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Linux 2.6 - man page for crontab (linux section 5)

CRONTAB(5)									       CRONTAB(5)

       crontab - tables for driving cron

       A crontab file contains instructions to the 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 usu-
       ally have their own crontabs, eliminating the need for explicitly running su(1) as part of
       a cron command.

       Blank  lines and leading spaces and tabs are ignored.  Lines whose first non-space charac-
       ter 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.

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

	   name = value

       where  the  spaces  around the equal-sign (=) are optional, and any subsequent non-leading
       spaces in value will be 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 blanks.
       To define an empty variable, quotes must be used. The value string is not parsed for envi-
       ronmental substitutions or replacement of variables, thus lines like

	   PATH = $HOME/bin:$PATH

       will not work as you might expect. And neither will this work

	   C=$A $B

       There will not be any subsitution for the defined variables in the last value.

       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 bash for your tasks you can use


       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.  PATH  is  set to "/usr/bin:/bin".  HOME, SHELL, and PATH may be overridden by set-
       tings in the crontab; LOGNAME is the user that the job is running from,	and  may  not  be

       (Another  note:	the LOGNAME variable is sometimes called USER on BSD systems...  on these
       systems, USER will be set also.)

       In addition to LOGNAME, HOME, and SHELL, 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.

       On  the	Debian GNU/Linux system, cron supports the pam_env module, and loads the environ-
       ment specified by /etc/environment and /etc/security/pam_env.conf.  It also  reads  locale
       information  from /etc/default/locale.  However, the PAM settings do NOT override the set-
       tings described above nor any settings in the 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

       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 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 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.

       The format of a cron command is very much the V7 standard, with a number of upward-compat-
       ible 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.

       Commands are executed by cron(8) when the minute, hour, and month of year fields match the
       current	time,  and when at least one of the two day fields (day of month, or day of week)
       match the current time (see ``Note'' below).  cron(8) examines  cron  entries  once  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 Sun, or use names)

       A field may be an asterisk (*), which always stands for ``first-last''.

       Ranges of numbers are allowed.  Ranges are two numbers separated with a hyphen.	The spec-
       ified range is inclusive.  For example, 8-11 for an ``hours'' entry specifies execution at
       hours 8, 9, 10 and 11.

       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 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''.

       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.

       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 "\".

       Note: The day of a command's execution can be specified by two fields -- day of month, and
       day of week.  If both fields are restricted (i.e., aren't *), 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. One can, however, achieve the desired result by adding a test to
       the command (see the last example in EXAMPLE CRON FILE below).

       Instead of the first five fields, one of eight special strings may appear:

	      string	     meaning
	      ------	     -------
	      @reboot	     Run once, at startup.
	      @yearly	     Run once a year, "0 0 1 1 *".
	      @annually      (same as @yearly)
	      @monthly	     Run once a month, "0 0 1 * *".
	      @weekly	     Run once a week, "0 0 * * 0".
	      @daily	     Run once a day, "0 0 * * *".
	      @midnight      (same as @daily)
	      @hourly	     Run once an hour, "0 * * * *".

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

       This has the username field, as used by /etc/crontab.
       # /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 other the crontabs do.


       # m h dom mon dow user	 command
       42 6 * * *	 root	 run-parts --report /etc/cron.daily
       47 6 * * 7	 root	 run-parts --report /etc/cron.weekly
       52 6 1 * *	 root	 run-parts --report /etc/cron.monthly
       # Removed invocation of anacron, as this is now handled by a
       # /etc/cron.d file

       cron(8), crontab(1)

       When specifying day of week, both day 0 and day 7 will be considered Sunday.  BSD and AT&T
       seem to disagree about this.

       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.

       Ranges can include "steps", so "1-9/2" is the same as "1,3,5,7,9".

       Months or days of the week can be specified by name.

       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.

       Command output is mailed to the crontab owner (BSD can't do this), can be mailed to a per-
       son  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).

       All of the `@' commands that can appear in place of the first five fields are extensions.

       The cron daemon runs with a defined timezone. It currently does not support per-user time-
       zones.  All  the  tasks: system's and user's will be run based on the configured timezone.
       Even if a user specifies the TZ environment variable in his crontab this will affect  only
       the commands executed in the crontab, not the execution of the crontab tasks themselves.

       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.

       Paul Vixie <paul@vix.com>

4th Berkeley Distribution		  19 April 2010 			       CRONTAB(5)

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