Ubuntu Manpage: crontab - tables for driving bcron

NAME

       crontab - tables for driving bcron

DESCRIPTION

A crontab file contains instructions to the bcron-sched(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.

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 bcron-exec(8) program.
SHELL is set to /bin/sh, and LOGNAME, USER, and HOME are set from the /etc/passwd line of
the crontab's owner.

In addition to LOGNAME, USER, HOME, and SHELL, bcron-exec(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
bcron-sched(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). Jobs scheduled during non-existent times, such as "missing hours"
during daylight savings conversion, will be scheduled at some point shortly after the non-
existent time. Jobs scheduled during repeating times, such as "duplicate hours" during
daylight savings conversion, will be scheduled only once (unless they would repeat
anyways, such as jobs that run every minute or hour).

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 will be executed by /bin/sh or by the shell specified in the
SHELL variable of the cronfile.

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 `bruce@example.com', no matter whose crontab this is
       MAILTO=bruce@example.com
       #
       # 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 bruce (above)
       15 14 1 * *     $HOME/bin/monthly
       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

       /etc/cron.d/        System crontab directory

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.

       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).

AUTHOR