CRONTAB man page on Oracle

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CRONTAB(5)			 File Formats			    CRONTAB(5)

       crontab - files used to schedule the execution of programs

       A crontab file contains instructions for the cron(8) daemon in the fol‐
       lowing simplified manner: "run this command at this time on this date".
       Each  user can define their own crontab.	 Commands defined in any given
       crontab are executed under the user who owns that  particular  crontab.
       Uucp and News usually have their own crontabs, eliminating the need for
       explicitly running su(1) as part of a cron command.

       Blank lines, leading spaces, and tabs are ignored.  Lines  whose	 first
       non-white space character is a pound-sign (#) are comments, and are not
       processed.  Note that comments are not allowed on the same line as cron
       commands,  since they are considered a part of the command.  Similarly,
       comments are not allowed on the same line as environment variable  set‐

       An  active line in a crontab is either an environment setting or a cron
       command.	 An environment setting is of the form:

	  name = value

       where the white spaces around the equal-sign (=) are optional, and  any
       subsequent  non-leading	white  spaces  in value is a part of the value
       assigned to name.  The value string may be placed in quotes (single  or
       double, but matching) to preserve leading or trailing white spaces.

       Several	environment  variables are set up automatically by the 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.  HOME and SHELL can be over‐
       ridden by settings in the crontab; LOGNAME can not.

       (Note: the LOGNAME variable is sometimes called USER on BSD systems and
       is also automatically set).

       In  addition  to	 LOGNAME, HOME, and SHELL, cron(8) looks at the MAILTO
       variable if a mail needs to be send as a result of running any commands
       in that particular crontab.  If MAILTO is defined (and non-empty), mail
       is sent to the specified address.   If  MAILTO  is  defined  but	 empty
       (MAILTO=""),  no mail is sent.  Otherwise, mail is sent to the owner of
       the crontab.  This option is useful if  you  decide  to	use  /bin/mail
       instead	of /usr/lib/sendmail as your mailer.  Note that /bin/mail does
       not provide aliasing and UUCP usually does not read its mail.  If MAIL‐
       FROM  is	 defined  (and	non-empty),  it is used as the envelope sender
       address, otherwise, ``root'' is used.

       By default, cron sends a mail using the 'Content-Type:'	header	of  of
       the  locale  in	which crond(8) is started up, i.e., either the default
       system locale, if no LC_* environment variables are set, or the	locale
       specified by the LC_* environment variables (see locale(7)).  Different
       character encodings can be used for mailing cron job outputs by setting
       the  CONTENT_TYPE  and CONTENT_TRANSFER_ENCODING variables in a crontab
       to the correct values of the mail headers of those names.

       The CRON_TZ variable specifies the time zone specific for the cron  ta‐
       ble.  The user should enter a time according to the specified time zone
       into the table.	The time used for writing into a  log  file  is	 taken
       from the local time zone, where the daemon is running.

       The  MLS_LEVEL  environment variable provides support for multiple per-
       job SELinux security contexts in the same crontab.   By	default,  cron
       jobs execute with the default SELinux security context of the user that
       created the crontab file.  When	using  multiple	 security  levels  and
       roles, this may not be sufficient, because the same user may be running
       in different roles or in different security levels.  For more  informa‐
       tion  about  roles  and SELinux MLS/MCS, see selinux(8) and the crontab
       example mentioned later on in this text.	 You  can  set	the  MLS_LEVEL
       variable to the SELinux security context string specifying the particu‐
       lar SELinux security context in which you want jobs to be  run.	 crond
       will  then set the execution context of those jobs that meet the speci‐
       fications of the particular security context.   For  more  information,
       see crontab(1) -s option.

       The RANDOM_DELAY variable allows delaying job startups by random amount
       of minutes with upper limit specified by the variable. The random scal‐
       ing  factor  is determined during the cron daemon startup so it remains
       constant for the whole run time of the daemon.

       The format of a cron command is similar to the V7 standard, with a num‐
       ber  of upward-compatible extensions.  Each line has five time-and-date
       fields followed by a username (if this is the system crontab file), and
       followed	 by  a	command.   Commands  are  executed by cron(8) when the
       'minute', 'hour', and 'month of the  year'  fields  match  the  current
       time, and at least one of the two 'day' fields ('day of month', or 'day
       of week') match the current time (see "Note" below).

