crontab.5

source code of crond

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.\" $Id: crontab.5,v 2.4 1994/01/15 20:43:43 vixie Exp $
.\" 
.TH CRONTAB 5 "24 January 1994"
.UC 4
.SH NAME
crontab \- tables for driving cron
.SH DESCRIPTION
A
.I crontab
file contains instructions to the
.IR 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
.IR su (1)
as part of a cron command.
.PP
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.
.PP
An active line in a crontab will be either an environment setting or a cron
command.  An environment setting is of the form,
.PP
    name = value
.PP
where the spaces around the equal-sign (=) are optional, and any subsequent
non-leading spaces in
.I value
will be part of the value assigned to
.IR name .
The
.I value
string may be placed in quotes (single or double, but matching) to preserve
leading or trailing blanks.
.PP
Several environment variables are set up
automatically by the
.IR 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.
.PP
(Another note: the LOGNAME variable is sometimes called USER on BSD systems...
on these systems, USER will be set also.)
.PP
In addition to LOGNAME, HOME, and SHELL,
.IR 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.
.PP
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
.IR cron (8)
when the minute, hour, and month of year fields match the current time,
.I and
when at least one of the two day fields (day of month, or day of week)
match the current time (see ``Note'' below).
.IR cron (8)
examines cron entries once every minute.
The time and date fields are:
.IP
.ta 1.5i
field	allowed values
.br
-----	--------------
.br
minute	0-59
.br
hour	0-23
.br
day of month	0-31
.br
month	0-12 (or names, see below)
.br
day of week	0-7 (0 or 7 is Sun, or use names)
.br
.PP
A field may be an asterisk (*), which always stands for ``first\-last''.
.PP
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.
.PP
Lists are allowed.  A list is a set of numbers (or ranges)
separated by commas.  Examples: ``1,2,5,9'', ``0-4,8-12''.
.PP
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''.
.PP
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.
.PP
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.
.PP
Note: The day of a command's execution can be specified by two
fields \(em day of month, and day of week.  If both fields are
restricted (ie, aren't *), the command will be run when
.I either
field matches the current time.  For example,
.br
``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.
.SH EXAMPLE CRON FILE
.nf

# 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"
.fi
.SH SEE ALSO
cron(8), crontab(1)
.SH EXTENSIONS
When specifying day of week, both day 0 and day 7 will be considered Sunday.
BSD and ATT seem to disagree about this.
.PP
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.
.PP
Ranges can include "steps", so "1-9/2" is the same as "1,3,5,7,9".
.PP
Names of months or days of the week can be specified by name.
.PP
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.
.PP
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).
.SH AUTHOR
.nf
Paul Vixie <[e-mail]>