1CRON(8)                      Cronie Users' Manual                      CRON(8)
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NAME

6       crond - daemon to execute scheduled commands
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SYNOPSIS

9       crond [-n | -p | -s | -c | -m<mailcommand>]
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11       crond -x [ext,sch,proc,pars,load,misc,test,bit]
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DESCRIPTION

14       Cron is started from /etc/rc.d/init.d or /etc/init.d It returns immedi‐
15       ately, thus, there is no need to need to start it with the '&'  parame‐
16       ter.
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18       Cron  searches  /var/spool/cron for crontab files which are named after
19       accounts in /etc/passwd; The found crontabs are loaded into the memory.
20       Cron also searches for /etc/anacrontab and any files in the /etc/cron.d
21       directory, which have a different format (see crontab(5)).  Cron  exam‐
22       ines  all  stored crontabs and checks each job to see if it needs to be
23       run in the current minute.  When  executing  commands,  any  output  is
24       mailed  to  the  owner  of the crontab (or to the user specified in the
25       MAILTO environment variable in the crontab, if such exists).   Any  job
26       output can also be sent to syslog by using the -s option.
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28       There  are  two  ways  how changes in crontables are checked. The first
29       method is checking the modtime of a file. The second  method  is  using
30       the  inotify  support.  Using of inotify is logged in the /var/log/cron
31       log after the daemon is started. The inotify support checks for changes
32       in  all  crontables  and  accesses  the hard disk only when a change is
33       detected.
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35       When using the modtime option, Cron  checks  its  crontables'  modtimes
36       every  minute to check for any changes and reloads the crontables which
37       have changed. There is no need  to  restart  Cron  after  some  of  the
38       crontables  were modified. The modtime option is also used when inotify
39       can not be initialized.
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41       Cron  checks  these  files  and  directories:  /etc/anacrontab   system
42       crontab,   usually  used  to  run  daily,  weekly,  monthly  jobs.  See
43       anacrontab(5) for more details.  /etc/cron.d/ directory  that  contains
44       system  cronjobs stored for different users.  /var/spool/cron directory
45       that contains user crontables created by the crontab command.
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47       Note that the crontab(1) command  updates  the  modtime  of  the  spool
48       directory whenever it changes a crontab.
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50   Daylight Saving Time and other time changes
51       Local  time  changes  of less than three hours, such as those caused by
52       the Daylight Saving Time changes, are handled in a special  way.   This
53       only applies to jobs that run at a specific time and jobs that run with
54       a granularity greater than one hour. Jobs that run more frequently  are
55       scheduled normally.
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57       If  time  was adjusted one hour forward, those jobs that would have run
58       in the interval that has been skipped will be  run  immediately.   Con‐
59       versely,  if  time was adjusted backward, running the same job twice is
60       avoided.
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62       Time changes of more than 3 hours are considered to be  corrections  to
63       the clock or the timezone, and the new time is used immediately.
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65       It  is  possible  to  use  different  time  zones  for  crontables. See
66       crontab(5) for more information.
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68   PAM Access Control
69       Cron supports access control with PAM if the system has PAM  installed.
70       For  more  information, see pam(8).  A PAM configuration file for crond
71       is installed in /etc/pam.d/crond.  The daemon loads the PAM environment
72       from  the  pam_env  module. This can be overridden by defining specific
73       settings in the appropriate crontab file.
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OPTIONS

76       -m     This option allows you to specify a shell  command  to  use  for
77              sending  Cron mail output instead of using sendmail(8) This com‐
78              mand must accept a fully formatted mail message  (with  headers)
79              on  standard  input and send it as a mail message to the recipi‐
80              ents specified in the mail headers. Specifying  the  string  off
81              (i.e. crond -m off) will disable the sending of mail.
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83       -n     Tells  the  daemon  to run in the foreground. This can be useful
84              when starting it out of init.
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86       -p     Allows Cron to accept any user set crontables.
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88       -c     This option enables clustering support, as described below.
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90       -s     This option will direct Cron to send the job output to the  sys‐
91              tem log using syslog(3).  This is useful if your system does not
92              have sendmail(8), installed or if mail is disabled.
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94       -x     This option allows you to set debug flags.
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SIGNALS

97       When the SIGHUP is received, the Cron daemon will close and reopen  its
98       log file.  This proves to be useful in scripts which rotate and age log
99       files.  Naturally, this is not relevant if Cron was built to  use  sys‐
100       log(3).
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CLUSTERING SUPPORT

103       In  this version of Cron it is possible to use a network-mounted shared
104       /var/spool/cron across a cluster of hosts and specify that only one  of
105       the  hosts  should  run  the  crontab jobs in this directory at any one
106       time. This is done by starting Cron with the -c option,  and  have  the
107       /var/spool/cron/.cron.hostname file contain just one line, which repre‐
108       sents the hostname of whichever host in  the  cluster  should  run  the
109       jobs.   If  this  file  does  not exist, or the hostname in it does not
110       match that returned by gethostname(2), then all crontab files  in  this
111       directory  are  ignored.   This has no effect on cron jobs specified in
112       the /etc/crontab file or on files in the /etc/cron.d  directory.  These
113       files are always run and considered host-specific.
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115       Rather than editing /var/spool/cron/.cron.hostname directly, use the -n
116       option of crontab(1) to specify the host.
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118       You should ensure that all hosts in a cluster, and the file server from
119       which  they  mount  the shared crontab directory, have closely synchro‐
120       nised clocks, e.g. using ntpd(8) , otherwise the results will  be  very
121       unpredictable.
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123       Using  cluster  sharing automatically disables inotify support, because
124       inotify cannot be relied on with network-mounted shared file systems.
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CAVEATS

127       All crontab files have to be  regular  files  or  symlinks  to  regular
128       files,  they must not be executable or writable for anyone else but the
129       owner.  This requirement can be overridden by using the  -p  option  on
130       the  crond  command line.  If inotify support is in use, changes in the
131       symlinked crontabs are not automatically noticed by  the  cron  daemon.
132       The  cron  daemon  must receive a SIGHUP signal to reload the crontabs.
133       This is a limitation of the inotify API.
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135       The syslog output will be used instead of mail, when  sendmail  is  not
136       installed.
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SEE ALSO

139       crontab(1), crontab(5), inotify(7), pam(8)
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AUTHOR

142       Paul Vixie <vixie@isc.org>
143       Marcela Mašláňová <mmaslano@redhat.com>
144       Colin Dean <colin@colin-dean.org>
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148Marcela Mašláňová                  July 2010                           CRON(8)
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