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 dis‐
               abled 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 pro‐
       grams   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  directory.
       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   executable,   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,  checking  each
       command  to  see if it should be run in the current minute.  When executing com‐
       mands, 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 whenever a crontab file  is  modified.
       Note  that  the  crontab(1)  command  updates 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 con‐
       trolled using the standard syslogd(8) facility.

ENVIRONMENT
       If configured in /etc/default/cron in Debian systems, the cron daemon  localisa‐
       tion  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 environment 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 time‐
       zone.

       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  available
       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 sys‐
       tem-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 be pass some san‐
       ity 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. Addi‐
       tionally, the file names must conform to the filename requirements of run-parts:
       they  must  be entirely made up of letters, digits and can only contain the spe‐
       cial 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 ending  in  .dpkg-dist,
       .dpkg-orig, and .dpkg-new).

       This  feature can be used by system administrators and packages to include tasks
       that will be run at defined intervals. Files created by packages in these direc‐
       tories 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 executable
       (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, underscores, 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  con‐
       form  to  the LSB namespace 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/default/cron which
       is read by the init.d script that launches the cron daemon. This file determines
       whether  cron will read the system's environment 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)