These are direct copy/pastes of the material in the openSUSE - KDE man pages for "crontab"
The material is an appendix to: HowTo: Use Cron Tables (Crontab) to Schedule Events in Suse Linux (openSUSE) 10.x
======================Start of section "User Commands"===========================
CRONTAB
Section: User Commands (1)
NAME
crontab - maintain crontab files for individual users (ISC
Cron V4.1)
SYNOPSIS
crontab [-u user] file
crontab [-u user] [-l | -r | -e]
DESCRIPTION
Crontab is the program used to install, deinstall or list the tables
used to drive the cron(8) daemon in ISC Cron. Each user can have their
own crontab, and though these are files in /var, they are not intended
to be edited directly.
If the cron.allow file exists, then you must be listed
therein in order to be allowed to use this command. If the cron.allow
file does not exist but the cron.deny file does exist, then you must
not be listed in the cron.deny file in order to use this command. If
neither of these files exists, only the super user will be allowed to
use this command.
If the -u option is given, it specifies the name of the user
whose crontab is to be tweaked. If this option is not given, crontab
examines "your" crontab, i.e., the crontab of the person executing the
command. Note that su(8) can confuse crontab and that if you are
running inside of su(8) you should always use the -u option for
safety's sake.
The first form of this command is used to install a new
crontab from some named file or standard input if the pseudo-filename
``-'' is given.
The -l option causes the current crontab to be displayed on
standard output.
The -r option causes the current crontab to be removed.
The -e option is used to edit the current crontab using the
editor specified by the VISUAL or EDITOR environment variables. After
you exit from the editor, the modified crontab will be installed
automatically.
SEE ALSO
crontab(5), cron(8)
FILES
/etc/cron.allow
/etc/cron.deny
STANDARDS
The crontab command conforms to IEEE Std1003.2-1992
(``POSIX''). This new command syntax differs from previous versions of
Vixie Cron, as well as from the classic SVR3 syntax.
DIAGNOSTICS
A fairly informative usage message appears if you run it with
a bad command line.
AUTHOR
Paul Vixie <vixie@isc.org>
======================End of section "User Commands"=============================
======================Start of section "File Formats"============================
CRONTAB
Section: File Formats (5)
NAME
crontab - tables for driving cron (ISC Cron V4.1)
DESCRIPTION
A crontab file contains instructions to the cron(8) daemon of
the general form: ``run this command at this time on this date''. Each
user has their own crontab, and commands in any given crontab will be
executed as the user who owns the crontab. Uucp and News will usually
have their own crontabs, eliminating the need for explicitly running
su(1) as part of a cron command.
Blank lines and leading spaces and tabs are ignored. Lines
whose first non-space character is a pound-sign (#) are comments, and
are ignored. Note that comments are not allowed on the same line as
cron commands, since they will be taken to be part of the command.
Similarly, comments are not allowed on the same line as environment
variable settings.
An active line in a crontab will be either an environment
setting or a cron command. An environment setting is of the form,
name = value
where the spaces around the equal-sign (=) are optional, and
any subsequent non-leading spaces in value will be part of the value
assigned to name. The value string may be placed in quotes (single or
double, but matching) to preserve leading or trailing blanks.
Several environment variables are set up automatically by the
cron(8) daemon. SHELL is set to /bin/sh, and LOGNAME and HOME are set
from the /etc/passwd line of the crontab's owner. HOME and SHELL may be
overridden by settings in the crontab; LOGNAME may not.
(Another note: the LOGNAME variable is sometimes called USER
on BSD systems... on these systems, USER will be set also.)
In addition to LOGNAME, HOME, and SHELL, cron(8) will look at
MAILTO if it has any reason to send mail as a result of running
commands in ``this'' crontab. If MAILTO is defined (and non-empty),
mail is sent to the user so named. If MAILTO is defined but empty
(MAILTO=""), no mail will be sent. Otherwise mail is sent to the owner
of the crontab. This option is useful if you decide on /bin/mail
instead of /usr/lib/sendmail as your mailer when you install cron --
/bin/mail doesn't do aliasing, and UUCP usually doesn't read its mail.
The format of a cron command is very much the V7 standard,
with a number of upward-compatible extensions. Each line has five time
and date fields, followed by a user name if this is the system crontab
file, followed by a command. Commands are executed by cron(8) when the
minute, hour, and month of year fields match the current time, and at
least one of the two day fields (day of month, or day of week) match
the current time (see ``Note'' below). Note that this means that
non-existent times, such as "missing hours" during daylight savings
conversion, will never match, causing jobs scheduled during the
"missing times" not to be run. Similarly, times that occur more than
once (again, during daylight savings conversion) will cause matching
jobs to be run twice.
cron(8) examines cron entries once every minute.
