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Linux 2.6 - man page for crontab (linux section 1posix)

CRONTAB(P)			    POSIX Programmer's Manual			       CRONTAB(P)

       crontab - schedule periodic background work

       crontab [file]

       crontab [ -e | -l | -r ]

       The crontab utility shall create, replace, or edit a user's crontab entry; a crontab entry
       is a list of commands and the times at which they shall be executed. The new crontab entry
       can  be input by specifying file or input from standard input if no file operand is speci-
       fied, or by using an editor, if -e is specified.

       Upon execution of a command from a  crontab  entry,  the  implementation  shall	supply	a
       default environment, defining at least the following environment variables:

       HOME   A pathname of the user's home directory.

	      The user's login name.

       PATH   A  string  representing a search path guaranteed to find all of the standard utili-

       SHELL  A pathname of the command interpreter. When crontab is invoked as specified by this
	      volume of IEEE Std 1003.1-2001, the value shall be a pathname for sh.

       The  values  of	these  variables  when	crontab is invoked as specified by this volume of
       IEEE Std 1003.1-2001 shall not affect the default values provided when the scheduled  com-
       mand is run.

       If  standard  output  and  standard error are not redirected by commands executed from the
       crontab entry, any generated output or errors shall  be	mailed,  via  an  implementation-
       defined method, to the user.

       Users   shall   be   permitted	to  use  crontab  if  their  names  appear  in	the  file
       /usr/lib/cron/cron.allow. If that file does not exist,  the  file  /usr/lib/cron/cron.deny
       shall  be checked to determine whether the user shall be denied access to crontab. If nei-
       ther file exists, only a process with appropriate privileges shall be allowed to submit	a
       job.  If  only  cron.deny  exists  and  is  empty,  global  usage shall be permitted.  The
       cron.allow and cron.deny files shall consist of one user name per line.

       The crontab utility shall conform to the Base Definitions volume of  IEEE Std 1003.1-2001,
       Section 12.2, Utility Syntax Guidelines.

       The following options shall be supported:

       -e     Edit  a copy of the invoking user's crontab entry, or create an empty entry to edit
	      if the crontab entry does not exist. When editing is complete, the entry	shall  be
	      installed as the user's crontab entry.

       -l     (The letter ell.) List the invoking user's crontab entry.

       -r     Remove the invoking user's crontab entry.

       The following operand shall be supported:

       file   The  pathname  of a file that contains specifications, in the format defined in the
	      INPUT FILES section, for crontab entries.

       See the INPUT FILES section.

       In the POSIX locale, the user or application shall ensure that a crontab entry is  a  text
       file  consisting  of  lines of six fields each. The fields shall be separated by <blank>s.
       The first five fields shall be integer patterns that specify the following:

	1. Minute [0,59]

	2. Hour [0,23]

	3. Day of the month [1,31]

	4. Month of the year [1,12]

	5. Day of the week ([0,6] with 0=Sunday)

       Each of these patterns can be either an asterisk (meaning all valid values),  an  element,
       or a list of elements separated by commas. An element shall be either a number or two num-
       bers separated by a hyphen (meaning an inclusive range). The specification of days can  be
       made  by  two  fields (day of the month and day of the week).  If month, day of month, and
       day of week are all asterisks, every day shall be matched. If either the month or  day  of
       month  is  specified  as an element or list, but the day of week is an asterisk, the month
       and day of month fields shall specify the days that match. If both month and day of  month
       are  specified as an asterisk, but day of week is an element or list, then only the speci-
       fied days of the week match. Finally, if either the month or day of month is specified  as
       an  element or list, and the day of week is also specified as an element or list, then any
       day matching either the month and day of month, or the day of week, shall be matched.

       The sixth field of a line in a crontab entry is a string that shall be executed by  sh  at
       the specified times. A percent sign character in this field shall be translated to a <new-
       line>. Any character preceded by a backslash (including the '%' ) shall cause that charac-
       ter  to be treated literally. Only the first line (up to a '%' or end-of-line) of the com-
       mand field shall be executed by the command interpreter. The other  lines  shall  be  made
       available to the command as standard input.

       Blank lines and those whose first non- <blank> is '#' shall be ignored.

