crontab中的%需转义

crontab条目中包含%号，最常见的取时间，如：date +%d，
对%需要使用\进行转义，否则不能按预期执行，正确做法为：
* * * * * echo "`date +\%d`" > /tmp/r1r.txt
而不能为
* * * * * echo "`date +%d`" > /tmp/r1r.txt

%是crontab的特殊字符，所有%后的被当作了标准输入，这可以通过“man 5 crontab查看到说明：
The entire command portion of the line, up to a newline or a "%" character,
will be executed by /bin/sh or by the shell specified in the SHELL variable of the cronfile.
A "%" character in the command, unless escaped with a backslash (\),
will be changed into newline char-acters,
and all data after the first % will be sent to the command as standard input.

示例：
$ cat /tmp/hello.txt
cat: /tmp/hello.txt: 没有那个文件或目录
$ echo -e "`crontab -l`\n* * * * * cat > /tmp/hello.txt % hello word"|crontab -
$ crontab -l|grep hello.txt
* * * * * cat > /tmp/hello.txt % hello word
$ cat /tmp/hello.txt
hello word