Crontab
The agony
Crontab can be a little tricky, but if you follow these simple
steps.... OK. They may not seem so simple to everyone, but really, just follow along, & you'll
have all sorts of annoying tasks lifted from your daily reutine all being done automatically
with little or no notice on your part.
Gotta tell ya, vi is, in my opinion, highly over rated. Sure, it
does a lot, & a coder can set it up to do lots of things automatically. But talk about bloat
-- & all the esoteric commands! Call me a weenie, but Pico's so much simpler, does everything
I've ever needed (even spell check) & more that I use nothing else. I don't even
use any office products -- unless I really have to. So, I say, don't use vi
unless you really want the hassles.
Now, by entering man 5 crontab at a prompt ($ man 5 crontab), the
first sentence tells us this:
"A crontab file contains instructions to the cron(8) daemon of the general
form: 'run this command at this time on this date'."
As the name implies, cron runs applications at specific times.
To set this up, you use crontab. At a prompt, enter crontab -e ($ crontab -e -- the -e is
necessary to tell crontab that you want to edit your existing crontab -- if you don't have one
yet, crontab creates one in your home directory).
There's a safer way to do it too: Either enter $ touch
crontab-temp or call it anything you'd like, then open the file in any text editor. Here
you'll enter this nifty guiding info below:
#minute (0-59),
#| hour (0-23),
#| | day of the month (1-31),
#| | | month of the year (1-12),
#| | | | day of the week (0-6 with 0=Sunday).
#| | | | | commands
MAILTO="> /dev/null"
This info, apart from the last line, is simply so you remember
what goes where. The last line is so that you won't get endless system e-mails telling you
that cron is doing what you've set it up to do.
Opening crontab with the -e option will open it in the vi
editor. As implied above, the vi editor is rather intimidating, but all one needs to know
about it is the Insert command (i), delete line (dd), & the Save & Exit (Esc : x) commands.
So, type crontab -e & open it up. It will likely be empty except for MAILTO=""^[ at the top
followed by ~'s all the way down the page.
Pico, which comes with Pine is clean & simple -- don't let any
ubergeek tell you otherwise. I'll tell you shortly how to make life easier -- &
safer -- using pico.
Here's an example crontab I'd created:
MAILTO="> /dev/null"
1,8,16,24,32,40,48,56 * * * * /usr/bin/fetchmail -a
0-59 * * * * /usr/games/fortune > ~/.fortune; cat ~/.sig ~/.fortune > ~/.signature
* 4 1,7,14,21,28 * * cat /dev/null > .procmail-log
~
~
~
~
~
"/tmp/crontab.5044" 2 lines, 112 characters
Before I explain what's happening, I gotta tell ya, I'm hardly
an expert at setting cron jobs (so, you might check out Nuintari's page for a better
explanation: (http://www.erg-mad.com/linux/tricks/cron.shtml),
but basically, it's composed of five asterisks, followed by the path to the application you
want to run, then any options that application uses, & any other things you want to happen
behind the scenes.
But first, "MAILTO="> /dev/null"" is very, very important. This
tells the cron daemon to send the e-mail it'll send you to send it to /dev/null (a nonexistant
system directory) instead, so that it's essentially being deleted. Otherwise, you'll get an
e-mail from cron each & every single time it does something. Trust me, you don't want
that.
The next line is for fetchmail with the -a (all) switch. In the
example, I have it set to check for mail every eight minutes. A full explanation of how to
enter the numbers is below
The asterisks following are for hour, day, month, & year. This
example means that every ten minutes of every hour of every day of every month of every year,
cron will "wake up" fetchmail to check my mail server & fetchmail will download anything new
it finds there (of course, I'm connected to the internet via DSL, & a modem connection would
need some other configuration -- hey, I've never connected a Linux box to the 'Net with an
analogue modem, so I dunno, but check out Scott's Configuring Pine page -- he may
get you connected).
My next entry does something really cool, & does it in a kind of
short-hand. Unlike the fetchmail line, this one is running not only every minute -- by 0-59
instead of entering 1,2,3, etc all the way to 60 -- it's also a set of complex commands.
First, it runs the program fortune, but instead of returning the fortune in a terminal window,
it then cats (writes) it to a file named ~/.sig (which is actually /home/username/.sig), then
takes that date & puts it in another file name .fortune (also in the user's home directory), &
finally writes it to a third file named .signature. Yeah, sounds like a lot of wasted effort,
but this was the only way I could get fortune to write to the .signature file so pine would
put a new signature from fortune into every e-mail I wrote. There may be an easier way to make
it work, but as this does work, I no longer mess with it.
So to further explain the numbers on the second line, instead of
all the cool things I did, for the first asterisk, I've entered those numbers on the second
line, "0-59", representing every minute in every (in this case) hour of the day. But, on the
third line, I've left the first asterisk to tell cron that (again, in this case) I only want
this job performed every 4 hour, or in crong-speak, at 4.00am. Following this, the third
asterisk -- for day of the month -- I have entered the numbers "1 7 14 21 28" & have separated
them by commas to tell cron to only perform this on the 1st, 7th,
14th, 21st, & the 28th days of the month.
