if [ ! -x /usr/bin/axel ]
then echo "axel is not installed,perform this?(y/n)"
    read ops
    case $ops in
     y) if apt-get install axel -y --force-yes
           then echo "axel installed"
        else echo "unable to install the axel. you are using sudo?" ; exit
        fi ;;
     n) echo "not possible usage apt-fast" ; exit ;;
    esac
fi

“fi”的用法是什么在if块的中间？