On Tue, 28 Aug 2001 16:05:32 +0200, Jorge Otero mumbled something
like:
>>I was wondering if it would be a good idea to backup the apache
>>conf files
>>several times a day using cron,
Once a day would be sufficient if you want to do this; but the GUI
keeps a copy of the config files in its postgresql database.
>>if there's any way to backup the
>>conf files
>>automatically when editing them with pico,
Before you start to edit the file, make a backup of it:
cp filename.ext filename.ext.bak
>>or if there's something
>>wrong
>>with doing this.
It's never a bad thing to CYA (cover your ass).
>>P.S.: I've never used cron
Well, here you go:
The crontab
As published on portico.org, 29 November 1998.
George Lewis (schvin@xxxxxxxxxx)
Okay, a certain moron friend of mine, aires, has requested that
someone explain the crontab to him.
Some of you may be familiar with at, a collection of programs that
lets you setup programs for programmed running, i.e., to run it at a
certain time, or to put various jobs in a queue. The cron daemon
(crond) allows you to assign times to various jobs.
There is a "crontab" for each user on your system, the list of
programs and when they should be executed. The cron system is really
a great thing, because it allows you to have the system do various
tasks for you at set times, even if you aren't there or logged in.
And like most other things under Linux, other users of your computer
can't mess with your settings, unless they happen to have root access
(administrative rights) on your computer.
I am running crond v2.2. Here are the basic options:
crontab -l user (list the crontab for that user)
crontab -e user (edit the crontab for that user)
crontab -d user (delete that user's crontab)
Now, let's create a situation, say I'm user joe and want to have my
system play an MP3, named paranoid_android.mp3 every morning at 6:30
AM.
I'd go to a prompt, and type crontab -e to open up your crontab.
Unless you are root, there will probably be nothing there, it will
just open up vi with your new crontab file.
If you are not familiar with vi, it is a very powerful text editor
that exists on nearly all unix-based computers out there. It is a
good thing to be able to use, if not know thoroughly. Now that you
have a file opened in it, you can press i to put you into insert
mode. Once you are done with a file in vi, press the escape key and
then type :x to exit and save the changes. If you want to discard the
changes, put in :q! instead.
Now, back to editing our crontab. The crontab's format goes like
this:
minutes
hour
day
month
day of week
command
For fields that aren't relevant, such as day of week in this example,
we can just put an asterisk (*). I play my MP3s with mpg123, so this
is how the line should be entered in the crontab:
30 6 * * * mpg123 /home/joe/paranoid_android.mp3
I put in 30 for minutes, and 6 for hours, and asterisks for the rest,
so it will run that command every day of the year at 6:30 AM. Note
that the time is in 24-hour format, so if you wanted it to play at
6:00 PM you would put 18 in the hours field. Those are just spaces in
between the fields, and several in between the day of week and
command. It is just sort of standard to put multiple spaces there,
but isn't really necessary as far as I know.
mpg123 is the program I'm using to play my MP3 again, and the file,
paranoid_android.mp3 is stored in my home directory /home/joe/ -- so
hopefully that makes sense.
Now, let's get fancy, what if you want the MP3 to play at 6:45 to
wake you up after you've ignored the first one. You can just add a
comma (without a space) and put the other minutes or hours that you
want it to play at. For example:
30,45 6 * * * mpg123 /home/joe/paranoid_android.mp3
By default, crond will email you the results of the cronjob, so if
you don't want an email every morning reminding you of how the
computer had the nerve to wake you up, you can have it port the
results to your system log. This is generally stored as
/var/log/messages.
To port something to your system log, you would just put the normal
command, and then at the end you would add >> /var/log/system. So
this is what our finished command would look like:
30,45 6 * * * mpg123 /home/joe/paranoid_android.mp3 >>
/var/log/messages
One thing to note about the porting thing, if you put one bracket
(>), it will completely overwrite the existing file, whereas, if you
put two brackets (>>), it will append the output to the end of the
existing file. That is now going to run at 6:30 and 6:45, and play
paranoid_android.mp3, and not tell us about it later, but just do it.
This should give you the basics of the crontab, it is a very useful
device. Besides an alarm clock, I have also use it to automatically
synchronize my system's time with the NIST servers, so that every day
my system time is accurate within a second or so. These are just
basic uses of the cron daemon, but it can be used for just about
anything.
--
CarrieB
"Wherever is found what is called a paternal government, there is
found state education. It has been discovered that the best way to
insure implicit obedience is to commence tyranny in the nursery."
--Benjamin Disraeli, British Prime Minister 1868 and 1874-1880