How does ${2:-} work in an 'if-then-else example' below?  Somehow I cannot help but think that being ${2:-} it means the second argument, but I am curious what the colon(:) and dash(-) after the digit 2 mean?  if [ ${2:-} = Y ]; then     prompt=Y/n     default=Y  elif [ ${2:-} = N ]; then     prompt=y/N     default=N  else     prompt=y/n     default=  fi  read -p $1 [$prompt]  REPLY </dev/tty
Variable with colon, dash in an if-then-else: `if [ ${2:-} = Y ]`
shell;arguments;variable substitution