path: root/cron.8

.\"/* Copyright 1988,1990,1993 by Paul Vixie
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.\" $Id: cron.8,v 2.2 1993/12/28 08:34:43 vixie Exp $
.\" 
.TH CRON 8 "19 April 2010"
.UC 4
.SH NAME
cron \- daemon to execute scheduled commands (Vixie Cron)
.SH SYNOPSIS
cron
.RB [ -f ]
.RB [ -l ]
.RB [ -L
.IR loglevel ]
.SH DESCRIPTION
.I cron
is started automatically from /etc/init.d on entering multi-user
runlevels.
.SH OPTIONS
.TP 8
.B -f
Stay in foreground mode, don't daemonize.
.TP
.B -l
Enable LSB compliant names for /etc/cron.d files. This setting, however, does
not affect the parsing of files under /etc/cron.hourly, /etc/cron.daily,
/etc/cron.weekly or /etc/cron.monthly.
.TP
.B -L loglevel 
Tell cron what to log about \fBjobs\fR (errors are logged regardless of this
value) as the sum of the following values:
.br
.RS 12
.IP \fB1\fR
will log the start of all cron jobs
.IP \fB2\fR
will log the end of all cron jobs
.IP \fB4\fR
will log all failed jobs (exit status != 0)
.IP \fB8\fR
will log the process number of all cron jobs
.RE
.IP
The default is to log the start of all jobs (1). Logging will be disabled
if \fIlevels\fR is set to zero (0). A value of fifteen (15) will select all
options.
.SH NOTES
.PP
.I cron
searches its spool area (/var/spool/cron/crontabs) for crontab
files (which are named after accounts in
/etc/passwd); crontabs found are loaded into memory.  Note that
crontabs in this directory should not be accessed directly -
the
.I crontab
command should be used to access and update them.

.I cron
also reads /etc/crontab, which is in a slightly different format (see
.IR crontab(5) ).
In Debian, the content of /etc/crontab is predefined
to run programs under /etc/cron.hourly, /etc/cron.daily,
/etc/cron.weekly and /etc/cron.monthly. This configuration is specific to
Debian, see the note under 
.B DEBIAN SPECIFIC
below.
in order 

Additionally, in Debian,
.I cron
reads the files in the /etc/cron.d directory.
.I cron
treats the files in /etc/cron.d as in the same way as the /etc/crontab file (they
follow the special format of that file, i.e. they include the 
.I user
field). However, they are independent of /etc/crontab: they do not, for 
example, inherit environment variable settings from it. This change is
specific to Debian see the note under 
.B DEBIAN SPECIFIC
below.

Like /etc/crontab, the files in the /etc/cron.d directory are
monitored for changes. In general, the system administrator should not use /etc/cron.d/,
but use the standard system crontab /etc/crontab.

/etc/crontab and the files in /etc/cron.d must be owned by root, and must not
be group- or other-writable. In contrast to the spool area, the files
under /etc/cron.d or the files under /etc/cron.hourly, /etc/cron.daily,
/etc/cron.weekly and /etc/cron.monthly may also be symlinks,
provided that both the symlink and the file it points to are owned by root. 
The files under /etc/cron.d do not need to be executable, while the files
under /etc/cron.hourly, /etc/cron.daily,
/etc/cron.weekly and /etc/cron.monthly do, as they are run by 
.I run-parts
(see
.IR run-parts(8) 
for more information).

.I cron
then wakes up every minute, examining all stored crontabs, checking
each command to see if it should be run in the current minute.  When
executing commands, any output is mailed to the owner of the crontab
(or to the user named in the MAILTO environment variable in the
crontab, if such exists).  The children copies of cron running these
processes have their name coerced to uppercase, as will be seen in the
syslog and ps output.
.PP
Additionally,
.I cron
checks each minute to see if its spool directory's modtime (or the modtime
on the 
.IR /etc/crontab
file)
has changed, and if it has,
.I cron
will then examine the modtime on all crontabs files and reload those which have
changed.  Thus
.I cron
need not be restarted whenever a crontab file is modified.  Note that the
.IR crontab (1)
command updates the modtime of the spool directory whenever it changes a
crontab.
.PP
Special considerations exist when the clock is changed by less than 3
hours, for example at the beginning and end of daylight savings
time. If the time has moved forwards, those jobs which would have
run in the time that was skipped will be run soon after the change. 
Conversely, if the time has moved backwards by less than 3 hours,
those jobs that fall into the repeated time will not be re-run.
.PP
Only jobs that run at a particular time (not specified as
@hourly, nor with '*' in the hour or minute specifier) are
affected. Jobs which are specified with wildcards are run based on the
new time immediately.
.PP
Clock changes of more than 3 hours are considered to be corrections to
the clock, and the new time is used immediately.
.PP
.I cron
logs its action to the syslog facility 'cron', and logging may be
controlled using the standard 
.IR syslogd(8) 
facility.
.SH ENVIRONMENT
If configured in
.I /etc/default/cron
in Debian systems, the
.I cron
daemon localisation settings environment can be managed through the use of 
.I /etc/environment
or through the use of
.I /etc/default/locale
with values from the latter overriding values from the former. These
files are read and they will be used to setup the LANG, LC_ALL, and
LC_CTYPE environment variables. These variables are then used to set the
charset of mails, which defaults to 'C'.
.PP
This does
.B NOT
affect the environment of tasks running under cron. For more information
on how to modify the environment of tasks, consult 
.IR crontab(5)
\.
.PP
The daemon will use, if present, the definition from
.I /etc/timezone
for the timezone.
.PP
The environment can be redefined in user's crontab definitions but
.I cron
will only handle tasks in a single timezone.