       The  text files /usr/lib/cron/cron.allow and /usr/lib/cron/cron.deny shall contain zero or
       more user names, one per line, of users who are, respectively, authorized or denied access
       to the service underlying the crontab utility.

       The following environment variables shall affect the execution of crontab:

       EDITOR Determine  the  editor  to be invoked when the -e option is specified.  The default
	      editor shall be vi.

       LANG   Provide a default value for the internationalization variables that  are	unset  or
	      null. (See the Base Definitions volume of IEEE Std 1003.1-2001, Section 8.2, Inter-
	      nationalization Variables for the precedence of internationalization variables used
	      to determine the values of locale categories.)

       LC_ALL If  set  to a non-empty string value, override the values of all the other interna-
	      tionalization variables.

	      Determine the locale for the interpretation of sequences of bytes of text  data  as
	      characters  (for	example, single-byte as opposed to multi-byte characters in argu-
	      ments and input files).

	      Determine the locale that should be used to affect the format and contents of diag-
	      nostic messages written to standard error.

	      Determine the location of message catalogs for the processing of LC_MESSAGES .


       If the -l option is specified, the crontab entry shall be written to the standard output.

       The standard error shall be used only for diagnostic messages.



       The following exit values shall be returned:

	0     Successful completion.

       >0     An error occurred.

       The user's crontab entry is not submitted, removed, edited, or listed.

       The following sections are informative.

       The  format of the crontab entry shown here is guaranteed only for the POSIX locale. Other
       cultures may be supported with substantially different  interfaces,  although  implementa-
       tions are encouraged to provide comparable levels of functionality.

       The  default settings of the HOME , LOGNAME , PATH , and SHELL variables that are given to
       the scheduled job are not affected by the settings of those variables when crontab is run;
       as  stated,  they  are  defaults.  The  text about "invoked as specified by this volume of
       IEEE Std 1003.1-2001" means that the implementation  may  provide  extensions  that  allow
       these  variables  to be affected at runtime, but that the user has to take explicit action
       in order to access the extension, such as give a new option flag or modify the  format  of
       the crontab entry.

       A  typical  user error is to type only crontab; this causes the system to wait for the new
       crontab entry on standard input.  If end-of-file is  typed  (generally  <control>-D),  the
       crontab	entry is replaced by an empty file. In this case, the user should type the inter-
       rupt character, which prevents the crontab entry from being replaced.

	1. Clean up core files every weekday morning at 3:15 am:

	   15 3 * * 1-5 find $HOME -name core 2>/dev/null | xargs rm -f

	2. Mail a birthday greeting:

	   0 12 14 2 * mailx john%Happy Birthday!%Time for lunch.

	3. As an example of specifying the two types of days:

	   0 0 1,15 * 1

       would run a command on the first and fifteenth of each month, as well as on every  Monday.
       To specify days by only one field, the other field should be set to '*' ; for example:

	      0 0 * * 1

       would run a command only on Mondays.

       All  references	to a cron daemon and to cron files have been omitted. Although historical
       implementations have used this arrangement, there is no reason to limit future implementa-

       This  description of crontab is designed to support only users with normal privileges. The
       format of the input is based on the System V crontab; however,  there  is  no  requirement
       here  that the actual system database used by the cron daemon (or a similar mechanism) use
       this format internally. For example, systems derived from BSD are likely to have an  addi-
       tional  field appended that indicates the user identity to be used when the job is submit-

       The -e option was adopted from the SVID as a user convenience, although it does not  exist
       in all historical implementations.



       Portions  of  this  text  are  reprinted  and  reproduced in electronic form from IEEE Std
       1003.1, 2003 Edition, Standard for Information Technology  --  Portable	Operating  System
       Interface  (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 by
       the Institute of Electrical and Electronics Engineers, Inc and  The  Open  Group.  In  the
       event  of  any  discrepancy  between this version and the original IEEE and The Open Group
       Standard, the original IEEE and The Open Group Standard is the referee document. The orig-
       inal Standard can be obtained online at http://www.opengroup.org/unix/online.html .

IEEE/The Open Group			       2003				       CRONTAB(P)

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