The third entry is just an example of something else you might
want to do. My .procmail log file gets huge because I subscribe to numerous
mailing lists & get tons of spam. I also had had a domain for many years & would get tons more
spam because of that. So, to get rid of this growing file, I would delete it periodically. In
this case, I don't do it on a minute basis, but rather every week & a specific time. So, the
minute spot is an astrisk, then, at 4.00am on the 1st, 7th,
14th, 21st, & 28th of the month, cron will delete (send the
file to /dev/null) the .procmail-log in my home directory.
After you enter the info, to save & exit with vi (trust me, it
took me a long time to find this), press your Esc key, then type a : (that's a full colon),
type x & press Enter. If there's anything not set properly, like a mis-type or spaces between
the commas, crontab will tell you.
In addition, you'll find that you're probably still getting mail
from cron. Even if you don't, you'll do well to do something more. Open a terminal window,
enter su, & type the root password (if you're not in X, ignore that, or if you've opened X
after logging in as root, just open the terminal window), type pico /etc/crontab & press
Enter. The crontab you've just opened is the system crontab & you'll probably find that
there's a line near the top reading MAILTO=""^[. Remove the ^[ from it, hold your CTRL key &
press x to save & close. Pico will ask if you want to save, press y & then Enter to save with
the name crontab. You're done.
I've been told that there's a security issue with this
/etc/crontab & that it may be best to delete it & any other crontab on the system but one. The
one you keep could be /root/crontab or /home/your-user-name/crontab. If you choose to save the
one in a user space, remember that some jobs may require super user permissions & you should
decide appropriately. I live dangerously, leave the one in /etc, create one for me & another
for root. I'm used to being called stupid, but you may not be. So think about what you really
wanna do.
Now, the easy way: Copy any existing crontab file you can find
on your system to your home directoy with a name like "crontab-temp", or anything you like.
Open that one in pico, edit it to only show what you want -- keeping the info at the top, of
course, etc. Then, save & close it with ctrl-x, &, at the prompt, enter "crontab crontab-temp"
($ crontab crontab-temp ). Much, much easier. And you get to fool with your cron jobs
witout fooling with vi, or even emacs all the time. Either way, if you've made any mistakes,
you'll find a message on the command line telling you what's wrong.
Finally, just for fun, I'm including another crontab I'd used
some time back with lots of backup entries. I forget why I was backing up so much -- maybe I
was losing data -- anyway, it'll get you a better idea of how to use crontab. You can copy it
directly to your system & edit it to suit your own needs:
#minute (0-59),
#| hour (0-23),
#| | day of the month (1-31),
#| | | month of the year (1-12),
#| | | | day of the week (0-6 with 0=Sunday).
#| | | | | commands
MAILTO="> /dev/null"
1,10,20,30,40,50 * * * * /usr/bin/fetchmail -a
0-59 * * * * /usr/games/fortune > ~/.fortune; cat ~/.sig ~/.fortune > ~/.signature
* 2 * * * rm /home/meph/mail/fetchlog
* 4 1,7,14,21,28 * * cat /dev/null > .procmail-log
1 22 * * * cp /home/meph/mbox /zip/mailspool
1 3 * * * cp /home/meph/mbox /zip/mailspool
2 3 * * * cp /home/meph/.addressbook* /zip/
3 3 * * * cp /home/meph/.bash* /zip/
4 3 * * * cp /home/meph/.fetchmailrc /zip/
5 3 * * * cp /home/meph/.fortune /zip/
6 3 * * * cp /home/meph/.forward /zip/
7 3 * * * cp /home/meph/.mailcap /zip/
8 3 * * * cp -Rf /home/meph/.mailnotify /zip/
9 3 * * * cp /home/meph/.mime.types /zip/
10 3 * * * cp /home/meph/.pinerc /zip/
11 3 * * * cp -Rf /home/meph/.procmail /zip/
12 3 * * * cp /home/meph/.procmailrc /zip/
13 3 * * * cp /home/meph/.sig* /zip/
14 3 * * * cp /home/meph/.svgalibrc /zip/
15 3 * * * cp /home/meph/.w3m-m17n/bookmark.html /zip/
16 3 * * * cp /home/meph/.w3m-m17n/config /zip/
17 3 * * * cp /home/meph/.w3m-m17n/cookie /zip/
18 3 * * * cp /home/meph/.w3m-m17n/history /zip/
19 3 * * * cp /home/meph/CoverLetter /zip/
20 3 * * * cp /home/meph/Resume* /zip/
If you look closely, you'll notice a couple of things: 1. One of
the commands is "cp -Rf /home/meph/.mailnotify /zip/". This one not only copies, but forces
the removal of the .mailnotify file. This is really handly for files you don't want growing to
massive size. 2. There's no command to zip the backups copied to /zip. I'm sure I had a reason
for this, but you could easily add two more lines:
21 3 * * * zip /zip/* /zip/zips/dailyzipofbackups.zip
23 3 * * * rm /zip/*
Or something to this effect. I'll check the zip man file to see
if there's a way to name the file with the date, so you get different names for each zip file
made & deposited in the /zip/zips/ directory. Of course, with the two lines above, you'll
still get an error saying that rm cannot remove the directory /zip/zips/, but you don't want
to delete that.
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