pkg-cron/trunk/cron.8

.\"/* Copyright 1988,1990,1993 by Paul Vixie
.\" * All rights reserved
.\" *
.\" * Distribute freely, except: don't remove my name from the source or
.\" * documentation (don't take credit for my work), mark your changes (don't
.\" * get me blamed for your possible bugs), don't alter or remove this
.\" * notice.  May be sold if buildable source is provided to buyer.  No
.\" * warrantee of any kind, express or implied, is included with this
.\" * software; use at your own risk, responsibility for damages (if any) to
.\" * 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
.\" */
.\" 
.\" $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
.TP
.B -L loglevel
Sets the loglevel for cron. The standard logging level (1) will log 
the start of all the cron jobs. A higher loglevel (2) will cause
cron to log also the end of all cronjobs, which can be useful to
audit the behaviour of tasks run by cron. Logging will be disabled
if the \fIloglevel\fR is set to zero (0).
.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) ). 
Additionally,
.I cron
reads the files in /etc/cron.d: it
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. The intended purpose
of this feature is to allow packages that require
finer control of their scheduling than the /etc/cron.{daily,weekly,monthly} 
directories to add a crontab file to /etc/cron.d. Such files
should be named after the package that supplies them. Files 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. If the 
.B -l
option is specified, then they must conform to the LSB namespace specification,
exactly as in the
.B --lsbsysinit
option in 
.IR run-parts .

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

In contrast to the spool area, files in /etc/cron.d may be symlinks, provided
that both the symlink and the file it points to are owned by root.

