Return to crontab.5 CVS log

Up to [local] / src / usr.sbin / cron

Support random offsets when using ranges with a step value in cron.
This extends the random range syntax to support step values.  Instead
of choosing a random number between the high and low values, the
field is treated as a range with a random offset less than the step
value.  This can be used to avoid thundering herd problems where
multiple machines contact a server all at the same time via cron jobs.

The syntax is similar to the existing range/step syntax but uses a
random range.  For example, instead of "0-59/10" in the minutes
field, "0~59/10" can be used to run a command every 10 minutes where
the first command starts at a random offset in the range [0,9].
The high and low numbers are optional, "~/10" can be used instead.

Requested by job@, OK phessler@

.\"/* Copyright 1988,1990,1993,1994 by Paul Vixie
.\" * All rights reserved
.\" */
.\"
.\" Copyright (c) 2004 by Internet Systems Consortium, Inc. ("ISC")
.\" Copyright (c) 1997,2000 by Internet Software Consortium, Inc.
.\"
.\" Permission to use, copy, modify, and distribute this software for any
.\" purpose with or without fee is hereby granted, provided that the above
.\" copyright notice and this permission notice appear in all copies.
.\"
.\" THE SOFTWARE IS PROVIDED "AS IS" AND ISC DISCLAIMS ALL WARRANTIES
.\" WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
.\" MERCHANTABILITY AND FITNESS.  IN NO EVENT SHALL ISC BE LIABLE FOR
.\" ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
.\" WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
.\" ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT
.\" OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
.\"
.\" $OpenBSD: crontab.5,v 1.42 2023/05/06 23:06:27 millert Exp $
.\"
.Dd $Mdocdate: May 6 2023 $
.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 at these times on these dates run this command .
There may be a system
.Nm
and each user may have their own
.Nm .
Commands in any given
.Nm
will be
executed either as the user who owns the
.Nm
or, in the case of the system
.Nm crontab ,
as the user specified on the command line.
.Pp
While a
.Nm
is a text file, it is not intended to be directly edited.
Creation, modification, and removal of a
.Nm
should be done using
.Xr crontab 1 .
.Pp
Blank lines, leading spaces, and tabs are ignored.
Lines whose first non-space character is a pound sign
.Pq Ql #
are comments, and are ignored.
Note that comments are not allowed on the same line as
.Xr cron 8
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
.Nm
is either an environment variable setting or a
.Xr cron 8
command.
.Pp
Environment variable settings create the environment
any command in the
.Nm
is run in.
An environment variable setting is of the form:
.Pp
.Dl name = value
.Pp
The spaces around the equal sign
.Pq Ql =
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
.Pq single or double , but matching
to preserve leading or trailing blanks.
.Pp
Lines in the system
.Nm
have six fixed fields, an optional flags field, and a command, in the form:
.Bd -ragged -offset indent
.Ar minute
.Ar hour
.Ar day-of-month
.Ar month
.Ar day-of-week
.Ar user
.Op Ar flags
.Ar command
.Ed
.Pp
While lines in a user
.Nm
have five fixed fields, an optional flags field, and a command, in the form:
.Bd -ragged -offset indent
.Ar minute
.Ar hour
.Ar day-of-month
.Ar month
.Ar day-of-week
.Op Ar flags
.Ar command
.Ed
.Pp
Fields are separated by blanks or tabs.
The command may be one or more fields long.
The allowed values for the fields are:
.Bl -column "day-of-month" "allowed values" -offset indent
.It Sy field Ta Sy allowed values
.It Ar minute Ta * or 0\(en59
.It Ar hour Ta * or 0\(en23
.It Ar day-of-month Ta * or 1\(en31
.It Ar month Ta * or 1\(en12 or a name (see below)
.It Ar day-of-week Ta * or 0\(en7 or a name (0 or 7 is Sunday)
.It Ar user Ta a valid username
.It Op Ar flags Ta runtime flags, denoted with '-'
.It Ar command Ta text
.El
.Pp
Lists are allowed.
A list is a set of numbers (or ranges) separated by commas.
For example,
.Dq 1,2,5,9
or
.Dq 0\(en4,8\(en12 .
.Pp
Ranges of numbers are allowed.
Ranges are two numbers separated with a hyphen.
The specified range is inclusive.
For example,
8\(en11 for an
.Ar hour
entry specifies execution at hours 8, 9, 10 and 11.
.Pp
A random value (within the legal range) may be obtained by using the
.Ql ~
character in a field.
The interval of the random value may be specified explicitly, for example
.Dq 0~30
will result in a random value between 0 and 30 inclusive.
If either (or both) of the numbers on either side of the
.Ql ~
are omitted, the appropriate limit (low or high) for the field will be used.
.Pp
Step values can be used in conjunction with ranges.
Following a range with
.No / Ns Ar number
specifies skips of