The time and date fields are:
field allowed
values
----- --------------
minute 0-59
hour 0-23
day of month 1-31
month 1-12
(or names, see below)
day of week 0-7 (0 or 7 is Sun,
or use names)
A field may be an asterisk (*), which always stands for
``first-last''.
Ranges of numbers are allowed. Ranges are two numbers
separated with a hyphen. The specified range is inclusive. For example,
8-11 for an ``hours'' entry specifies execution at hours 8, 9, 10 and
11.
Lists are allowed. A list is a set of numbers (or ranges)
separated by commas. Examples: ``1,2,5,9'', ``0-4,8-12''.
Step values can be used in conjunction with ranges. Following
a range with ``/<number>'' specifies skips of the
number's value through the range. For example, ``0-23/2'' can be used
in the hours field to specify command execution every other hour (the
alternative in the V7 standard is ``0,2,4,6,8,10,12,14,16,18,20,22'').
Steps are also permitted after an asterisk, so if you want to say
``every two hours'', just use ``*/2''.
Names can also be used for the ``month'' and ``day of week''
fields. Use the first three letters of the particular day or month
(case doesn't matter). Ranges or lists of names are not allowed.
The ``sixth'' field (the rest of the line) specifies the
command to be run. The entire command portion of the line, up to a
newline or % character, will be executed by /bin/sh or by the shell
specified in the SHELL variable of the cronfile. Percent-signs (%) in
the command, unless escaped with backslash (\), will be changed into
newline characters, and all data after the first % will be sent to the
command as standard input.
Note: The day of a command's execution can be specified by
two fields — day of month, and day of week. If both fields
are restricted (ie, aren't *), the command will be run when either
field matches the current time. For example,
``30 4 1,15 * 5'' would cause a command to be run at 4:30 am
on the 1st and 15th of each month, plus every Friday.
EXAMPLE CRON FILE
# use /bin/sh to run commands, no matter what /etc/passwd says
SHELL=/bin/sh
# mail any output to `paul', no matter whose crontab this is
MAILTO=paul
#
# run five minutes after midnight, every day
5 0 * * *
$HOME/bin/daily.job >> $HOME/tmp/out 2>&1
# run at 2:15pm on the first of every month -- output mailed to paul
15 14 1 * * $HOME/bin/monthly
# run at 10 pm on weekdays, annoy Joe
0 22 * * 1-5 mail -s "It's 10pm" joe%Joe,%%Where
are your kids?%
23 0-23/2 * * * echo "run 23 minutes after midn, 2am, 4am ..., everyday"
5 4 * * sun echo "run at 5
after 4 every sunday"
FILES
/etc/crontab System crontab file
SEE ALSO
cron(8), crontab(1)
EXTENSIONS
When specifying day of week, both day 0 and day 7 will be
considered Sunday. BSD and ATT seem to disagree about this.
Lists and ranges are allowed to co-exist in the same field.
"1-3,7-9" would be rejected by ATT or BSD cron -- they want to see
"1-3" or "7,8,9" ONLY.
Ranges can include "steps", so "1-9/2" is the same as
"1,3,5,7,9".
Names of months or days of the week can be specified by name.
Environment variables can be set in the crontab. In BSD or
ATT, the environment handed to child processes is basically the one
from /etc/rc.
If the uid of the owner is 0 (root), he can put a "-" as
first character of a crontab entry. This will prevent cron from writing
a syslog message about this command getting executed.
Command output is mailed to the crontab owner (BSD can't do
this), can be mailed to a person other than the crontab owner (SysV
can't do this), or the feature can be turned off and no mail will be
sent at all (SysV can't do this either).
These special time specification "nicknames" are supported,
which replace the 5 initial time and date fields, and are prefixed by
the '@' character:
@reboot :
Run once, at startup.
@yearly :
Run once a year, ie. "0 0 1 1 *".
@annually : Run once a year,
ie. "0 0 1 1 *".
@monthly : Run once a
month, ie. "0 0 1 * *".
@weekly :
Run once a week, ie. "0 0 * * 0".
@daily
: Run once a day, ie.
"0 0 * * *".
@hourly :
Run once an hour, ie. "0 * * * *".
CAVEATS
In this version of cron, /etc/crontab must not be writable by
any user other than root. No crontab files may be links, or linked to
by any other file. No crontab files may be executable, or be writable
by any user other than their owner.
AUTHOR
Paul Vixie <vixie@isc.org>
======================End of section "File Formats"==============================