1ANACRONTAB(5) Cronie Users' Manual ANACRONTAB(5)
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6 crontab - files used to schedule the execution of programs
7
9 A crontab file contains instructions for the cron(8) daemon in the fol‐
10 lowing simplified manner: "run this command at this time on this date".
11 Each user can define their own crontab. Commands defined in any given
12 crontab are executed under the user who owns that particular crontab.
13 Uucp and News usually have their own crontabs, eliminating the need for
14 explicitly running su(1) as part of a cron command.
15 Blank lines, leading spaces, and tabs are ignored. Lines whose first
17 non-white space character is a pound-sign (#) are comments, and are
18 note processed. Note that comments are not allowed on the same line as
19 cron commands, since they are considered a part of the command. Simi‐
20 larly, comments are not allowed on the same line as environment vari‐
21 able settings.
22 An active line in a crontab is either an environment setting or a cron
24 command. An environment setting is of the form:
25 name = value
27 where the white spaces around the equal-sign (=) are optional, and any
29 subsequent non-leading white spaces in value is a part of the value
30 assigned to name. The value string may be placed in quotes (single or
31 double, but matching) to preserve leading or trailing white spaces.
32 Several environment variables are set up automatically by the cron(8)
34 daemon. SHELL is set to /bin/sh, and LOGNAME and HOME are set from the
35 /etc/passwd line of the crontab´s owner. HOME and SHELL can be over‐
36 ridden by settings in the crontab; LOGNAME can not.
37 (Note: the LOGNAME variable is sometimes called USER on BSD systems and
39 is also automatically set).
40 In addition to LOGNAME, HOME, and SHELL, cron(8) looks at the MAILTO
42 variable if a mail needs to be send as a result of running any commands
43 in that particular crontab. If MAILTO is defined (and non-empty), mail
44 is sent to the specified address. If MAILTO is defined but empty
45 (MAILTO=""), no mail is sent. Otherwise, mail is sent to the owner of
46 the crontab. This option is useful if you decide to use /bin/mail
47 instead of /usr/lib/sendmail as your mailer. Note that /bin/mail does
48 not provide aliasing and UUCP usually does not read its mail. If MAIL‐
49 FROM is defined (and non-empty), it is used as the envelope sender
50 address, otherwise, ``root'' is used.
51 By default, cron sends a mail using the 'Content-Type:' header of
53 'text/plain' with the 'charset=' parameter set to the 'charmap/codeset'
54 of the locale in which crond(8) is started up - i.e. either the default
55 system locale, if no LC_* environment variables are set, or the locale
56 specified by the LC_* environment variables (see locale(7)). Different
57 character encodings can be used for mailing cron job outputs by setting
58 the CONTENT_TYPE and CONTENT_TRANSFER_ENCODING variables in a crontab
59 to the correct values of the mail headers of those names.
60 The CRON_TZ variable specifies the time zone specific for the cron ta‐
62 ble. The user should enter a time according to the specified time zone
63 into the table. The time used for writing into a log file is taken
64 from the local time zone, where the daemon is running.
65 The MLS_LEVEL environment variable provides support for multiple per-
67 job SELinux security contexts in the same crontab. By default, cron
68 jobs execute with the default SELinux security context of the user that
69 created the crontab file. When using multiple security levels and
70 roles, this may not be sufficient, because the same user may be running
71 in different roles or in different security levels. For more informa‐
72 tion about roles and SELinux MLS/MCS, see selinux(8) and the crontab
73 example mentioned later on in this text. You can set the MLS_LEVEL
74 variable to the SELinux security context string specifying the particu‐
75 lar SELinux security context in which you want jobs to be run. crond
76 will then set the execution context of those jobs that meet the speci‐
77 fications of the particular security context. For more information,
78 see crontab(1) -s option.
79 The format of a cron command is similar to the V7 standard, with a num‐
81 ber of upward-compatible extensions. Each line has five time-and-date
82 fields followed by a username (if this is the system crontab file), and
83 followed by a command. Commands are executed by cron(8) when the
84 'minute', 'hour', and 'month of the year' fields match the current
85 time, and at least one of the two 'day' fields ('day of month', or 'day
86 of week') match the current time (see "Note" below).
87 Note that this means that non-existent times, such as the "missing
89 hours" during the daylight savings time conversion, will never match,
90 causing jobs scheduled during the "missing times" not to be run. Simi‐
91 larly, times that occur more than once (again, during the daylight sav‐
92 ings time conversion) will cause matching jobs to be run twice.
93 cron(8) examines cron entries every minute.
95 The time and date fields are:
97 field allowed values
99 ----- --------------
100 minute 0-59
101 hour 0-23
102 day of month 1-31
103 month 1-12 (or names, see below)
104 day of week 0-7 (0 or 7 is Sunday, or use names)
105 A field may contain an asterisk (*), which always stands for
107 "first-last".