       Note that this means that non-existent  times,  such  as	 the  "missing
       hours"  during  the daylight savings time conversion, will never match,
       causing jobs scheduled during the "missing times" not to be run.	 Simi‐
       larly, times that occur more than once (again, during the daylight sav‐
       ings time conversion) will cause matching jobs to be run twice.

       cron(8) examines cron entries every minute.

       The time and date fields are:

	      field	     allowed values
	      -----	     --------------
	      minute	     0-59
	      hour	     0-23
	      day of month   1-31
	      month	     1-12 (or names, see below)
	      day of week    0-7 (0 or 7 is Sunday, or use names)

       A  field	 may  contain  an  asterisk  (*),  which  always  stands   for

       Ranges of numbers are allowed.  Ranges are two numbers separated with a
       hyphen.	The specified range is inclusive.  For example, 8-11 for an

       Lists are allowed.  A list is a set of numbers (or ranges) separated by
       commas.	Examples: "1,2,5,9", "0-4,8-12".

       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 spec‐
       ify command execution for every other hour (the alternative in  the  V7
       standard	 is  "0,2,4,6,8,10,12,14,16,18,20,22").	  Step values are also
       permitted after an asterisk, so if specifying a job to be run every two
       hours, you can use "*/2".

       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  does  not
       matter).	 Ranges or lists of names are not allowed.

       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 a  "%"
       character, will be executed by /bin/sh or by the shell specified in the
       SHELL variable of the cronfile.	A "%" character in the command, unless
       escaped	with a backslash (\), will be changed into newline characters,
       and all data after the first % will be sent to the command as  standard

       Note:  The day of a command's execution can be specified in the follow‐
       ing two fields — 'day of month', and 'day of week'.  If both fields are
       restricted  (i.e.,  do not contain the "*" character), the command will
       be run when either field matches the current time.  For example,
       "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.

       # use /bin/sh to run commands, no matter what /etc/passwd says
       # mail any output to `paul', no matter whose crontab this is
       # 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"

Jobs in /etc/cron.d/
       The  jobs  in  cron.d  and /etc/crontab are system jobs, which are used
       usually for more than one  user,	 thus,	additionaly  the  username  is
       needed.	MAILTO on the first line is optional.

EXAMPLE OF A JOB IN /etc/cron.d/job
       #login as root
       #create job with preferred editor (e.g. vim)
       * * * * * root touch /tmp/file

SELinux with multi level security (MLS)
       In a crontab, it is important to specify a security level by crontab -s
       or specifying the required level on the	first  line  of	 the  crontab.
       Each  level  is specified in /etc/selinux/targeted/seusers.  When using
       crontab in the MLS mode, it is especially important to:
       - check/change the actual role,
       - set correct role for directory, which is used for input/output.

       # login as root
       newrole -r sysadm_r
       mkdir /tmp/SystemHigh
       chcon -l SystemHigh /tmp/SystemHigh
       crontab -e
       # write in crontab file
       0-59 * * * * id -Z > /tmp/SystemHigh/crontest

       /etc/crontab main system crontab file.	/var/spool/cron/  a  directory
       for  storing  crontabs  defined by users.  /etc/cron.d/ a directory for
       storing system crontabs.

       cron(8), crontab(1)

       These special time specification "nicknames" which replace the  5  ini‐
       tial time and date fields, and are prefixed with the '@' character, are

       @reboot	  :    Run once after reboot.
       @yearly	  :    Run once a year, ie.  "0 0 1 1 *".
       @annually  :    Run once a year, ie.  "0 0 1 1 *".
       @monthly	  :    Run once a month, ie. "0 0 1 * *".
       @weekly	  :    Run once a week, ie.  "0 0 * * 0".
       @daily	  :    Run once a day, ie.   "0 0 * * *".
       @hourly	  :    Run once an hour, ie. "0 * * * *".

       crontab files have to be regular files or symlinks  to  regular	files,
       they  must not be executable or writable for anyone else but the owner.
       This requirement can be overridden by using the -p option on the	 crond
       command	line.	If inotify support is in use, changes in the symlinked
       crontabs are not automatically noticed by the cron  daemon.   The  cron
       daemon  must receive a SIGHUP signal to reload the crontabs.  This is a
       limitation of the inotify API.

       Paul Vixie ⟨⟩

cronie				  2012-11-22			    CRONTAB(5)

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