Default variable values with zsh

zsh has two modifiers (${:-} and ${:=}) to handle fallback for empty values.

echo ${ahead:-0} will display the variable $ahead, or display 0 if the variable is empty.

echo ${ahead:=0} (note the := instead of :-) will assign 0 to the variable $ahead, and then display it.

They are pretty similar, and in that example the result is the same. But with :=, the variable will still be set to 0 afterward, while with :- it's a one-off thing.