crontab(5) - phpMan

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CRONTAB(5)                                 File Formats                                CRONTAB(5)



NAME
       crontab - files used to schedule the execution of programs

DESCRIPTION
       A  crontab  file  contains instructions for the cron(8) daemon in the following 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
       settings.

       An  active line in a crontab is either an environment setting or a cron command.  An envi-
       ronment 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 overridden by settings in the crontab; LOGNAME can not.

       (Note:  the LOGNAME variable is sometimes called USER on BSD systems and is also automati-
       cally 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 MAILFROM 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 cor-
       rect values of the mail headers of those names.

       The CRON_TZ variable specifies the time zone specific for the cron table.  The user should
       enter a time according to the specified time zone into the table.  The time used for writ-
       ing 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 dif-
       ferent roles or in different security  levels.   For  more  information  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
       particular SELinux security context in which you want jobs to be run.  crond will then set
       the execution context of those jobs that meet the specifications of the  particular  secu-
       rity 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 scaling 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 number of upward-com-
       patible 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 cur-
       rent time (see "Note" below).

       Note that this means that non-existent times, such as the "missing hours" during the  day-
       light  savings time conversion, will never match, causing jobs scheduled during the "miss-
       ing times" not to be run.  Similarly, times that occur more than once (again,  during  the
       daylight savings 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 "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

       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 for every other hour (the alterna-
       tive in the V7 standard is "0,2,4,6,8,10,12,14,16,18,20,22").  Step values are  also  per-
       mitted  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 let-
       ters  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 com-
       mand  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 input.

       Note: The day of a command's execution can be specified in the  following  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.

EXAMPLE CRON FILE
       # use /bin/sh to run commands, no matter what /etc/passwd says
       SHELL=/bin/sh
       # mail any output to `paul', no matter whose crontab this is
       MAILTO=paul
       #
       CRON_TZ=Japan
       # 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)
       MAILTO=root
       * * * * * 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.

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

FILES
       /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.

SEE ALSO
       cron(8), crontab(1)

EXTENSIONS
       These  special  time  specification  "nicknames" which replace the 5 initial time and date
       fields, and are prefixed with the '@' character, are supported:

       @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 * * * *".

CAVEATS
       crontab files have to be regular files or symlinks to regular files, they must not be exe-
       cutable  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.

AUTHOR
       Paul Vixie <vixie AT isc.org>



cronie                                      2012-11-22                                 CRONTAB(5)

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