19.md

解答

$ printf xxxx-xxxx 1<> cardno

別解

別解1（山田）$ IFS=-;a=($(<cardno));echo xxxx-xxxx-${a[2]}-${a[3]} > cardno
別解2（上田）$ a=$(<cardno);echo xxxx-xxxx-${a:10} > cardno
別解3（上田）$ ( read a ; echo xxxx-xxxx-${a:10} > cardno ) < cardno