIR must come before the line using PROJDIR. Otherwise, PROJDIR will not yet be defined when we use it in the PYTHON_DIR definition.

The PATH environment variable

An important environment variable to know and understand is the PATH. Your terminal uses the PATH environment variable to find programs to execute. Try the following in a new terminal window:

export PATH=

Now try using ls in the terminal. It doesn't work! Try a few other commands like man or chgrp. None of them work. That's because commands like ls are just programs stored in a file somewhere in your filesystem. The reason we don't normally need to give the full path to the ls command when we use it is because ls is a file found in one of the folders that are specified on the path.

Open a fresh terminal window. The ls command should be working again. Now use the which command to see where on the path ls is coming from:

which ls

Typically, the ls command is located in the /bin directory (though if you're using oh-my-zsh, the command may be aliased to ls -G). Let's change our PATH environment variable again, but this time, let's assign it to /bin:

export PATH=/bin

Now try to use the ls command. It should still work! That's because ls can now be found in one of the folders of the PATH, specifically in /bin. Other commands still don't work however. The man command still isn't working because it is not found on the PATH. Let's add a few more directories to the PATH in the same terminal window. We want to add /usr/bin, /usr/sbin, and /sbin, but we don't want to rewrite the PATH variable completely. Instead, let's add to the PATH that already exists. In order to do that, we reference the PATH environment variable using the $ and separate multiple paths using a colon:

export PATH=$PATH:/usr/bin:/usr/sbin:/sbin

Now if you do echo $PATH, you should see the following output:

/bin:/usr/bin:/usr/sbin:/sbin

And commands like man should work. Now that you understand the PATH, close the terminal window and open a fresh window, so that your PATH will be set up correctly.