Default values for variables

The single most useful parameter expansion is giving a variable a fallback value when it isn't set. You'll write this pattern in every script from now on.

The four default operators

${var:-default}     # if var is unset or empty, use "default" (var itself unchanged)
${var:=default}     # same, but also assigns "default" to var
${var:?message}     # if var is unset or empty, fail with "message"
${var:+alternate}   # opposite: if var IS set, use "alternate" instead

Each is a single expression. No braces or if statements needed.

The everyday pattern

DB_URL="${DB_URL:-postgres://localhost:5432/dev}"
LOG_DIR="${LOG_DIR:-/var/log/myapp}"
PORT="${PORT:-8080}"

Read as: "use $DB_URL if the caller set it, otherwise fall back to the local dev URL". This is how nearly every well-behaved script handles configuration from environment variables.

:= for one-time assignment

name=${name:=guest}
echo "$name"        # guest
name=${name:=admin}
echo "$name"        # guest (still, because it was already set)

:= sets the variable AND returns the value, but only the first time. Handy in loops where you want to remember what got assigned.

:? for required variables

API_KEY="${API_KEY:?API_KEY must be set before running this script}"

If API_KEY is unset, the script exits with your message. No if [[ -z ... ]] boilerplate needed. Perfect for production scripts where a missing value should stop everything before it causes silent damage.

:+ for optional flags

DEBUG="${DEBUG:+--verbose}"
some-command $DEBUG

If DEBUG is set to anything, the value becomes --verbose. If unset, the value is empty and the command runs normally. Useful for optional flags controlled by an env var.

Why the colon matters

The colon in :-, :=, etc. means "unset OR empty". Without the colon, only truly unset variables trigger the fallback:

name=""
echo "${name:-guest}"      # guest (name is empty, colon triggers fallback)
echo "${name-guest}"       # (empty string, name is set even though empty)

Almost always use the colon form. "Empty string" and "unset" usually mean the same thing in a script's mental model.