crontab

Schedule a command to run at a later date or time.

     A crontab file contains instructions to the cron daemon of the general
     form: `run this command at this time on this date'.  Each user has
     their own crontab, and commands in any given crontab will be executed as
     the user who owns the crontab.  Uucp and News will usually have their own
     crontabs, eliminating the need for explicitly running su as part of a
     cron command.

     Blank lines and leading spaces and tabs are ignored.  Lines whose first
     non-space character is a pound-sign (#) are comments, and are ignored.
     Note that comments are not allowed on the same line as cron commands,
     since they will be taken to be part of the command.  Similarly, comments
     are not allowed on the same line as environment variable settings.

     An active line in a crontab will be either an environment setting or a
     cron command.  An environment setting is of the form,

	 name = value

     where the spaces around the equal-sign (=) are optional, and any subse-
     quent non-leading spaces in value will be part of the value assigned to
     name.  The value string may be placed in quotes (single or double, but
     matching) to preserve leading or trailing blanks.	The name string may
     also be placed in quote (single or double, but matching) to preserve
     leading, traling or inner blanks.

     Several environment variables are set up automatically by the cron
     daemon.  SHELL is set to /bin/sh, and LOGNAME and HOME are set from the
     /etc/passwd line of the crontab's owner.  HOME and SHELL may be overrid-
     den by settings in the crontab; LOGNAME may not.

     (Another note: the LOGNAME variable is sometimes called USER on BSD sys-
     tems...  on these systems, USER will be set also).

     In addition to LOGNAME, HOME, and SHELL, cron will look at MAILTO if
     it has any reason to send mail as a result of running commands in
     `this' crontab.	If MAILTO is defined (and non-empty), mail is sent to
     the user so named.	 If MAILTO is defined but empty (MAILTO=""), no mail
     will be sent.  Otherwise mail is sent to the owner of the crontab.	 This
     option is useful if you decide on /bin/mail instead of /usr/lib/sendmail
     as your mailer when you install cron -- /bin/mail doesn't do aliasing,
     and UUCP usually doesn't read its mail.

     The format of a cron command is very much the V7 standard, with a number
     of upward-compatible extensions.  Each line has five time and date
     fields, followed by a user name (with optional `:group' and
     `/login-class' suffixes) if this is the system crontab file, followed
     by a command.  Commands are executed by cron when the minute, hour,
     and month of year fields match the current time, and when at least one of
     the two day fields (day of month, or day of week) match the current time
     (see `Note' below).  cron examines cron entries once every minute.
     The time and date fields are:

	   field	 allowed values
	   -----	 --------------
	   minute	 0-59
	   hour		 0-23
	   day of month	 1-31
	   month	 1-12 (or names, see below)
	   day of week	 0-7 (0 or 7 is Sun, or use names)

     A field may be an asterisk (*), which always stands for `first-last'.

     Ranges of numbers are allowed.  Ranges are two numbers separated with a
     hyphen.  The specified range is inclusive.	 For example, 8-11 for an
     ``hours'' entry specifies execution at hours 8, 9, 10 and 11.

     Lists are allowed.	 A list is a set of numbers (or ranges) separated by
     commas.  Examples: `1,2,5,9', `0-4,8-12'.

     Step values can be used in conjunction with ranges.  Following a range
     with `/number' specifies skips of the number's value through the
     range.  For example, `0-23/2' can be used in the hours field to specify
     command execution every other hour (the alternative in the V7 standard is
     `0,2,4,6,8,10,12,14,16,18,20,22').  Steps are also permitted after an
     asterisk, so if you want to say `every two hours', just use `*/2'.

     Names can also be used for the `month' and `day of week' fields.  Use
     the first three letters of the particular day or month (case doesn't mat-
     ter).  Ranges or lists of names are not allowed.

     The `sixth' field (the rest of the line) specifies the command to be
     run.  The entire command portion of the line, up to a newline or % char-
     acter, will be executed by /bin/sh or by the shell specified in the SHELL
     variable of the cronfile.	Percent-signs (%) in the command, unless
     escaped with backslash (\), will be changed into newline characters, and
     all data after the first % will be sent to the command as standard input.

     The command can optionally be prefixed by `@AppleNotOnBattery ' to tell
     cron not to run the command when functioning on battery power.  For exam-
     ple, the `sixth' field when using this option would appear something
     like `@AppleNotOnBattery /usr/bin/touch /tmp/foo'

     Note: The day of a command's execution can be specified by two fields --
     day of month, and day of week.  If both fields are restricted (ie, aren't
     *), the command will be run when either field matches the current time.
     For example, `30 4 1,15 * 5' would cause a command to be run at 4:30 am
     on the 1st and 15th of each month, plus every Friday.

     Instead of the first five fields, one of eight special strings may
     appear:

	   string	   meaning
	   ------	   -------
	   @reboot	   Run once, at startup.
	   @yearly	   Run once a year, "0 0 1 1 *".
	   @annually	   (sames as @yearly)
	   @monthly	   Run once a month, "0 0 1 * *".
	   @weekly	   Run once a week, "0 0 * * 0".
	   @daily	   Run once a day, "0 0 * * *".
	   @midnight	   (same as @daily)
	   @hourly	   Run once an hour, "0 * * * *".

EXAMPLE CRON FILE

     # use /bin/sh to run commands, overriding the default set by cron
     SHELL=/bin/sh
     #
     # mail any output to `paul', no matter whose crontab this is
     MAILTO=paul
     #
     # run five minutes after midnight, every day
     5 0 * * *	     $HOME/bin/daily.job >> $HOME/tmp/out 2>&1
     #
     # run at 2:15pm on the first of every month -- output mailed to paul
     15 14 1 * *     $HOME/bin/monthly
     #
     # run at 10 pm on weekdays, annoy Joe
     0 22 * * 1-5    mail -s "It's 10pm" joe%Joe,%%Where are your kids?%
     23 0-23/2 * * * echo "run 23 minutes after midn, 2am, 4am ..., everyday"
     5 4 * * sun     echo "run at 5 after 4 every sunday"

Notes
When specifying day of week, both day 0 and day 7 will be considered Sunday. BSD and ATT seem to disagree about this.

Lists and ranges are allowed to co-exist in the same field. "1-3,7-9" would be rejected by ATT or BSD cron -- they want to see "1-3" or "7,8,9" ONLY.

Ranges can include "steps", so "1-9/2" is the same as "1,3,5,7,9".

Names of months or days of the week can be specified by name. Environment variables can be set in the crontab. In BSD or ATT, the environment handed to child processes is basically the one from /etc/rc.

Command output is mailed to the crontab owner (BSD can't do this), can be mailed to a person other than the crontab owner (SysV can't do this), or the feature can be turned off and no mail will be sent at all (SysV can't do this either).

All of the `@' commands that can appear in place of the first five fields are extensions.

If you're in one of the 70-odd countries that observe Daylight Savings Time, jobs scheduled during the rollback or advance will be affected. In general, it's not a good idea to schedule jobs during this period.

“Wisdom is the power to put our time and our knowledge to the proper use” ~ Thomas J. Watson

Related:

crontab man page - Apple.com
cron - Daemon to execute scheduled commands.
disown - Unbind a job from the current login session
jobs - List active jobs


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