NAME¶
crontab - tables for driving bcron
DESCRIPTION¶
A
crontab file contains instructions to the
bcron-sched(8) 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.
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 subsequent
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.
Several environment variables are set up automatically by the
bcron-exec(8) program. SHELL is set to /bin/sh, and LOGNAME, USER, and
HOME are set from the
/etc/passwd line of the crontab's owner.
In addition to LOGNAME, USER, HOME, and SHELL,
bcron-exec(8) 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 if this is the system crontab file, followed by a
command. Commands are executed by
bcron-sched(8) when the minute, hour,
and month of year fields match the current time,
and at least one of
the two day fields (day of month, or day of week) match the current time (see
``Note'' below). Jobs scheduled during non-existent times, such as
"missing hours" during daylight savings conversion, will be
scheduled at some point shortly after the non-existent time. Jobs scheduled
during repeating times, such as "duplicate hours" during daylight
savings conversion, will be scheduled only once (unless they would repeat
anyways, such as jobs that run every minute or hour).
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 matter).
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 will be executed by /bin/sh or by the shell
specified in the SHELL variable of the cronfile.
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.
EXAMPLE CRON FILE¶
# use /bin/sh to run commands, no matter what /etc/passwd says
SHELL=/bin/sh
# mail any output to `bruce@example.com', no matter whose crontab this is
MAILTO=bruce@example.com
#
# 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 bruce (above)
15 14 1 * * $HOME/bin/monthly
23 0-23/2 * * * echo "run 23 minutes after midn, 2am, 4am ..., everyday"
5 4 * * sun echo "run at 5 after 4 every sunday"
FILES¶
- /etc/crontab
- System crontab file
- /etc/cron.d/
- System crontab directory
SEE ALSO¶
bcron-sched(8),
bcron-spool(8),
bcrontab(1)
EXTENSIONS¶
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).
AUTHOR¶
Paul Vixie <vixie@isc.org>
Charles Cazabon <charlesc-cronman @ discworld.dyndns.org>
Bruce Guenter <bruce@untroubled.org>