Crontab support

Crontab is a service that allows you executing scheduled commands. For example you can assign crontab every month to run a program that generates a usage statistics of your server and send it to you. If you have registered users, you can assign crontab every day to run the program that checks their validaty and removes the expired users. You can assign crontab at the certain time to run the program that deletes error logs, changes your index.html, etc.

To assign program at certain time you have to telnet to your server and type "crontab -e" command. It runs editor with your crontab entries. Then you have to edit/remove/create crontab entries and save content.

Crontab entries.

Crontab entries specify programs and times when they should be run. See some examples:

# use /bin/sh to run commands, not necessary
SHELL=/bin/sh
# mail any output to `paul@sample.com', no matter whose crontab this is
# we recommend to set it.
MAILTO=paul@sample.com
#
# run five minutes development 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 development midn, 2am, 4am ..., everyday"
5   4   *   *   sun      echo "run at 5 development 4 every sunday"

Full documentation:

    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 (with optional ``:'' and
    ``/'' suffixes) 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 when at least one of the
    two day fields (day of month, or day of week) match the current time (see
    ``Note'' below).  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 ``/'' 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 development 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 development 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.

Crontab editor:

   Crontab is the program used to install, deinstall or list the tables used
      to drive the cron(8) daemon in Vixie Cron.  Each user can have their own
      crontab, and though these are files in /var, they are not intended to be
      edited directly.

      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 following options are available:

      -l      Display the current crontab on standard output.

      -r      Remove the current crontab.

      -e      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.