Complete Environment Variable Guide — .bashrc · .zshrc · export · env

# Set (current session only)
export MY_VAR="hello"
export PORT=3000

# Inspect
echo $MY_VAR
printenv MY_VAR

# List all environment variables
env
printenv
export -p

# Unset
unset MY_VAR

# Default if the variable is unset
echo ${MY_VAR:-"default"}

# Check whether the variable exists
[[ -z "$MY_VAR" ]] && echo "unset" || echo "set"

# Add to ~/.bashrc or ~/.zshrc
export PATH="$HOME/.local/bin:$PATH"
export JAVA_HOME="/usr/lib/jvm/java-21-openjdk-amd64"
export EDITOR="vim"

# Apply
source ~/.bashrc
# or
. ~/.zshrc

# Add a directory to PATH
export PATH="$HOME/bin:$PATH"        # prepend (higher priority)
export PATH="$PATH:/opt/custom/bin"  # append

# Show current PATH
echo $PATH | tr ':' '\n'

# Locate a command
which python3
type -a python3    # show all locations

# Prefixing applies only to that command
NODE_ENV=production PORT=8080 node server.js

# Same thing via env
env NODE_ENV=production PORT=8080 node server.js

# Run with a clean environment
env -i PATH=$PATH node server.js

# Example .env file
DB_HOST=localhost
DB_PORT=5432
DB_USER=app
DB_PASS=secret
NODE_ENV=development

# Load a .env file (bash)
set -a; source .env; set +a

# Load only variable assignments
export $(grep -v '^#' .env | xargs)

# Apply .env to the current session
while IFS= read -r line; do
  [[ "$line" =~ ^[^#] ]] && export "$line"
done < .env

# /etc/systemd/system/myapp.service
[Service]
Environment=NODE_ENV=production
Environment=PORT=3000
EnvironmentFile=/etc/myapp/.env   # load from a file

systemctl daemon-reload
systemctl restart myapp
# Verify
systemctl show myapp --property=Environment