Linux的cron与%

这个cron不能执行：

* * * * * /bin/echo `/bin/date +"%Y-%m-%d-%T"` >> /home/adminuser/test.txt 2>&1

需要修改为： 因为%在cron中需要被转意

*/1 * * * * /bin/echo /bin/date +"\%Y-\%m-\%d-\%T" >> /home/adminuser/test.txt 2>&1

检测disk和folder的使用率：

*/15 * * * * echo `date +"\%Y-\%m-\%d-\%T"` /home/adminuser `/bin/df -h /home/adminuser | /bin/grep /dev/sda1 | awk -F' ' '{print "free:"$3" usage:"$4}'` >> /home/adminuser/df.txt
*/30 * * * * echo `date +"\%Y-\%m-\%d-\%T"` `/usr/bin/du -h --max-depth=0 /home/adminuser` `date +"\%Y-\%m-\%d-\%T"` >> /home/adminuser/du.txt

详细解释：

The purpose of this document is to explain how to cope with the percentage sign (%) in a crontab entry.

Usually, a % is used to denote a new line in a crontab entry. The first % is special in that it denotes the start of STDIN for the crontab entry's command. A trivial example is:

* * * * * cat - % another minute has passed

This would output the text

another minute has passed

After the first %, all other %s in a crontab entry indicate a new line. So a slightly different trivial example is:

* * * * * cat - % another % minute % has % passed
This would output the text

another
minute
has
passed

Note how the % has been used to indicate a new line.

The problem is how to use a % in a crontab line to as a % and not as a new line. Many manuals will say escape it with a \. This certainly stops its interpretation as a new line but the shell running the cron job can leave the \ in. For example:

* * * * * echo '\% another \% minute \% has \% passed'

would output the text

\% another \% minute \% has \% passed

Clearly, not what was intended.

A solution is to pass the text through sed. The crontab example now becomes:

* * * * * echo '\% another \% minute \% has \% passed'| sed -e 's|\\||g'

This would output the text

% another % minute % has % passed

which is what was intended.

This technique is very useful when using a MySQL command within a crontab. MySQL command can often have a % in them. Some example are:

SET @monyy=DATE_FORMAT(NOW(),"%M %Y")

SELECT * FROM table WHERE name LIKE 'fred%'

So, to have a crontab entry to run the MySQL command

mysql -vv -e "SELECT * FROM table WHERE name LIKE Fred%'" member_list

would have to appear in the crontab as

echo "SELECT * FROM table WHERE name LIKE 'Fred\%'" | sed -e 's|\\||g' | mysql -vv member_list