CodingNic
Sign In Book Session

Environment, Process and Finding

Terminal Environment

Environment, Process and Finding 18 min read

Terminal Environment

Objectives

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

  • Describe what a terminal environment is
  • Create and use environment variables, including reading the PATH
  • Save an environment variable so it survives closing your terminal

💡 Why this matters: Environment variables quietly control how your shell behaves — which programs it can find, what your prompt looks like, where temporary files go. When a command mysteriously “isn’t found,” or a setting resets every time you open a new window, this is almost always why.

What Is Your Environment?

Your terminal’s environment is a list of settings that programs can read. Run env right now and you’ll see a long list like this:

text
TERM=xterm-256color
SHELL=/bin/zsh
USER=sam
PWD=/Users/sam/Projects/practice
PATH=/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin
...

Everything to the left of each = is a variable name; everything to the right is its value.

Reading a Variable

echo, with a $ in front of the variable name, prints its value:

bash
echo $PWD

That should match whatever pwd on its own prints — PWD is just the environment’s copy of your current directory.

Creating Your Own Variable

Make a folder to work with:

bash
mkdir ~/Projects

Now define a variable that points at it, using export:

bash
export PROJDIR=/Users/sam/Projects

Notice there’s no $ when you define a variable — only when you read one. Try it out:

bash
cd ~
cd $PROJDIR

That should drop you straight into ~/Projects. One catch: close this terminal window and the variable is gone. Every new shell session starts with a fresh environment.

Making It Permanent

To keep a variable around across sessions, add it to your shell’s configuration file — ~/.zshrc for Zsh, ~/.bash_profile for Bash. Open that file and add:

bash
export PROJDIR=/Users/$USER/Projects

Save it, quit every terminal window, and reopen one. echo $PROJDIR should now print the path without you having to redefine anything.

Notice $USER inside that line — a variable can reference another variable. That means you can build on variables you’ve already defined:

bash
export PYTHON_DIR=$PROJDIR/python

The only rule: the line defining PROJDIR has to come before the line that uses it, or PROJDIR won’t exist yet when PYTHON_DIR is being set.

The PATH Variable

PATH is the environment variable your shell uses to find programs to run. To see what happens when it’s empty, try this in a fresh terminal window (so you don’t lose your working PATH permanently):

bash
export PATH=

Now try ls. It fails — along with man, chgrp, and almost everything else. That’s because commands like ls are just program files sitting somewhere on disk, and without PATH, your shell has no idea where to look.

Open a new terminal window to get a working PATH back, then check where ls actually lives:

bash
which ls

That’s usually /bin/ls. Now try narrowing PATH down to just that folder:

bash
export PATH=/bin

ls still works — /bin is on the PATH again — but man doesn’t, since it lives elsewhere. Rather than overwriting PATH completely, you can extend it, using $PATH to keep what’s already there and a colon to separate entries:

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

man works again, and echo $PATH shows every folder your shell now searches, colon-separated. Once you’re done experimenting, close this window and open a fresh one so your real PATH is back to normal.

Try It

Environment variables are shell state, not files — there’s no sandbox for this, so work through these in your own terminal:

  1. Create a variable called MY_NAME set to your name, then print it.
  2. Add a line to your shell config file that sets a variable permanently, then confirm it survives a fresh terminal window.
  3. Run which on two or three commands you use often (ls, cat, grep) and note which folder each one comes from.

Recap

  • The environment is a list of NAME=value settings your shell and programs can read.
  • Define a variable with export NAME=value; read one with echo $NAME.
  • A variable set in a terminal window disappears when you close it — add it to ~/.zshrc or ~/.bash_profile to make it permanent.
  • PATH is the list of folders your shell searches for programs; break it and even ls stops working.

Next lesson: what a process actually is, and how to see — and stop — one from Terminal.