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