File: crontab.5

package info (click to toggle)
cron 3.0pl1-128+deb9u1
  • links: PTS, VCS
  • area: main
  • in suites: stretch
  • size: 548 kB
  • sloc: ansic: 4,242; perl: 733; sh: 250; makefile: 94
file content (357 lines) | stat: -rw-r--r-- 13,160 bytes parent folder | download | duplicates (5)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
.\"/* Copyright 1988,1990,1993,1994 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: crontab.5,v 2.4 1994/01/15 20:43:43 vixie Exp $
.\" 
.TH CRONTAB 5 "19 April 2010"
.UC 4
.SH NAME
crontab \- tables for driving cron
.SH DESCRIPTION
A
.I crontab
file contains instructions to the
.IR cron (8)
daemon of the general form: ``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
.IR 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 hash-sign (#) 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.  The crontab file is parsed from top to bottom, so any environment
settings will affect only the cron commands below them in the file.
An environment setting is of the form,
.PP
    name = value
.PP
where the spaces around the equal-sign (=) are optional, and any subsequent
non-leading spaces in
.I value
will be part of the value assigned to
.IR name .
The
.I value
string may be placed in quotes (single or double, but matching) to preserve
leading or trailing blanks. To define an empty variable, quotes
.B must
be used. The  
.I value
string is 
.B not
parsed for environmental substitutions or replacement of variables, thus lines
like
.PP
    PATH = $HOME/bin:$PATH
.PP
will not work as you might expect. And neither will this work
.PP
    A=1
    B=2
    C=$A $B
.PP
There will not be any subsitution for the defined variables in the
last value.
.PP
An alternative for setting up the commands path is using the fact that
many shells will treat the tilde(~) as substitution of $HOME, so if you use 
.I bash
for your tasks you can use this:
.PP
     SHELL=/bin/bash
     PATH=~/bin:/usr/bin/:/bin
.PP
Several environment variables are set up automatically by the
.IR cron (8)
daemon.
SHELL is set to /bin/sh, and LOGNAME and HOME are set from the /etc/passwd 
line of the crontab's owner. PATH is set to "/usr/bin:/bin".
HOME, SHELL, and PATH may be overridden by settings in the crontab;
LOGNAME is the user that the job is running from, and may not be changed.
.PP
(Another note: the LOGNAME variable is sometimes called USER on BSD systems...
on these systems, USER will be set also.)
.PP
In addition to LOGNAME, HOME, and SHELL,
.IR cron (8)
will look at MAILTO if it has any reason to send mail as a result of running
commands in ``this'' crontab.  If MAILTO is defined (and non-empty), mail is
sent to the user so named.  MAILTO may also be used to direct mail to multiple
recipients by separating recipient users with a comma. If MAILTO is defined
but empty (MAILTO=""), no mail will be sent.  Otherwise mail is sent to the
owner of the crontab.
.PP
On the Debian GNU/Linux system, cron supports the
.B pam_env
module, and loads the environment specified by 
.IR /etc/environment
and
.IR /etc/security/pam_env.conf .
It also reads locale information from
.IR /etc/default/locale .
However, the PAM settings do
.B NOT
override the settings described above nor any settings in the 
.I crontab
file itself. Note in particular that if you want a PATH other than
"/usr/bin:/bin", you will need to set it in the crontab file.
.PP
By default, cron will send mail using the mail "Content-Type:" header of
"text/plain" with the "charset=" parameter set to the charmap / codeset of the
locale in which
.IR crond (8)
is started up - ie. either the default system locale, if no LC_* environment
variables are set, or the locale specified by the LC_* environment variables 
( see
.IR locale (7) ).
You can use different character encodings for mailed cron job output by
setting the CONTENT_TYPE and CONTENT_TRANSFER_ENCODING variables in crontabs,
to the correct values of the mail headers of those names.
.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 command, followed by a newline character ('\\n').  
The system crontab (/etc/crontab) uses the same format, except that
the username for the command is specified after the time and
date fields and before the command. The fields may be separated
by spaces or tabs. The maximum permitted length for the command field is
998 characters.
.PP
Commands are executed by
.IR cron (8)
when the minute, hour, and month of year fields match the current time,
.I and
when at least one of the two day fields (day of month, or day of week)
match the current time (see ``Note'' below).
.IR cron (8)
examines cron entries once every minute.
The time and date fields are:
.IP
.ta 1.5i
field	allowed values
.br
-----	--------------
.br
minute	0-59
.br
hour	0-23
.br
day of month	1-31
.br
month	1-12 (or names, see below)
.br
day of week	0-7 (0 or 7 is Sun, or use names)
.br
.PP
A field may be an asterisk (*), which always stands for ``first\-last''.
.PP
Ranges of numbers are allowed.  Ranges are two numbers separated
with a hyphen.  The specified range is inclusive.  For example,
8-11 for an ``hours'' entry specifies execution at hours 8, 9, 10
and 11.
.PP
Lists are allowed.  A list is a set of numbers (or ranges)
separated by commas.  Examples: ``1,2,5,9'', ``0-4,8-12''.
.PP
Step values can be used in conjunction with ranges.  Following
a range with ``/<number>'' specifies skips of the number's value
through the range.  For example, ``0-23/2'' can be used in the hours
field to specify command execution every other hour (the alternative
in the V7 standard is ``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 ``every two
hours'', just use ``*/2''.
.PP
Names can also be used for the ``month'' and ``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 ``sixth'' field (the rest of the line) specifies the command to be
run.
The entire command portion of the line, up to a newline or %
character, will be executed by /bin/sh or by the shell
specified in the SHELL variable of the crontab file.
Percent-signs (%) in the command, unless escaped with backslash
(\\), will be changed into newline characters, and all data
after the first % will be sent to the command as standard
input. There is no way to split a single command line onto multiple
lines, like the shell's trailing "\\".
.PP
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
.I either
field matches the current time.  For example,
.br
``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. One can, however, achieve the desired result
by adding a test to the command (see the last example in EXAMPLE CRON FILE
below).
.PP
Instead of the first five fields, one of eight special strings may appear:
.IP
.ta 1.5i
string	meaning
.br
------	-------
.br
@reboot	Run once, at startup.
.br
@yearly	Run once a year, "0 0 1 1 *".
.br
@annually	(same as @yearly)
.br
@monthly	Run once a month, "0 0 1 * *".
.br
@weekly	Run once a week, "0 0 * * 0".
.br
@daily	Run once a day, "0 0 * * *".
.br
@midnight	(same as @daily)
.br
@hourly	Run once an hour, "0 * * * *".
.br
.PP
Please note that startup, as far as @reboot is concerned, is the time when
the
.IR cron (8)
daemon startup.  In particular, it may be before some system daemons,
or other facilities, were startup.  This is due to the boot order
sequence of the machine.