108 Ranges of numbers are allowed. Ranges are two numbers separated with a
110 hyphen. The specified range is inclusive. For example, 8-11 for an
111 'hours' entry specifies execution at hours 8, 9, 10, and 11.
112 Lists are allowed. A list is a set of numbers (or ranges) separated by
114 commas. Examples: "1,2,5,9", "0-4,8-12".
115 Step values can be used in conjunction with ranges. Following a range
117 with "/<number>" specifies skips of the number's value through the
118 range. For example, "0-23/2" can be used in the 'hours' field to spec‐
119 ify command execution for every other hour (the alternative in the V7
120 standard is "0,2,4,6,8,10,12,14,16,18,20,22"). Step values are also
121 permitted after an asterisk, so if specifying a job to be run every two
122 hours, you can use "*/2".
123 Names can also be used for the 'month' and 'day of week' fields. Use
125 the first three letters of the particular day or month (case does not
126 matter). Ranges or lists of names are not allowed.
127 The "sixth" field (the rest of the line) specifies the command to be
129 run. The entire command portion of the line, up to a newline or a "%"
130 character, will be executed by /bin/sh or by the shell specified in the
131 SHELL variable of the cronfile. A "%" character in the command, unless
132 escaped with a backslash (\), will be changed into newline characters,
133 and all data after the first % will be sent to the command as standard
134 input.
135 Note: The day of a command's execution can be specified in the follow‐
137 ing two fields — 'day of month', and 'day of week'. If both fields are
138 restricted (i.e., do not contain the "*" character), the command will
139 be run when either field matches the current time. For example,
140 "30 4 1,15 * 5" would cause a command to be run at 4:30 am on the 1st
141 and 15th of each month, plus every Friday.
142
144 # use /bin/sh to run commands, no matter what /etc/passwd says
145 SHELL=/bin/sh
146 # mail any output to `paul', no matter whose crontab this is
147 MAILTO=paul
148 #
149 CRON_TZ=Japan
150 # run five minutes after midnight, every day
151 5 0 * * * $HOME/bin/daily.job >> $HOME/tmp/out 2>&1
152 # run at 2:15pm on the first of every month -- output mailed to paul
153 15 14 1 * * $HOME/bin/monthly
154 # run at 10 pm on weekdays, annoy Joe
155 0 22 * * 1-5 mail -s "It's 10pm" joe%Joe,%%Where are your kids?%
156 23 0-23/2 * * * echo "run 23 minutes after midn, 2am, 4am ..., everyday"
157 5 4 * * sun echo "run at 5 after 4 every sunday"
158
160 The jobs in cron.d and /etc/crontab are system jobs, which are used
161 usually for more than one user, thus, the username is needed. MAILTO on
162 the first line is optional.
163
165 #login as root
166 #create job with preferred editor (e.g. vim)
167 MAILTO=root
168 * * * * * root touch /tmp/file
169
171 In a crontab, it is important to specify a security level by crontab -s
172 or specifying the required level on the first line of the crontab. Each
173 level is specified in /etc/selinux/targeted/seusers. When using crontab
174 in the MLS mode, it is especially important to:
175 - check/change the actual role,
176 - set correct role for directory, which is used for input/output.
177
179 # login as root
180 newrole -r sysadm_r
181 mkdir /tmp/SystemHigh
182 chcon -l SystemHigh /tmp/SystemHigh
183 crontab -e
184 # write in crontab file
185 MLS_LEVEL=SystemHigh
186 0-59 * * * * id -Z > /tmp/SystemHigh/crontest
187
189 /etc/anacrontab system crontab file for jobs like cron.daily, weekly,
190 monthly. /var/spool/cron/ a directory for storing crontabs defined by
191 users. /etc/cron.d/ a directory for storing system crontables.
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194 cron(8), crontab(1)
195
197 These special time specification "nicknames" which replace the 5 ini‐
198 tial time and date fields, and are prefixed with the '@' character, are
199 supported:
200 @reboot : Run once after reboot.
201 @yearly : Run once a year, ie. "0 0 1 1 *".
202 @annually : Run once a year, ie. "0 0 1 1 *".
203 @monthly : Run once a month, ie. "0 0 1 * *".
204 @weekly : Run once a week, ie. "0 0 * * 0".
205 @daily : Run once a day, ie. "0 0 * * *".
206 @hourly : Run once an hour, ie. "0 * * * *".
207
209 crontab files have to be regular files or symlinks to regular files,
210 they must not be executable or writable for anyone else but the owner.
211 This requirement can be overridden by using the -p option on the crond
212 command line. If inotify support is in use, changes in the symlinked
213 crontabs are not automatically noticed by the cron daemon. The cron
214 daemon must receive a SIGHUP signal to reload the crontabs. This is a
215 limitation of the inotify API.
216
219 Paul Vixie <vixie@isc.org>
220Marcela Mašláňová July 2010 ANACRONTAB(5)