\" $NetBSD: crontab.5,v 1.11 2022/02/26 17:02:47 christos Exp $
\"
\"/* Copyright 1988,1990,1993,1994 by Paul Vixie
\" * All rights reserved
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\" $OpenBSD: crontab.5,v 1.36 2018/06/13 13:27:37 jmc Exp $
\"
Dd February 26, 2022
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 plus 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
Ar command
Ed
Pp
While lines in a user
Nm
have five fixed fields plus a command, in the form:
Bd -ragged -offset indent
Ar minute
Ar hour
Ar day-of-month
Ar month
Ar day-of-week
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 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 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
Step values can be used in conjunction with ranges (but not random ranges
which represent a single number).
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 .
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
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 may be modified as follows:
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
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 "CRON_WITHIN"
It Ev CRON_TZ
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.
It Ev CRON_WITHIN
The
Ev CRON_WITHIN
variable should indicate the number of seconds within a job's
scheduled time that it should still be run.
For example if a job is scheduled for 12:30pm and
Ev CRON_WITHIN
is
Dv 120
(2 minutes), then the job will not be run if the system attempts
to start it past 12:32pm.
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
Pq 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, that is
the job will always run, even if it is going to start at a time
long past its scheduled time.
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 -- 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"
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
Months or days of the week can be specified by name.
It
Mailing after a successful run can be suppressed with
Fl n .
It
Logging can be suppressed with
Fl q .
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.
El
Sh AUTHORS
Nm
was written by
An Paul Vixie Aq Mt [email protected] .

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