Answer

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.
Blank lines and leading spaces and tabs are ignored. Lines whose first non-space character is a pound-sign (#) are comments, and are ignored.
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 (siingle or double, but matching) to preserve leading or trailing blanks.

The format of a cron command is as follows:
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 when 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-existant 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.

The time and date fields are:

field allowed values
----- --------------
minute 0-59
hour 0-23
day of month 0-31
month 0-12 (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. Fol-lowing 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 execu-tion every other hour 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 partic-ular 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 4:30am 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, run cleanup.sh
0 22 * * 1-5 $HOME/bin/cleanup.sh
23 0-23/2 * * * echo "run 23 minutes after midn, 2am, 4am ..., everyday"
5 4 * * sun echo "run at 5 after 4 every sunday"