crontab

출처: http://www.debianhelp.co.uk/crontab.htm

Run programs at startup via crontab

There is an easy way to start a program during system boot. Just put this in your crontab: 

@reboot /path/to/my/program

The command will be executed on every (re)boot. Crontab can be modified by running 

#crontab -e

Other available Options

string             meaning
------           -----------
@reboot      Run once, at startup.
@yearly       Run once a year, "0 0 1 1 *".
@annually    (same as @yearly)
@monthly    Run once a month, "0 0 1 * *".
@weekly     Run once a week, "0 0 * * 0".
@daily        Run once a day, "0 0 * * *".
@midnight   (same as @daily)
@hourly      Run once an hour, "0 * * * *".

More information about crontab options is available in the man page check here

How to Use 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 

Pulling the crontab entry apart there is: 

the echo command sends the MySQL command to STDOUT where it is piped into 
sed which removes any back slashes before sending the output to STDOUT where it is piped into 
the mysql command processor which reads its commands from STDIN