Tip #1: Crontab syntax
I never remember the crontab syntax, so here it is for quick reference:
minute hour day of month month day of week command
* * * * * echo hello world!
Tip #2: ISP Disconnection Problem
One day my ISP gave me a dynamic IP (an IP that changes every time you
connect to the Internet). This was not a good solution from my point of
view, as I ran a webserver behind this DSL connection. To make things
even worse, my ISP disconnects me after X minutes of inactivity (and my
IP address changes again). I solved this problem by adding these lines
to my crontab:
0 0,2,4,6,8,10,12,14,16,18,20,22 * * * ping -c 3 server1.com >/dev/null 2>&1
0 1,3,5,7,9,11,13,15,17,19,21,23 * * * ping -c 3 server2.com >/dev/null 2>&1
This pings two servers, one each even hour and the other each odd hour,
hence preventing the disconnection. This is, of course, not the ideal
solution since it generates extra traffic and load and the administrators of
the site you're pinging will probably not like this.
Tip #3: File/Directory Dump
I wanted to automatically generate a file which would consist of a list
of all files and subdirectories under a certain directory. Since this
directory was a FTP upload directory for users, the contents of it changed
constantly. I decided to generate a simple text file using ls
every six hours. Here is what I inserted into my crontab:
0 0,6,12,18 * * * ls -lhABGR /mnt/data/upload/ > /mnt/data/upload/index.txt
This way the file is always overwritten. The format of this file is
similar to this:
-rw-r--r-- 1 ftp 63k May 14 2001 07.htm
-rw-r--r-- 1 ftp 36k May 14 2001 08.htm
-rw-r--r-- 1 ftp 33k May 14 2001 09.htm
-rw-r--r-- 1 ftp 1161k May 14 2001 mycar.mpg
See the man page for ls for more information.
Tip #4: Temporary File Upload Directory
I had the need for a directory where FTP users could upload temporary files
that would be deleted at a certain point, so that the temporary files
wouldn't take up all my disk space, as I noticed that users wouldn't delete
them anyway. I decided that one week would be a suitable amount of time for
these files, so each sunday at 11pm (when the user load is light), I delete
the entire contents of this directory with the cron job:
0 23 * * sun rm -r /mnt/data/temp/*; cp /var/www/docs/dumpheader.html /mnt/data/temp/HEADER.html >/dev/null 2>&1
Note that I also copy over a new Apache header file each time from a "safe"
location (i.e. from outside the temporary files' upload directory structure,
which is entirely wiped out each time). Of course, if you upload a file on,
for example, wednesday, it will be stored for only one day.
Tip #5: Backing up a directory using Tar
Since I do not have RAID harddrives mirroring each other all the time, I had
to invent some other way of creating backups of a specific directory. I also
decided to compress the data using Tar in order to save space.
Here is what I inserted in my crontab:
0 23 * * wed,sun tar -cf >/dev/null 2>&1
I decided that backing up twice per week (Wednesday and Sunday) would be
sufficient, since I did not have any mission critical data stored in this
directory.
Tip #6: Periodically mirroring a directory
Since I do not have RAID harddrives mirroring each other all the time, I had
to invent some other way of creating backups of a specific directory, and
besides, this way the server you are uploading to can be located on the other
side of the world, if so desired. Here I'm using lftp to upload the
files to another server (reverse mirroring). Here is what I inserted in my
crontab:
0 23 * * wed,sun lftp -e 'open www.mysecondserver.com -u username,password -p 21 &&
mirror --continue --reverse --delete /my/local/directory /my/remote/directory' >/dev/null 2>&1
I decided that backing up twice per week (Wednesday and Sunday) would be
sufficient, since I did not have any mission critical data stored in this
directory. Combined with the tip above, you could save some space. See the
lftp manual page for more info. You would probably want to use
rsync if there are larger amounts of data in question.
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