%m month (01..12)

%M minute (00..59)

%n a newline

%N nanoseconds (000000000..999999999)

%p locale’s equivalent of either AM or PM; blank if not known

%P like %p, but lower case

%r locale’s 12-hour clock time (e.g., 11:11:04 PM)

%R 24-hour hour and minute; same as %H:%M

%s seconds since 1970-01-01 00:00:00 UTC

%S second (00..60)

…

%n就是纳秒数，%s是1970以来的秒数，如果只是想要毫秒就自己cut一下就ok了。

1秒s = 1000 毫秒ms

1毫秒ms = 1000 微秒us

1微秒us = 1000 纳秒ns

1纳秒ns = 1000 皮秒ps

1毫秒 就等于 1000 * 1000 纳秒

下面这个我是懒得cut了，直接除以以一百万就是了。

cat count.sh

START=`date +%s%N`;