Shell Variable Default Values: Using ${var:-default} and ${var:=default}

Parameter Expansion for Default Values

Bash provides parameter expansion operators that let you handle unset or empty variables gracefully. These are essential for writing robust scripts that handle missing configuration or arguments.

The Four Operators

Syntax	Meaning
${var:-default}	Use default if var is unset or empty
${var:=default}	Assign default if var is unset or empty
${var:+alternate}	Use alternate if var IS set and non-empty
${var:?error msg}	Exit with error if var is unset or empty

${var:-default} — Use Default

# Use /tmp if TMPDIR is not set
workdir="${TMPDIR:-/tmp}/myapp"
mkdir -p "$workdir"

# Default port
port="${PORT:-8080}"
echo "Listening on port $port"

${var:=default} — Assign Default

# Set and use the default
: "${LOG_LEVEL:=info}"
echo "Log level: $LOG_LEVEL"
# LOG_LEVEL is now "info" if it was previously unset

${var:?error} — Require Variable

# Exit with error if required variables are missing
: "${DATABASE_URL:?DATABASE_URL must be set}"
: "${API_KEY:?API_KEY is required}"

# Safe rm with required variable
rm -rf "${DEPLOY_DIR:?DEPLOY_DIR not set}/old_release"

${var:+alternate} — Conditional Value

# Only add --verbose flag if DEBUG is set
curl ${DEBUG:+--verbose} https://api.example.com

# Conditional configuration
docker run ${NETWORK:+--network "$NETWORK"} myimage

Without the Colon

Dropping the colon (${var-default}) only checks if the variable is unset, not if it is empty. This is useful when empty string is a valid value.