.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
.IR /etc/crontab)
has changed, and if it has,
.I cron
will then examine the modtime on all crontabs 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 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
whichever is available (with the latter one taking precedence). These
files are read and they will use to setup the LANG, LC_ALL, and
LC_CTYPE environment variables.
.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 "SEE ALSO"
crontab(1), crontab(5)
.SH AUTHOR
.nf
Paul Vixie <paul@vix.com>
1	.\"/* Copyright 1988,1990,1993 by Paul Vixie
2	.\" * All rights reserved
3	.\" *
4	.\" * Distribute freely, except: don't remove my name from the source or
5	.\" * documentation (don't take credit for my work), mark your changes (don't
6	.\" * get me blamed for your possible bugs), don't alter or remove this
7	.\" * notice. May be sold if buildable source is provided to buyer. No
8	.\" * warrantee of any kind, express or implied, is included with this
9	.\" * software; use at your own risk, responsibility for damages (if any) to
10	.\" * anyone resulting from the use of this software rests entirely with the
11	.\" * user.
12	.\" *
13	.\" * Send bug reports, bug fixes, enhancements, requests, flames, etc., and
14	.\" * I'll try to keep a version up to date. I can be reached as follows:
15	.\" * Paul Vixie <paul@vix.com> uunet!decwrl!vixie!paul
16	.\" */
17	.\"
18	.\" $Id: cron.8,v 2.2 1993/12/28 08:34:43 vixie Exp $
19	.\"
20	.TH CRON 8 "19 April 2010"
21	.UC 4
22	.SH NAME
23	cron \- daemon to execute scheduled commands (Vixie Cron)
24	.SH SYNOPSIS
25	cron
26	.RB [ -f ]
27	.RB [ -l ]
28	.RB [ -L
29	.IR loglevel ]
30	.SH DESCRIPTION
31	.I cron
32	is started automatically from /etc/init.d on entering multi-user
33	runlevels.
34	.SH OPTIONS
35	.TP 8
36	.B -f
37	Stay in foreground mode, don't daemonize.
38	.TP
39	.B -l
40	Enable LSB compliant names for /etc/cron.d files
41	.TP
42	.B -L loglevel
43	Sets the loglevel for cron. The standard logging level (1) will log
44	the start of all the cron jobs. A higher loglevel (2) will cause
45	cron to log also the end of all cronjobs, which can be useful to
46	audit the behaviour of tasks run by cron. Logging will be disabled
47	if the \fIloglevel\fR is set to zero (0).
48	.SH NOTES
49	.PP
50	.I cron
51	searches its spool area (/var/spool/cron/crontabs) for crontab
52	files (which are named after accounts in
53	/etc/passwd); crontabs found are loaded into memory. Note that
54	crontabs in this directory should not be accessed directly -
55	the
56	.I crontab
57	command should be used to access and update them.
58
59	.I cron
60	also reads /etc/crontab, which is in a slightly different format (see
61	.IR crontab(5) ).
62	Additionally,
63	.I cron
64	reads the files in /etc/cron.d: it
65	treats the files in /etc/cron.d as in the same way as the /etc/crontab file (they
66	follow the special format of that file, i.e. they include the
67	.I user
68	field). However, they are independent of /etc/crontab: they do not, for
69	example, inherit environment variable settings from it. The intended purpose
70	of this feature is to allow packages that require
71	finer control of their scheduling than the /etc/cron.{daily,weekly,monthly}
72	directories to add a crontab file to /etc/cron.d. Such files
73	should be named after the package that supplies them. Files must
74	conform to the same naming convention as used by
75	.IR run-parts(8) :
76	they
77	must consist solely of upper- and lower-case letters, digits, underscores,
78	and hyphens. If the
79	.B -l
80	option is specified, then they must conform to the LSB namespace specification,
81	exactly as in the
82	.B --lsbsysinit
83	option in
84	.IR run-parts .
85
86	Like /etc/crontab, the files in the /etc/cron.d directory are
87	monitored for changes. In general, the admin should not use /etc/cron.d/,
88	but use the standard system crontab /etc/crontab.
89
90	In contrast to the spool area, files in /etc/cron.d may be symlinks, provided
91	that both the symlink and the file it points to are owned by root.
92
93	.I cron
94	then wakes up every minute, examining all stored crontabs, checking
95	each command to see if it should be run in the current minute. When
96	executing commands, any output is mailed to the owner of the crontab
97	(or to the user named in the MAILTO environment variable in the
98	crontab, if such exists). The children copies of cron running these
99	processes have their name coerced to uppercase, as will be seen in the
100	syslog and ps output.
101	.PP
102	Additionally,
103	.I cron
104	checks each minute to see if its spool directory's modtime (or the modtime
105	on
106	.IR /etc/crontab)
107	has changed, and if it has,
108	.I cron
109	will then examine the modtime on all crontabs and reload those which have
110	changed. Thus
111	.I cron
112	need not be restarted whenever a crontab file is modified. Note that the
113	.IR crontab (1)
114	command updates the modtime of the spool directory whenever it changes a
115	crontab.
116	.PP
117	Special considerations exist when the clock is changed by less than 3
118	hours, for example at the beginning and end of daylight savings
119	time. If the time has moved forwards, those jobs which would have
120	run in the time that was skipped will be run soon after the change.
121	Conversely, if the time has moved backwards by less than 3 hours,
122	those jobs that fall into the repeated time will not be re-run.
123	.PP
124	Only jobs that run at a particular time (not specified as
125	@hourly, nor with '*' in the hour or minute specifier) are
126	affected. Jobs which are specified with wildcards are run based on the
127	new time immediately.
128	.PP
129	Clock changes of more than 3 hours are considered to be corrections to
130	the clock, and the new time is used immediately.
131	.PP
132	.I cron
133	logs its action to the syslog facility 'cron', and logging may be
134	controlled using the standard syslogd(8) facility.
135	.SH ENVIRONMENT
136	If configured in
137	.I /etc/default/cron
138	in Debian systems, the
139	.I cron
140	daemon localisation settings environment can be managed through the use of
141	.I /etc/environment
142	or through the use of
143	.I /etc/default/locale
144	whichever is available (with the latter one taking precedence). These
145	files are read and they will use to setup the LANG, LC_ALL, and
146	LC_CTYPE environment variables.
147	.PP
148	The daemon will use, if present, the definition from
149	.I /etc/timezone
150	for the timezone.
151	.PP
152	The environment can be redefined in user's crontab definitions but
153	.I cron
154	will only handle tasks in a single timezone.
155	.SH "SEE ALSO"
156	crontab(1), crontab(5)
157	.SH AUTHOR
158	.nf
159	Paul Vixie <paul@vix.com>