cron/dist/crontab.5

.\"	$NetBSD: crontab.5,v 1.3 2010/07/15 22:18:20 christos Exp $
.\"
.\"/* Copyright 1988,1990,1993,1994 by Paul Vixie
.\" * All rights reserved
.\" *
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.\" * anyone resulting from the use of this software rests entirely with the
.\" * user.
.\" *
.\" * Send bug reports, bug fixes, enhancements, requests, flames, etc., and
.\" * I'll try to keep a version up to date.  I can be reached as follows:
.\" * Paul Vixie          <paul@vix.com>          uunet!decwrl!vixie!paul
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.\"
.\" Id: crontab.5,v 2.4 1994/01/15 20:43:43 vixie Exp
.\"
.Dd July 15, 2010
.Dt CRONTAB 5
.Os
.Sh NAME
.Nm crontab
.Nd tables for driving cron
.Sh DESCRIPTION
A
.Nm
file contains instructions to the
.Xr cron 8
daemon of the general form:
.Dq 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
.Xr 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
.Pq Sq #
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,
.Bd -literal
    name = value
.Ed
where the spaces around the equal-sign
.Pq Sq =
are optional, and any subsequent non-leading spaces in
.Ar value
will be part of the value assigned to
.Ar name .
The
.Ar value
string may be placed in quotes (single or double, but matching) to
preserve leading or trailing blanks.
The
.Ar name
string may also be placed in quotes (single or double, but matching)
to preserve leading, trailing or inner blanks.
.Pp
Several environment variables are set up automatically by the
.Xr cron 8
daemon.
.Ev SHELL
is set to
.Pa /bin/sh ,
and
.Ev LOGNAME
and
.Ev HOME
are set from the
.Pa /etc/passwd
line of the crontab's owner.
.Ev HOME
and
.Ev SHELL
may be overridden by settings in the crontab;
.Ev LOGNAME
may not.
.Pp
(Another note: the
.Ev LOGNAME
variable is sometimes called
.Ev USER
on BSD systems...  on these systems,
.Ev USER
will be set also.)
.Pp
In addition to
.Ev LOGNAME ,
.Ev HOME ,
and
.Ev SHELL ,
.Xr cron 8
will look at
.Ev MAILTO
if it has any reason to send mail as a result of running commands in
.Dq this
crontab.
If
.Ev MAILTO
is defined (and non-empty), mail is sent to the user so named.
If
.Ev MAILTO
is defined but empty
.Pq Ev MAILTO Ns = Ns \&"" ,
no mail will be sent.
Otherwise mail is sent to the owner of the crontab.
This option is useful if you decide on
.Xr mail 1
instead of
.Xr sendmail 1
as your mailer when you install cron --
.Xr mail 1
doesn't do aliasing, and UUCP usually doesn't read its mail.
.Pp
In order to provide finer control over when jobs execute, users
can also set the environment variables
.Ev CRON_TZ
and
.Ev CRON_WITHIN .
The
.Ev CRON_TZ
variable can be set to an alternate time zone in order to affect
when the job is run.
Note that this only affects the scheduling of the job, not the time
zone that the job perceives when it is run.
If
.Ev CRON_TZ
is defined but empty
.Pq Ev CRON_TZ Ns = Ns \&"" ,
jobs are scheduled with respect to the local time zone.
.Pp
The
.Ev CRON_WITHIN
variable should indicate the number of seconds within a job's
scheduled time that it should still be run.
On a heavily loaded system, or on a system that has just been
.Dq woken up ,
jobs will sometimes start later than originally intended, and by
skipping non-critical jobs because of delays, system load can be
lightened.
If
.Ev CRON_WITHIN
is defined but empty
.Pa Ev CRON_WITHIN Ns = Ns \&""
or set to some non-positive value (0, a negative number, or a
non-numeric string), it is treated as if it was unset.
.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
.Xr cron 8
when the minute, hour, and month of year fields match the current
time,
.Em and
when at least one of the two day fields (day of month, or day of week)
match the current time (see
.Dq Note
below).
.Xr cron 8
examines cron entries once every minute.
