Lab 18: Environment Variables

Objective

Understand, inspect, set, and use environment variables: PATH, HOME, USER, export, .bashrc, .profile, and how environment variables control program behaviour and pass configuration to applications.

Time: 25 minutes | Level: Foundations | Docker: docker run -it --rm ubuntu:22.04 bash

---

Step 1: Viewing Environment Variables

printenv | head -8

📸 Verified Output:

HOSTNAME=b77d3fb36284
PWD=/
HOME=/root
SHLVL=0
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
_=/usr/bin/printenv

echo "HOME:  $HOME"
echo "PATH:  $PATH"

📸 Verified Output:

HOME:  /root
PATH:  /usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin

💡 Variables are accessed by prefixing with $. $HOME expands to /root. Curly braces ${HOME} are equivalent and required when concatenating: ${HOME}_backup.

---

Step 2: The PATH Variable

PATH is a colon-separated list of directories searched when you type a command:

echo $PATH | tr ':' '\n'

📸 Verified Output:

/usr/local/sbin
/usr/local/bin
/usr/sbin
/usr/bin
/sbin
/bin

which bash
which ls
which python3

📸 Verified Output:

/usr/bin/bash
/usr/bin/ls
/usr/bin/python3

💡 The shell searches PATH left-to-right. Putting a directory first gives it priority. Security note: attackers who control a directory earlier in PATH can plant fake ls, cat, etc. — always verify PATH is trusted on compromised systems.

---

Step 3: Setting and Exporting Variables

# Local variable — only this shell session
MY_VAR="hello"
echo $MY_VAR

📸 Verified Output:

hello

# Export — child processes inherit it
export APP_ENV="production"
export DB_HOST="localhost"
export DB_PORT="5432"

env | grep -E '^(APP_ENV|DB_HOST|DB_PORT)'

📸 Verified Output:

APP_ENV=production
DB_HOST=localhost
DB_PORT=5432

💡 Without export, child processes (scripts you call) cannot see the variable. With export, it's in the environment — all child processes inherit it automatically.

---

Step 4: Variable Expansion and Defaults

# Default if unset
echo "${UNDEFINED:-fallback_value}"

📸 Verified Output:

fallback_value

# Command substitution
TODAY=$(date +%Y-%m-%d)
HOST=$(hostname)
echo "Date: $TODAY  Host: $HOST"

📸 Verified Output:

Date: 2026-03-05  Host: b77d3fb36284

# Arithmetic
X=10; Y=3
echo "Sum: $((X + Y))  Product: $((X * Y))  Modulo: $((X % Y))"

📸 Verified Output:

Sum: 13  Product: 30  Modulo: 1

---

Step 5: Unsetting Variables

export TESTVAR=42
echo "Set:   $TESTVAR"

unset TESTVAR
echo "Unset: [${TESTVAR:-empty}]"

📸 Verified Output:

Set:   42
Unset: [empty]

---

Step 6: Persistent Variables — .bashrc and .profile

# View current .bashrc (non-comment lines)
grep -v '^#' /root/.bashrc | grep -v '^$' | head -8

📸 Verified Output:

[ -z "$PS1" ] && return
HISTCONTROL=ignoredups:ignorespace
shopt -s histappend
HISTSIZE=1000
HISTFILESIZE=2000
shopt -s checkwinsize

# Add a persistent export
echo 'export MY_APP_KEY="dev-key-12345"' >> /root/.bashrc
grep MY_APP_KEY /root/.bashrc

📸 Verified Output:

export MY_APP_KEY="dev-key-12345"

💡 .bashrc loads for every new interactive shell. .profile loads for login shells. Changes take effect in new sessions — or run source ~/.bashrc to apply immediately in the current shell.

---

Step 7: Per-Command Environment

cat > /tmp/show_env.sh << 'EOF'
#!/bin/bash
echo "APP_ENV=${APP_ENV:-not set}"
echo "DB_HOST=${DB_HOST:-not set}"
echo "LOG_LEVEL=${LOG_LEVEL:-not set}"
EOF
chmod +x /tmp/show_env.sh

# Pass variables only to this one command
APP_ENV=staging DB_HOST=db.staging.example LOG_LEVEL=DEBUG /tmp/show_env.sh
echo ""
# They don't persist in the parent shell
echo "In parent: APP_ENV=${APP_ENV:-still not set}"

📸 Verified Output:

APP_ENV=staging
DB_HOST=db.staging.example
LOG_LEVEL=DEBUG

In parent: APP_ENV=still not set

---

Step 8: Capstone — 12-Factor App Config Validator

cat > /tmp/config_check.sh << 'SCRIPT'
#!/bin/bash
# 12-Factor App: store ALL config in environment variables
echo "=== Configuration Validation ==="

REQUIRED_VARS="DATABASE_URL SECRET_KEY"
OPTIONAL_VARS="LOG_LEVEL PORT"
MISSING=0

for var in $REQUIRED_VARS; do
    if [ -z "${!var}" ]; then
        echo "  ❌ MISSING (required): $var"
        MISSING=$((MISSING + 1))
    else
        MASKED="${!var:0:4}****"
        echo "  ✅ $var = $MASKED"
    fi
done

for var in $OPTIONAL_VARS; do
    VAL="${!var:-<default>}"
    echo "  ℹ️  $var = $VAL"
done

echo ""
if [ $MISSING -gt 0 ]; then
    echo "  ❌ $MISSING required variable(s) missing. Cannot start."
    exit 1
else
    echo "  ✅ All required config present. Starting app..."
fi
SCRIPT
chmod +x /tmp/config_check.sh

echo "--- Without vars ---"
/tmp/config_check.sh; echo ""

echo "--- With vars set ---"
DATABASE_URL="postgres://user:pass@localhost/mydb" \
SECRET_KEY="s3cr3tk3y-abc123" \
PORT=8080 \
/tmp/config_check.sh

📸 Verified Output:

--- Without vars ---
=== Configuration Validation ===
  ❌ MISSING (required): DATABASE_URL
  ❌ MISSING (required): SECRET_KEY
  ℹ️  LOG_LEVEL = <default>
  ℹ️  PORT = <default>

  ❌ 2 required variable(s) missing. Cannot start.

--- With vars set ---
=== Configuration Validation ===
  ✅ DATABASE_URL = post****
  ✅ SECRET_KEY = s3cr****
  ℹ️  LOG_LEVEL = <default>
  ℹ️  PORT = 8080

  ✅ All required config present. Starting app...

---

Summary

Command / Syntax	Purpose
echo $VAR	Print variable
export VAR=val	Set + export to child processes
unset VAR	Delete variable
printenv	Print all environment
env	Same as printenv
${VAR:-default}	Use default if unset
$(command)	Command substitution
VAR=x cmd	Set variable for one command only
source ~/.bashrc	Reload config in current shell
which cmd	Find command location in PATH