CRON(8)                  (19 April 2010)                  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 follow-
                  ing 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 differ-
          ent format (see crontab(5)). In Debian, the content of

     Page 1                       Plan 9             (printed 5/23/22)

     CRON(8)                  (19 April 2010)                  CRON(8)

          /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 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 adminis-
          trator 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 cron-
          tabs, checking 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) from the owner of the crontab (or from the email
          address given in the MAILFROM environment variable in the
          crontab, if such exists).  The children copies of cron run-
          ning 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 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 mod-
          time 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

     Page 2                       Plan 9             (printed 5/23/22)

     CRON(8)                  (19 April 2010)                  CRON(8)

          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 cor-
          rections to the clock, and the new time is used immediately.

          cron logs its action to the syslog facility 'cron', and log-
          ging may be controlled using the standard syslogd(8) facil-
          ity.

     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 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 timezone.

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

     DEBIAN SPECIFIC
          Debian introduces some changes to cron that were not origi-
          nally 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 cron-
               tabs),

          -    PAM support,

     Page 3                       Plan 9             (printed 5/23/22)

     CRON(8)                  (19 April 2010)                  CRON(8)

          -    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 exe-
          cute 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 exe-
          cutable, be owned by root, not be writable by group or other
          and, if symlinks, point to files owned by root.  Addition-
          ally, the file names must conform to the filename require-
          ments of run-parts: they must be entirely made up of let-
          ters, digits and can only contain the special signs under-
          scores ('_') and hyphens ('-').  Any file that does not con-
          form 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, .dpkg-
          old, and .dpkg-new).

          This feature can be used by system administrators and pack-
          ages to include 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 cron-
          tab spool.  This directory can contain any file defining
          tasks following the format used in /etc/crontab, i.e. unlike

     Page 4                       Plan 9             (printed 5/23/22)

     CRON(8)                  (19 April 2010)                  CRON(8)

          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 con-
          vention 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 modi-
          fied for Debian by Steve Greenland, Javier Fernandez-
          Sanguino and Christian Kastner.

     Page 5                       Plan 9             (printed 5/23/22)