cron(8)



CRON(8)                     System Manager's Manual                    CRON(8)

NAME
       cron - daemon to execute scheduled commands (Vixie Cron)

SYNOPSIS
       cron [-f] [-l] [-L loglevel]

DESCRIPTION
       cron  is  started automatically from /etc/init.d on entering multi-user
       runlevels.

OPTIONS
       -f      Stay in foreground mode, don't daemonize.

       -l      Enable LSB compliant names for /etc/cron.d files. This setting,
               however,   does   not   affect   the  parsing  of  files  under
               /etc/cron.hourly,    /etc/cron.daily,    /etc/cron.weekly    or
               /etc/cron.monthly.

       -n      Include the FQDN in the subject when sending mails. By default,
               cron will abbreviate the hostname.

       -L loglevel
               Tell cron what to log about jobs (errors are logged  regardless
               of this value) as the sum of the following values:

                   1      will log the start of all cron jobs

                   2      will log the end of all cron jobs

                   4      will log all failed jobs (exit status != 0)

                   8      will log the process number of all cron jobs

               The  default  is to log the start of all jobs (1). Logging will
               be disabled if levels is set to zero (0). A  value  of  fifteen
               (15) will select all options.

NOTES
       cron  searches  its  spool  area (/var/spool/cron/crontabs) for crontab
       files (which are named after accounts in /etc/passwd);  crontabs  found
       are  loaded  into  memory.  Note that crontabs in this directory should
       not be accessed directly - the crontab command should be used to access
       and update them.

       cron  also  reads /etc/crontab, which is in a slightly different format
       (see crontab(5)).  In Debian, the content of /etc/crontab is predefined
       to    run    programs    under    /etc/cron.hourly,    /etc/cron.daily,
       /etc/cron.weekly and /etc/cron.monthly. This configuration is  specific
       to Debian, see the note under DEBIAN SPECIFIC below.

       Additionally, in Debian, cron reads the files in the /etc/cron.d direc-
       tory.  cron treats the files in /etc/cron.d as in the same way  as  the
       /etc/crontab  file  (they  follow the special format of that file, i.e.
       they  include  the  user  field).  However,  they  are  independent  of
       /etc/crontab:  they  do  not, for example, inherit environment variable
       settings from it. This change is specific to Debian see the note  under
       DEBIAN SPECIFIC below.

       Like /etc/crontab, the files in the /etc/cron.d directory are monitored
       for changes. In  general,  the  system  administrator  should  not  use
       /etc/cron.d/, but use the standard system crontab /etc/crontab.

       /etc/crontab  and  the  files in /etc/cron.d must be owned by root, and
       must not be group- or other-writable. In contrast to  the  spool  area,
       the  files  under  /etc/cron.d  or  the  files  under /etc/cron.hourly,
       /etc/cron.daily, /etc/cron.weekly and  /etc/cron.monthly  may  also  be
       symlinks,  provided that both the symlink and the file it points to are
       owned by root.  The files under /etc/cron.d do  not  need  to  be  exe-
       cutable,  while  the  files  under  /etc/cron.hourly,  /etc/cron.daily,
       /etc/cron.weekly and /etc/cron.monthly do, as they are run by run-parts
       (see run-parts(8) for more information).

       cron  then wakes up every minute, examining all stored crontabs, check-
       ing each command to see if it should be  run  in  the  current  minute.
       When  executing  commands,  any  output  is  mailed to the owner of the
       crontab (or to the user named in the MAILTO environment variable in the
       crontab,  if  such  exists).  The children copies of cron running these
       processes have their name coerced to uppercase, as will be seen in  the
       syslog and ps output.

       Additionally,  cron  checks each minute to see if its spool directory's
       modtime (or the modtime on the /etc/crontab file) has changed,  and  if
       it  has,  cron  will then examine the modtime on all crontabs files and
       reload those which have changed.  Thus cron need not be restarted when-
       ever  a crontab file is modified.  Note that the crontab(1) command up-
       dates the modtime of the spool directory whenever it changes a crontab.

       Special considerations exist when the clock is changed by less  than  3
       hours,  for  example at the beginning and end of daylight savings time.
       If the time has moved forwards, those jobs which would have run in  the
       time  that  was skipped will be run soon after the change.  Conversely,
       if the time has moved backwards by less than 3 hours, those  jobs  that
       fall into the repeated time will not be re-run.

       Only  jobs that run at a particular time (not specified as @hourly, nor
       with '*' in the hour or minute specifier) are affected. Jobs which  are
       specified with wildcards are run based on the new time immediately.

       Clock  changes of more than 3 hours are considered to be corrections to
       the clock, and the new time is used immediately.

       cron logs its action to the syslog facility 'cron', and logging may  be
       controlled using the standard syslogd(8) facility.