.SH EXAMPLE CRON FILE

The following lists an example of a user crontab file.

.nf

# use /bin/bash to run commands, instead of the default /bin/sh
SHELL=/bin/bash
# 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"
# Run on every second Saturday of the month
0 4 8-14 * *    test $(date +\\%u) \-eq 6 && echo "2nd Saturday"
.fi
.SH EXAMPLE SYSTEM CRON FILE

The following lists the content of a regular system-wide crontab file. Unlinke a
user's crontab, this file has the username field, as used by /etc/crontab.

.nf
# /etc/crontab: system-wide crontab
# Unlike any other crontab you don't have to run the `crontab'
# command to install the new version when you edit this file
# and files in /etc/cron.d. These files also have username fields,
# that none of the other crontabs do.

SHELL=/bin/sh
PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin

# m h dom mon dow user	command
17 * * * *  root  cd / && run-parts \-\-report /etc/cron.hourly
25 6 * * *  root  test \-x /usr/sbin/anacron || ( cd / && run-parts \-\-report /etc/cron.daily )
47 6 * * 7  root  test \-x /usr/sbin/anacron || ( cd / && run-parts \-\-report /etc/cron.weekly )
52 6 1 * *  root  test \-x /usr/sbin/anacron || ( cd / && run-parts \-\-report /etc/cron.monthly )
#
.fi
.SH SEE ALSO
cron(8), crontab(1)
.SH EXTENSIONS
When specifying day of week, both day 0 and day 7 will be considered Sunday.
BSD and AT&T seem to disagree about this.
.PP
Lists and ranges are allowed to co-exist in the same field.  "1-3,7-9" would
be rejected by AT&T or BSD cron -- they want to see "1-3" or "7,8,9" ONLY.
.PP
Ranges can include "steps", so "1-9/2" is the same as "1,3,5,7,9".
.PP
Months or days of the week can be specified by name.
.PP
Environment variables can be set in the crontab.  In BSD or AT&T, the
environment handed to child processes is basically the one from /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 `@' commands that can appear in place of the first five fields
are extensions.
.SH LIMITATIONS
The
.I cron
daemon runs with a defined timezone. It currently does not support 
per-user timezones. All the tasks: system's and user's will be run based on the
configured timezone. Even if a user specifies the 
.I TZ
environment variable in his
.I crontab
this will affect only the commands executed in the crontab, not the execution
of the crontab tasks themselves.

The 
.I crontab
syntax does not make it possible to define all possible periods one could 
image off. For example, it is not straightforward to define the last
weekday of a month. If a task needs to be run in a specific period of time
that cannot be defined in the 
.I crontab
syntaxs the best approach would be to have the program itself check the
date and time information and continue execution only if the period
matches the desired one.

If the program itself cannot do the checks then a wrapper script would be
required. Useful tools that could be used for date analysis are 
.I ncal
or
.I calendar 
For example, to run a program the last Saturday of every month you could use
the following wrapper code:

.nf
0 4 * * Sat   [ "$(date +\\%e)" = "`ncal | grep $(date +\\%a | sed \-e 's/.$//') | sed \-e 's/^.*\\s\\([0-9]\\+\\)\\s*$/\\1/'`" ] && echo "Last Saturday" && program_to_run
.fi


.SH DIAGNOSTICS
cron requires that each entry in a crontab end in a newline character. If the
last entry in a crontab is missing a newline (ie, terminated by EOF), cron will
consider the crontab (at least partially) broken. A warning will be written to
syslog.

.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.