.SH DEBIAN SPECIFIC
.PP
Debian introduces some changes to 
.I cron
that were not originally available upstream. The most significant
changes introduced are:

.IP \(em
Support for /etc/cron.{hourly,daily,weekly,monthly} via /etc/crontab,
.IP \(em
Support for /etc/cron.d (drop-in dir for package crontabs),
.IP \(em
PAM support,
.IP \(em
SELinux support,
.IP \(em
auditlog support,
.IP \(em
DST and other time-related changes/fixes,
.IP \(em
SGID crontab(1) instead of SUID root,
.IP \(em
Debian-specific file locations and commands,
.IP \(em
Debian-specific configuration (/etc/default/cron),
.IP \(em
numerous other smaller features and fixes.

.PP
Support for /etc/cron.hourly, /etc/cron.daily, /etc/cron.weekly and
/etc/cron.monthly is provided in Debian through the default setting
of the /etc/crontab file (see the system-wide example in 
.IR crontab(5) ).
The default sytem-wide crontab contains four tasks: run every hour, every
day, every week and every month. Each of these tasks will execute 
.B run-parts
providing each one of the directories as an argument. These tasks are disabled if
.B anacron 
is installed (except for the hourly task) to prevent conflicts between
both daemons.

As described above, the files under these directories have to be pass
some sanity checks including the following: be executable, be owned by root,
not be writable by group or other and, if symlinks, point to files owned by
root. Additionally, the file names must conform to the filename requirements
of 
.BR run-parts :
they must be entirely made up of letters, digits and can only contain the
special signs underscores ('_') and hyphens ('-'). Any file that does
not conform to these requirements will not be executed by 
.BR run-parts . 
For example, any file containing dots will be ignored.
This is done to prevent cron from running any of the files
that are left by the Debian package management system when handling files in
/etc/cron.d/ as configuration files (i.e. files ending in .dpkg-dist, .dpkg-orig, 
and .dpkg-new).

This feature can be used by system administrators and packages to include
tasks that will be run at defined intervals. Files created by packages in these
directories should be named after the package that supplies them.

.PP
Support for /etc/cron.d is included in the
.I cron 
daemon itself, which handles this location as the system-wide crontab spool.
This directory can contain any file defining tasks following the format
used in /etc/crontab, i.e. unlike the user cron spool, these files must
provide the username to run the task as in the task definition.

Files in this directory have to be owned by root, do not need to be executable
(they are configuration files, just like /etc/crontab) and 
must conform to the same naming convention as used by 
.IR run-parts(8) :
they
must consist solely of upper- and lower-case letters, digits, underscores,
and hyphens. This means that they 
.B cannot
contain any dots. 
If the 
.B -l
option is specified to 
.I cron
(this option can be setup through /etc/default/cron, see below), then they must
conform to the LSB namespace specification, exactly as in the
.B --lsbsysinit
option in 
.IR run-parts .

The intended purpose
of this feature is to allow packages that require
finer control of their scheduling than the /etc/cron.{hourly,daily,weekly,monthly} 
directories to add a crontab file to /etc/cron.d. Such files
should be named after the package that supplies them.


Also, the default configuration of 
.I cron
is controlled by
.I /etc/default/cron
which is read by the init.d script that launches the
.I cron
daemon. This file determines whether 
.I cron
will read the system's environment variables and makes it possible to add 
additional options to the 
.I cron
program before it is executed, either to configure its logging or to define how
it will treat the files under /etc/cron.d.

.SH "SEE ALSO"
crontab(1), crontab(5), run-parts(8)
.SH AUTHOR
Paul Vixie <paul@vix.com> is the author of 
.I cron
and original creator of this manual page. This page has also been modified for
Debian by Steve Greenland, Javier Fernandez-Sanguino and Christian Kastner.