[K12OSN] cron & crontab (was Re: Presentation Freezing)
hick518 at yahoo.com
Tue Feb 22 11:42:27 UTC 2005
I've been editing /etc/crontab to do things like
performing nightly backups and updates. It works, but
I guess you are saying it's the wrong way to do it,
and that I should be using root's "personal" crontab.
Could you give an example of what should be in
/etc/crontab? I see your example of /etc/cron.d. My
system has both /etc/crontab and /etc/cron.d so I
guess I'm not sure what each of their purpose is.
--- Michael C Wescott <wescott at sc.rr.com> wrote:
> cron(8) is a program designed to run other programs
> periodically. It is
> controlled by some configuration files called
> "crontabs" that come in
> two flavors. One type of crontab file lives as
> /etc/crontab or as a file
> in the /etc/cron.d directory. The second type lives
> in the
> /var/spool/cron directory and takes the name of it's
> crontabs of the first kind have the 6th field
> indicating the user whose
> privileges with which the command will run. These
> crontabs should be
> considered as system files. Don't change them unless
> you really know
> what you are doing. The ones in /etc/cron.d are
> usually specific to some
> package. I don't see any in a k12ltsp system I've
> got available but my
> home system (Debian) shows several all associated
> with a particular
> ~ $ ls /etc/cron.d
> anacron logcheck sendmail uucp
> ~ $
> Having a directory with a package-specific crontab
> makes it easy for a
> package to install cron entries without trying to
> edit one or more
> standard crontabs.
> These system crontabs can be editted with any text
> A user (including root) should find his crontab in
> These crontabs don't use the username in the sixth
> field since all
> commands in that file will be run with the user's
> privileges. These
> crontabs should be installed, removed and editted
> using the crontab(1)
> command. For details on the format see crontab(5),
> but briefly there are
> two types of lines (other than empty or comment
> lines). One sets an
> enviroment variable and looks like:
> ENVVAR = some value
> The other type has at least 6 whitespace separated
> fields. The first 5
> determine when the command should be run. The 6th
> (and following) are
> treated as a command line and executed by a shell,
> much as if it had
> been run with /bin/sh -c "command line".
> The rules for exactly when to run the command can be
> fairly complicated
> so read the man page crontab(5) more than once.
> The current version of cron(8) is quite good about
> "doing the right
> thing" but it wasn't always so. In olden times (i.e
> some Unix systems)
> had some very interesting bugs that popped up when a
> command was
> scheduled to be run when daylight savings time
> On Fri, 2005-02-18 at 13:11, Rob Owens wrote:
> > No commas in the crontab file (at least not in
> A comma is a separtor within a time field but not
> between different
> fields. Whitespace is used between fields.
> 15,45 9-11,13-16 * * 1-5 echo
> "working hours"
> Will execute the command 'echo "working hours"'
> every hour at 15 and 45
> minutes after the hour between 9 and 5 pm (not
> including a lunch break
> at noon) on every weekday.
> > Here is a sample that I pulled from "man crontab".
> > my crontab file, the 6th field is usually "root"
> > the user whose account will run the specified
> Err ... no, the sixth field only shows up in the
> special crontabs that
> you really don't want to mess with.
> > The ">> $HOME/tmp/out 2>&1" part means to send all
> > output, errors and otherwise, to a file:
> Close. The ">> $HOME/tmp/out" part means to append
> stdout (the normal
> output of the command) to $HOME/tmp/out. The 2>&1
> part means to do to
> stderr (usually error messages) whatever was done to
> > Brandon Kovach <bkovach at lrhs.greene-r8.k12.mo.us>
> > This will be my first effort at cron, so please
> > confirm that this is how
> > the cron file should be setup.
> > 0, 23, *, *, 1-5, del /tmp/*.*
> Lose the commas. Don't you mean "rm" rather than
> "del"? And *.* will
> match any file with a dot in it, except those
> starting with a dot. I
> think you probably want
> 0 23 * * 1-5 /bin/rm -f /tmp/*
> or more likely
> 0 23 * * 1-5 /bin/rm -r -f /tmp/*
> The bad part about this kind of blanket removal of
> files from /tmp is
> that it is difficult to know when other programs,
> say other cron jobs,
> might be using /tmp.
> > Then I would name the file tmpremove.cron
> > Then I would do a crontab -l to make sure I had no
> > files of the same name.
> > Then I would crontab tmpremove.cron to activate
> > file.
> > Is this all correct?
> Nope. First create a copy of your current crontab;
> then edit it; then
> install it as the new version:
> [root at drbob root]# crontab -l > xxx.cron
> [root at drbob root]# vi xxx.cron
> [root at drbob root]# crontab xxx.cron
> or combine it all into one step:
> [root at drbob root]# VISUAL=vi crontab -e
> Mike Wescott
> K12OSN mailing list
> K12OSN at redhat.com
> For more info see <http://www.k12os.org>
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