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

Provided by: cron_3.0pl1-124ubuntu2_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.

       -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. in order

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 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 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 sytem-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 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, 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.