CRONTAB(5) - phpMan

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 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-
       tings.
       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
       'text/plain' with the 'charset=' parameter set to the 'charmap/codeset'
       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
       "first-last".
       Ranges of numbers are allowed.  Ranges are two numbers separated with a
       hyphen.   The  specified  range is inclusive.  For example, 8-11 for an
       'hours' entry specifies execution at hours 8, 9, 10, and 11. The  first
       number must be less than or equal to the second one.
       Randomization of the execution time within a range can be used.  A ran-
       dom number within a range specified as two  numbers  separated  with  a
       tilde  is picked.  The specified range is inclusive.  For example, 6~15
       for a 'minutes' entry picks a random minute within 6 to 15 range.   The
       random  number is picked when crontab file is parsed.  The first number
       must be less than or equal to the second one. You  might  omit  one  or
       both  of  the numbers specifying the range.  For example, ~ for a 'min-
       utes' entry picks a random minute within 0 to 59 range.
       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.
       If  the  UID of the owner is 0 (root), the first character of a crontab
       entry can be "-" character. This will prevent cron from writing a  sys-
       log message about the command being executed.
       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
       input.
       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.
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,  additionally  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  ini-
       tial 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 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.
AUTHOR
       Paul Vixie <vixie AT isc.org>
cronie                            2012-11-22                        CRONTAB(5)