CRON(8)

CRON(8) | System Manager's Manual | CRON(8)

# NAME

cron — daemon to execute scheduled commands (ISC Cron V4.1)

# SYNOPSIS

cron | [-n] [-x debugflags]

# DESCRIPTION

cron is normally started during system boot by rc.d(8) framework, if cron is switched on in rc.conf(5).

It will return immediately so you don't have to start it with ‘&’.

cron searches /var/cron/tabs for crontab files which are named after accounts in /etc/passwd. Crontabs found are loaded into memory. cron also searches for /etc/crontab which is in a different format (see crontab(5)). Finally cron looks for crontabs in /etc/cron.d if it exists, and executes each file as a crontab.

When cron looks in a directory for crontabs (either in /var/cron/tabs or /etc/cron.d) it will not process files that:

- Start with a ‘.’ or a ‘#’.
- End with a ‘~’ or with “.rpmsave”, “.rpmorig”, or “.rpmnew”.
- Are of zero length.
- Their length is greater than MAXNAMLEN.

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).

Events such as START and FINISH are recorded in the /var/log/cron log file with date and time details. This information is useful for a number of reasons, such as determining the amount of time required to run a particular job. By default, root has an hourly job that rotates these log files with compression to preserve disk space.

Additionally, cron checks each minute to see if its spool directory's modtime (or the modtime on /etc/crontab or /etc/cron.d) 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.

The following options are available:

-x
  This flag turns on some debugging flags. debugflags is comma-separated list of debugging flags to turn on. If a flag is turned on, cron writes some additional debugging information to system log during its work. Available debugging flags are:
  sch
    scheduling
  proc
    process control
  pars
    parsing
  load
    database loading
  misc
    miscellaneous
  test
    test mode - do not actually execute any commands
  bit
    show how various bits are set (long)
  ext
    print extended debugging information
-n
  Stay in the foreground and don't daemonize cron.

# Daylight Saving Time and other time changes

Local time changes of less than three hours, such as those caused by the start or end of Daylight Saving Time, are handled specially. This only applies to jobs that run at a specific time and jobs that are run with a granularity greater than one hour. Jobs that run more frequently are scheduled normally.

If time has moved forward, those jobs that would have run in the interval that has been skipped will be run immediately. Conversely, if time has moved backward, care is taken to avoid running jobs twice.

Time changes of more than 3 hours are considered to be corrections to the clock or timezone, and the new time is used immediately.

# SIGNALS

On receipt of a SIGHUP, the cron daemon will close and reopen its log file. This is useful in scripts which rotate and age log files. Naturally this is not relevant if cron was built to use syslog(3).

# FILES

/var/cron/tabs
  cron spool directory
/etc/crontab
  system crontab file
/etc/cron.d/
  system crontab directory
/var/log/cron
  log file for cron events

# SEE ALSO

crontab(1), crontab(5)

# AUTHORS

Paul Vixie <vixie@isc.org>

October 12, 2011 | NetBSD 7.0