man > 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] [-n fqdn] [-x debugflags]

       cron [-N]

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

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

       -P      Don't set PATH for child processes.  Let it inherit instead.

       -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.

       -N      Run cron jobs Now, immediately, and exit. This option is useful to perform tests.

       -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.

       -x debugflags
               Tell cron to be more verbose and output debugging information; debugflags is the  sum
               of those values:

                   1      "ext": ...

                   2      "sch": ...

                   4      "proc": ...

                   8      "pars": ...

                   16     "load": ...

                   32     "misc": ...

                   64     "test": ...

                   128    "bit": ...

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 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.  The
       system administrator may create cron jobs in /etc/cron.d/ with file  names  like  "local"  or
       "local-foo".


       /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  un‐
       der  /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 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 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 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.  Con‐
       versely, 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/de‐
       fault/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/localtime 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 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 pro‐
       vided in Debian through the default setting of the /etc/crontab file (see the system-wide ex‐
       ample  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 includ‐
       ing 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
       filename requirements of run-parts: they must be entirely made up of letters, 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  con‐
       taining  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 packages to include tasks that will be
       run at defined intervals.  Files created by packages in these directories should be named af‐
       ter 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 con‐
       figuration 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 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 AT 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)