Shell Default Values

The generic here for defining a value with a fallback looks like:

# If $may_exist is there, that will be used as the value.
# Otherwise, it's $fallback.
some_value=${may_exist:-fallback}

You can use this in functions to define default parameters:

print_price() {
	price=${1:-0}

	echo "\$${price} to buy this!"
}

print_price 5
print_price

Note

The syntax ${maybe:-fallback} will evaluate to fallback and do nothing else, BUT ${maybe:=fallback} will evaluate to fallback AND assign the value of maybe to fallback. As in:

# The value MAYBE is intentionally left unset
DEFINITELY=${MAYBE:="yeah"}

# Now it's set!
echo "$MAYBE" # "yeah"