Environment variables

What you will be able to do

• Print HOME and PATH and explain what PATH is for
• Set a variable for the current session with export
• Know where API keys should and should not go

Why this matters for DE and AI

Programs look up configuration in the environment: where Python lives, which GPU is visible, what the warehouse password is. Later you will see CUDA_VISIBLE_DEVICES, AWS_PROFILE, and .env files. The rule starts now: secrets are not files you commit, and not text you paste into a website.

Concepts

An environment variable is a named value the shell and child processes can read.

echo $HOME
echo $PATH

$NAME expands the variable. HOME is your home directory. PATH is a list of directories, separated by :, where the shell looks for programs when you type a name.

That is why ./scripts/hello.sh needed ./. The scripts folder is not on PATH.

Set a variable for this terminal only:

export DCUBES_LAB=~/dcubes/linux-lab
echo $DCUBES_LAB

export makes it visible to programs you launch from this shell. Close the window, it is gone — unless you add the line to a startup file (.bashrc or .zshrc). We will not edit startup files in this lesson. One bad line can change every terminal.

.env files (you will meet them in Python) are a convention: a local file of NAME=value lines that tools load. They must stay out of git. Treat them like passwords.

Practice on your machine

echo $HOME
echo $USER
echo $PATH

What you should see: your home path, your username, and a long colon-separated list of directories.

export DCUBES_LAB=~/dcubes/linux-lab
ls $DCUBES_LAB

You should see data, logs, and scripts.

export GREETING=hello
echo $GREETING
bash -c 'echo $GREETING'

Because you exported it, the inner bash should also print hello.

Without export (try in a new thought experiment): GREETING=hi then bash -c 'echo $GREETING' may print nothing. Export is the difference.

Confirm hello.sh is not on PATH:

which hello.sh || echo "not on PATH — use ./scripts/hello.sh"

Common mistakes

• Spaces around =. export FOO = bar is wrong. export FOO=bar is right.
• Putting secrets in lesson notes or screenshots. Redact.
• Editing .bashrc by copying a random blog post. Skip that until you can read the file with less and know how to undo.

Next

What’s running, what’s full — disk, memory, and the two classic DE/AI failures.