#!/bin/sh
echo "Your username: $USER"
echo "Your home directory: $HOME"
echo "Current directory: $PWD"

#!/bin/sh
# A regular shell variable — only visible here
SECRET="I am local"
echo "Parent sees SECRET: $SECRET"

# A child shell cannot see it
echo "Child sees SECRET:"
sh -c 'echo "  ${SECRET:-<not visible>}"'

# Export promotes it to an environment variable
export SECRET
echo ""
echo "After export:"
echo "Parent sees SECRET: $SECRET"

# Now the child shell can see it
echo "Child sees SECRET:"
sh -c 'echo "  $SECRET"'

#!/bin/sh
# Method 1: Shell-only variable
DB_NAME="mydb"
echo "Method 1 - shell variable:"
echo "  Parent: DB_NAME=$DB_NAME"
sh -c 'echo "  Child:  DB_NAME=${DB_NAME:-<not visible>}"'

# Method 2: Set and export in one step
echo ""
echo "Method 2 - export in one step:"
export DB_HOST="localhost"
echo "  Parent: DB_HOST=$DB_HOST"
sh -c 'echo "  Child:  DB_HOST=$DB_HOST"'

# Method 3: Set first, export later
echo ""
echo "Method 3 - export on a separate line:"
DB_PORT="5432"
export DB_PORT
echo "  Parent: DB_PORT=$DB_PORT"
sh -c 'echo "  Child:  DB_PORT=$DB_PORT"'

#!/bin/sh
echo "USER:  $USER"
echo "HOME:  $HOME"
echo "PWD:   $PWD"
echo "PATH:  $PATH"
echo "SHELL: $SHELL"
echo "TERM:  $TERM"
echo "LANG:  $LANG"

#!/bin/sh
# Test flags for conditional logic
echo "Checking with test flags:"
if [ -n "$HOME" ]; then
    echo "  HOME is set to: $HOME"
fi
if [ -z "$UNDEFINED_VAR" ]; then
    echo "  UNDEFINED_VAR is not set"
fi

# ${VAR:-default} — provide a fallback without changing VAR
echo ""
echo "Fallback values with \${VAR:-default}:"
echo "  DB_HOST: ${DB_HOST:-localhost}"
echo "  DB_PORT: ${DB_PORT:-5432}"

# ${VAR:=default} — provide a fallback AND assign it
echo ""
echo "Assign defaults with \${VAR:=default}:"
: "${CACHE_DIR:=/tmp/cache}"
echo "  CACHE_DIR is now: $CACHE_DIR"

# ${VAR:+alternate} — use a value only if VAR is set
echo ""
echo "Alternate values with \${VAR:+alternate}:"
echo "  USER is set: ${USER:+yes}"
echo "  MISSING is set: ${MISSING_VAR:+yes}"

# ${VAR:?message} — require a variable or exit
# Uncommenting the line below would exit the script
# if REQUIRED_CONFIG is not set:
#   : "${REQUIRED_CONFIG:?must be set in environment}"
echo ""
echo "\${VAR:?message} exits if VAR is unset (commented out above)"

#!/bin/sh
# Inline assignment: VAR exists only for the command
echo "Inline assignment (VAR=value command):"
GREETING="Hola" sh -c 'echo "  During: GREETING=$GREETING"'
echo "  After:  GREETING=${GREETING:-<gone>}"

# The env utility does the same thing
echo ""
echo "Using env:"
env COLOR="blue" sh -c 'echo "  During: COLOR=$COLOR"'
echo "  After:  COLOR=${COLOR:-<gone>}"

# Modifying PATH for the current session
echo ""
echo "Modifying PATH temporarily:"
echo "  PATH has $(echo "$PATH" | tr ':' '\n' | wc -l | tr -d ' ') entries"
PATH="$HOME/bin:$PATH"
echo "  After prepend, PATH starts with: ${PATH%%:*}"

#!/bin/sh
# unset removes a variable entirely
export TEMP_VAR="temporary"
echo "Before unset: TEMP_VAR=$TEMP_VAR"
unset TEMP_VAR
echo "After unset:  TEMP_VAR=${TEMP_VAR:-<removed>}"

# env with no arguments lists all exported variables
echo ""
echo "Exported variables (first 5):"
env | sort | head -5

# env -i runs a command with an empty environment
# Here we pass only PATH so the child can find commands
echo ""
echo "Clean environment with env -i:"
env -i PATH="$PATH" sh -c '
    count=$(env | wc -l | tr -d " ")
    echo "  Variables in clean env: $count"
    echo "  PATH is set: ${PATH:+yes}"
    echo "  USER is set: ${USER:-<not set>}"
'

#!/bin/sh
# Create a sample config file
cat > /tmp/app.env <<'EOF'
APP_HOST=0.0.0.0
APP_PORT=3000
APP_DEBUG=false
EOF

# Source without set -a: variables are shell-only
. /tmp/app.env
echo "Without set -a:"
echo "  Parent sees APP_HOST: $APP_HOST"
sh -c 'echo "  Child sees APP_HOST:  ${APP_HOST:-<not visible>}"'

# Source with set -a: variables are auto-exported
set -a
. /tmp/app.env
set +a

echo ""
echo "With set -a:"
echo "  Parent sees APP_HOST: $APP_HOST"
sh -c 'echo "  Child sees APP_HOST:  $APP_HOST"'

# A config function with overridable defaults
echo ""
echo "Config function with defaults:"
show_config() {
    echo "  APP_PORT=${APP_PORT:-3000}"
    echo "  APP_HOST=${APP_HOST:-0.0.0.0}"
    echo "  APP_DEBUG=${APP_DEBUG:-false}"
}

show_config
echo ""
echo "Overridden via inline variables:"
APP_PORT=8080 APP_DEBUG=true show_config

rm /tmp/app.env

#!/bin/sh
# Demonstrating the risk: child processes see everything
export API_KEY="secret123"
echo "Child process can see the secret:"
sh -c 'echo "  API_KEY=$API_KEY"'
unset API_KEY

# Better approach: use a file with restricted permissions
echo ""
echo "Storing a secret in a file instead:"
echo "my_secret_token" >/tmp/secret.txt
chmod 600 /tmp/secret.txt

# Only the file owner can read it
echo "  Permissions: $(ls -l /tmp/secret.txt | cut -d' ' -f1)"

# Read the secret only when needed
SECRET=$(cat /tmp/secret.txt)
echo "  Secret read from file: ${SECRET}"

rm /tmp/secret.txt
echo "  Cleaned up secret file"