ENVIRONMENT
       If  configured  in /etc/default/cron in Debian systems, the cron daemon
       localisation settings environment can be managed  through  the  use  of
       /etc/environment  or through the use of /etc/default/locale with values
       from the latter overriding values from the former. These files are read
       and  they will be used to setup the LANG, LC_ALL, and LC_CTYPE environ-
       ment variables. These variables are then used to  set  the  charset  of
       mails, which defaults to 'C'.

       This  does  NOT affect the environment of tasks running under cron. For
       more information on how to modify the  environment  of  tasks,  consult
       crontab(5)

       The  daemon will use, if present, the definition from /etc/timezone for
       the timezone.

       The environment can be redefined in user's crontab definitions but cron
       will only handle tasks in a single timezone.

DEBIAN SPECIFIC
       Debian  introduces some changes to cron that were not originally avail-
       able upstream. The most significant changes introduced are:

       --     Support    for    /etc/cron.{hourly,daily,weekly,monthly}    via
              /etc/crontab,

       --     Support for /etc/cron.d (drop-in dir for package crontabs),

       --     PAM support,

       --     SELinux support,

       --     auditlog support,

       --     DST and other time-related changes/fixes,

       --     SGID crontab(1) instead of SUID root,

       --     Debian-specific file locations and commands,

       --     Debian-specific configuration (/etc/default/cron),

       --     numerous other smaller features and fixes.

       Support  for  /etc/cron.hourly,  /etc/cron.daily,  /etc/cron.weekly and
       /etc/cron.monthly is provided in Debian through the default setting  of
       the /etc/crontab file (see the system-wide example in crontab(5)).  The
       default system-wide crontab contains four tasks: run every hour,  every
       day,  every week and every month. Each of these tasks will execute run-
       parts providing each one of the directories as an argument. These tasks
       are  disabled  if  anacron is installed (except for the hourly task) to
       prevent conflicts between both daemons.

       As described above, the files under these directories have to pass some
       sanity checks including the following: be executable, be owned by root,
       not be writable by group or other and,  if  symlinks,  point  to  files
       owned  by  root. Additionally, the file names must conform to the file-
       name requirements of run-parts: they must be entirely made up  of  let-
       ters,  digits  and can only contain the special signs underscores ('_')
       and hyphens ('-'). Any file that does not conform to these requirements
       will  not  be  executed by run-parts.  For example, any file containing
       dots will be ignored.  This is done to prevent cron from running any of
       the  files  that  are left by the Debian package management system when
       handling files in /etc/cron.d/ as configuration files (i.e. files  end-
       ing in .dpkg-dist, .dpkg-orig, and .dpkg-new).

       This  feature  can be used by system administrators and packages to in-
       clude tasks that will be run at defined  intervals.  Files  created  by
       packages  in  these  directories should be named after the package that
       supplies them.

       Support for /etc/cron.d is included in the cron  daemon  itself,  which
       handles this location as the system-wide crontab spool.  This directory
       can contain any file  defining  tasks  following  the  format  used  in
       /etc/crontab, i.e. unlike the user cron spool, these files must provide
       the username to run the task as in the task definition.

       Files in this directory have to be owned by root, do not need to be ex-
       ecutable  (they  are  configuration  files, just like /etc/crontab) and
       must conform to the same naming convention  as  used  by  run-parts(8):
       they  must consist solely of upper- and lower-case letters, digits, un-
       derscores, and hyphens. This means that they cannot contain  any  dots.
       If the -l option is specified to cron (this option can be setup through
       /etc/default/cron, see below), then they must conform to the LSB  name-
       space  specification,  exactly  as  in  the --lsbsysinit option in run-
       parts.

       The intended purpose of this feature is to allow packages that  require
       finer      control      of      their      scheduling      than     the
       /etc/cron.{hourly,daily,weekly,monthly} directories to  add  a  crontab
       file  to /etc/cron.d. Such files should be named after the package that
       supplies them.

       Also, the default configuration  of  cron  is  controlled  by  /etc/de-
       fault/cron  which  is  read by the init.d script that launches the cron
       daemon. This file determines whether cron will read the system's  envi-
       ronment  variables  and  makes it possible to add additional options to
       the cron program before it is executed, either to configure its logging
       or to define how it will treat the files under /etc/cron.d.

SEE ALSO
       crontab(1), crontab(5), run-parts(8)

AUTHOR
       Paul Vixie <paul@vix.com> is the author of cron and original creator of
       this manual page. This page has also been modified for Debian by  Steve
       Greenland, Javier Fernandez-Sanguino and Christian Kastner.

4th Berkeley Distribution        19 April 2010                         CRON(8)

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