.Ar number
through the range.
For example,
.Dq 0\(en23/2
can be used in the
.Ar hour
field to specify command execution every other hour.
Steps are also permitted after an asterisk, so to say
.Dq every two hours ,
just use
.Dq */2 .
A step value after a random range will execute the command at a random
offset less than the step size.
For example, to avoid a thundering herd at the top and bottom of the hour,
.Dq 0~59/30
.Po
or simply
.Dq ~/30
.Pc
can be used in the
.Ar minute
field to specify that command execution happen twice an hour at
consistent intervals.
.Pp
An asterisk
.Pq Ql *
is short form for a range of all allowed values.
.Pp
Names can be used in the
.Ar month
and
.Ar 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
Some
.Ar flags
relating to process operation can be provided before the
.Ar command
field.
Flags are denoted with '-' and may be combined.
.Bl -tag -width Ds
.It Fl n Ar command
No mail is sent after a successful run.
The execution output will only be mailed if the command exits with a non-zero
exit code.
The
.Fl n
option is an attempt to cure potentially copious volumes of mail coming from
.Xr cron 8 .
.It Fl q Ar command
Execution will not be logged.
.It Fl s Ar command
Only a single instance of
.Ar command
will be run concurrently.
Additional instances of
.Ar command
will not be scheduled until the earlier one completes.
.El
.Pp
The
.Ar command
field (the rest of the line) is the command to be
run.
The entire command portion of the line, up to a newline or %
character, will be executed by
.Pa /bin/sh
or by the shell
specified in the
.Ev SHELL
variable of the
.Nm crontab .
Percent signs
.Pq Ql %
in the command, unless escaped with a backslash
.Pq Ql \e ,
will be changed into newline characters, and all data
after the first
.Ql %
will be sent to the command as standard input.
.Pp
Commands are executed by
.Xr cron 8
when the
.Ar minute ,
.Ar hour ,
and
.Ar month
fields match the current time,
.Em and
when at least one of the two day fields
.Po Ar day-of-month
or
.Ar day-of-week Pc ,
match the current time.
.Pp
Note: The day of a command's execution can be specified by two
fields \(em
.Ar day-of-month
and
.Ar 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,
.Pp
.Dl 30 4 1,15 * 5
.Pp
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 "@midnight" "meaning" -offset indent
.It Sy string Ta Sy meaning
.It @reboot Ta Run once, at startup.
.It @yearly Ta Run every January 1 (0 0 1 1 *).
.It @annually Ta The same as @yearly.
.It @monthly Ta Run the first day of every month (0 0 1 * *).
.It @weekly Ta Run every Sunday (0 0 * * 0).
.It @daily Ta Run every midnight (0 0 * * *).
.It @midnight Ta The same as @daily.
.It @hourly Ta Run every hour, on the hour (0 * * * *).
.El
.Sh ENVIRONMENT
.Bl -tag -width "LOGNAMEXXX"
.It Ev HOME
Set from the user's
.Pa /etc/passwd
entry.
May be overridden by settings in the
.Nm .
.It Ev LOGNAME
Set from the user's
.Pa /etc/passwd
entry.
May not be overridden by settings in the
.Nm .
.It Ev MAILTO
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 = Qq ,
no mail will be sent.
Otherwise mail is sent to the owner of the
.Nm .
This is useful for pseudo-users that lack an alias
that would otherwise redirect the mail to a real person.
.It Ev SHELL
Set to
.Pa /bin/sh .
May be overridden by settings in the
.Nm .
.It Ev USER
Set from the user's
.Pa /etc/passwd
entry.
May not be overridden by settings in the
.Nm .
.El
.Sh FILES
.Bl -tag -width "/var/cron/tabs/<user>XXX" -compact
.It Pa /etc/crontab
System crontab.
.It Pa /var/cron/tabs/ Ns Aq Ar user
User crontab.
.El
.Sh EXAMPLES
.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 >> $HOME/tmp/out 2>&1

# run at 2:15pm on the first of every month -- job output will be sent
# to paul, but only if $HOME/bin/monthly exits with a non-zero exit code
15 14 1 * *     -n $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"

# run hourly at a random time within the first 30 minutes of the hour
0~30 * * * *   /usr/libexec/spamd-setup
.Ed
.Sh SEE ALSO
.Xr crontab 1 ,
.Xr cron 8
.Sh STANDARDS
The
.Nm
file format is compliant with the
.St -p1003.1-2008
specification.
The behaviours described below are all extensions to that standard:
.Bl -dash
.It
The
.Ar day-of-week
field may use 7 to represent Sunday.
.It
Ranges may include
.Dq steps .
.It
Random intervals are supported using the
.Ql ~
character.
.It
Months or days of the week can be specified by name.
.It
Environment variables can be set in a crontab.
.It
Command output can be mailed to a person other than the crontab
owner, or the feature can be turned off and no mail will be sent
at all.
.It
All of the
.Ql @
commands that can appear in place of the first five fields.
.It
All of the
.Op Fl nqs
flags.
.El
.Sh AUTHORS
.Nm
was written by
.An Paul Vixie Aq Mt vixie@isc.org .