The time and date fields are:
.Bl -column -offset indent "day of month" "0-7 (0 or 7 is Sun, or use names)"
.It Em field Ta Em allowed values
.It minute Ta 0-59
.It hour Ta 0-23
.It day of month Ta 1-31
.It month Ta 1-12 (or names, see below)
.It day of week Ta 0-7 (0 or 7 is Sun, or use names)
.El
.Pp
A field may be an asterisk
.Pq Sq * ,
which always stands for
.Dq first\-last .
.Pp
Ranges of numbers are allowed.
Ranges are two numbers separated with a hyphen.
The specified range is inclusive.
For example,
.Dq 8-11
for an
.Dq hours
entry specifies execution at hours 8, 9, 10, and 11.
.Pp
A field may begin with a question mark
.Pq Sq \&? ,
which indicates a single value randomly selected when the crontab
file is read.
If the field contains only a question mark, the value is randomly
selected from the range of all possible values for the field.
If the question mark precedes a range, the value is randomly selected
from the range.
For example,
.Dq ? ?2-5 * * *
specifies that a task will be performed daily between 2:00am and
and 5:59am at a time randomly selected when the crontab file is
first read.
As just one example, this feature can be used to prevent a large
number of hosts from contacting a server simultaneously and
overloading it by staggering the time at which a download script
is executed.
.Pp
Lists are allowed.
A list is a set of numbers (or ranges) separated by commas.
Examples:
.Dq 1,2,5,9 ,
.Dq 0-4,8-12 .
.Pp
Step values can be used in conjunction with ranges.
Following a range with
.Dq / Ns Aq number
specifies skips of the number's value through the range.
For example,
.Dq 0-23/2
can be used in the hours field to specify command execution every
other hour (the alternative in the V7 standard is
.Dq 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
.Dq every two hours ,
just use
.Dq */2 .
.Pp
Names can also be used for the
.Dq month
and
.Dq 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
If the
.Nm
file is the system crontab
.Pa /etc/crontab ,
then the next (
.Dq sixth )
field contains the username to run the command as.
.Pp
The
.Dq sixth
field (or the
.Dq seventh
one for
.Pa /etc/crontab )
(the rest of the line) specifies the command to be run.
The entire command portion of the line, up to a newline or percent
signs
.Pq Sq % ,
will be executed by
.Xr sh 1
or by the shell specified in the
.Ev SHELL
variable of the cronfile.
Percent signs
.Pq Sq %
in the command, unless escaped with backslash
.Pq Sq \e ,
will be changed into newline characters, and all data after the
first % will be sent to the command as standard input.
.Pp
.Em 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 (i.e., aren't *), the command will
be run when
.Em either
field matches the current time.
For example,
.Dq 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.
.Pp
Instead of the first five fields, one of eight special strings may appear:
.Bl -column -offset indent "@annually" "Run once a month, 0 0 1 * *."
.It Sy string Ta Sy meaning
.It @reboot Ta Run once, at startup.
.It @yearly Ta Run once a year, Dq 0 0 1 1 * .
.It @annually Ta (same as @yearly)
.It @monthly Ta Run once a month, Dq 0 0 1 * * .
.It @weekly Ta Run once a week, Dq 0 0 * * 0 .
.It @daily Ta Run once a day, Dq 0 0 * * * .
.It @midnight Ta (same as @daily)
.It @hourly Ta Run once an hour, Dq 0 * * * * .
.El
.Ss EXAMPLE CRON FILE
.Bd -literal
# 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 \*[Gt]\*[Gt] $HOME/tmp/out 2\*[Gt]\*[Am]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"
? ?2-4 1,15 * * echo "random between 2am-4:59am on the 1st and 15th"
.Ed
.Sh SEE ALSO
.Xr crontab 1 ,
.Xr cron 8
.Sh STANDARDS
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.
.Dq 1-3,7-9
would be rejected by ATT or BSD cron -- they want to see
.Dq 1-3
or
.Dq 7,8,9
ONLY.
.Pp
Ranges can include
.Dq steps ,
so
.Dq 1-9/2
is the same as
.Dq 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
.Pa /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).
.Pp
All of the
.Sq @
commands that can appear in place of the first five fields are
extensions.
.Sh AUTHORS
.An Paul Vixie Aq paul@vix.com