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Linux Full Tutorial -- Part 10: Environment Variables

By Suraj Ahir 2025-12-21 11 min read

Environment variables are how Linux processes receive configuration. Your PATH tells the shell where to find commands. DATABASE_URL tells your application how to connect to the database. AWS_ACCESS_KEY_ID gives the AWS CLI your credentials. Understanding how environment variables work, where to set them, and how they flow from shell to process is essential Linux knowledge.

Viewing Environment Variables

env                    # Show all environment variables
printenv               # Same as env
printenv PATH          # Show specific variable
echo $PATH             # Print variable value
echo $HOME             # Home directory
echo $USER             # Current username
echo $HOSTNAME         # Machine hostname
echo $SHELL            # Current shell
echo $PWD              # Current directory
echo $?                # Exit code of last command
echo $$                # Current process ID

Setting and Exporting Variables

# Set in current shell only
MY_VAR="hello"
echo $MY_VAR    # hello

# Export makes it available to child processes
export MY_VAR="hello"
export DATABASE_URL="postgresql://user:pass@localhost/mydb"
export PORT=8000

# Set and export in one step
export APP_ENV="production"

# Unset a variable
unset MY_VAR

# Set for single command only (no export needed)
DEBUG=true python3 app.py  # DEBUG only available to this command

The PATH Variable

echo $PATH
# /usr/local/bin:/usr/bin:/bin:/usr/local/sbin:/usr/sbin

# Add a directory to PATH
export PATH="$HOME/.local/bin:$PATH"   # Prepend (higher priority)
export PATH="$PATH:/usr/local/myapp"   # Append (lower priority)

# Which directory does a command come from?
which python3      # /usr/bin/python3
which pip3         # /usr/local/bin/pip3
type -a python3    # Shows all matches in PATH order

Configuration Files

# ~/.bashrc -- interactive non-login shells (most terminals)
# ~/.bash_profile or ~/.profile -- login shells (SSH sessions)
# /etc/environment -- system-wide, all users (simple KEY=VALUE format)
# /etc/profile.d/*.sh -- system-wide scripts

# Add to ~/.bashrc for permanent user settings:
echo 'export MY_VAR="value"' >> ~/.bashrc
echo 'export PATH="$HOME/.local/bin:$PATH"' >> ~/.bashrc

# Reload without logging out
source ~/.bashrc
. ~/.bashrc       # Same thing, shorter syntax

# System-wide environment (/etc/environment)
# Format: KEY=VALUE (no export, no $)
PATH="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin"
LANG="en_US.UTF-8"

Environment Variables for Applications

# .env file (never commit to git)
DATABASE_URL=postgresql://user:pass@localhost/mydb
SECRET_KEY=your-secret-key-here
DEBUG=false
PORT=8000

# Load in bash script
set -a           # Mark all variables for export
source .env
set +a

# Or use export with each line
export $(grep -v "^#" .env | xargs)

# In systemd service -- add environment file
[Service]
EnvironmentFile=/home/ubuntu/myapp/.env
ExecStart=/usr/bin/python3 app.py

Frequently Asked Questions

In Part 11, we cover disk management -- understanding and managing storage on Linux servers.

Written by

Suraj Ahir

Cloud & DevOps engineer running four live production services on my own AWS infrastructure. I write everything on this site myself — no ghostwriters, no AI filler.

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Environment Variables in Docker and Kubernetes

# Docker: pass environment variables
docker run -e DATABASE_URL=postgresql://... myapp

# Docker: load from file
docker run --env-file .env myapp

# Docker Compose: environment section
services:
  myapp:
    image: myapp:latest
    environment:
      - DATABASE_URL=${DATABASE_URL}   # From host environment
      - APP_ENV=production              # Literal value
      - PORT=8000
    env_file:
      - .env.production

# Kubernetes: ConfigMap for non-sensitive config
apiVersion: v1
kind: ConfigMap
metadata:
  name: myapp-config
data:
  APP_ENV: production
  PORT: "8000"
  LOG_LEVEL: INFO

# Kubernetes: Secret for sensitive config
apiVersion: v1
kind: Secret
metadata:
  name: myapp-secrets
type: Opaque
data:
  DATABASE_PASSWORD: c2VjcmV0MTIz  # base64 encoded

# Use in deployment
env:
  - name: APP_ENV
    valueFrom:
      configMapKeyRef:
        name: myapp-config
        key: APP_ENV
  - name: DB_PASSWORD
    valueFrom:
      secretKeyRef:
        name: myapp-secrets
        key: DATABASE_PASSWORD

Debugging Environment Variable Issues

# Check environment of a running process
cat /proc/$(pgrep myapp)/environ | tr '' '
'

# Verify variable is set before using it
: "${DATABASE_URL:?DATABASE_URL must be set}"  # Exits with error if not set

# Variable substitution with defaults
DB_HOST="${DATABASE_HOST:-localhost}"
DB_PORT="${DATABASE_PORT:-5432}"

# Debug: show all environment variables sorted
env | sort

# Check if variable is exported (available to child processes)
export -p | grep MY_VAR

# Print a variable with surrounding context for debugging
echo "DATABASE_URL is: [${DATABASE_URL}]"  # Brackets show whitespace