Shellのハイフンは標準入出力を表す
http://tldp.org/LDP/abs/html/special-chars.html#DASHREF2
CRONTAB
(1)だと
-
はpseud-filenameと言われている
The first form of this command is used to install a new crontab from some named file or standard input if the pseudo-filename `-' is given.
#Shell