Ubuntu Manpage: cron - daemon to execute scheduled commands (Vixie Cron)

Provided by: cron_3.0pl1-136ubuntu1_amd64

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 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 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 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 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 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 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 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 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 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 packages 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 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 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.

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.