crontab - tables for driving cron
A crontab file contains instructions to the cron(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. Uucp and News will usu-
ally have their own crontabs, eliminating the need for explicitly running su(1) as part of
a cron command.
Blank lines and leading spaces and tabs are ignored. Lines whose first non-space charac-
ter 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 cron(8) 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 overridden by settings in the crontab; LOGNAME may not.
(Another note: the LOGNAME variable is sometimes called USER on BSD systems... on these
systems, USER will be set also.)
In addition to LOGNAME, HOME, and SHELL, cron(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-compat-
ible 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 cron(8) 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). Note that this means that non-existant times, such as "missing hours" during day-
light savings conversion, will never match, causing jobs scheduled during the "missing
times" not to be run. Similarly, times that occur more than once (again, during daylight
savings conversion) will cause matching jobs to be run twice.
cron(8) examines cron entries once every minute.
The time and date fields are:
field allowed values
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 spec-
ified 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:
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, up to a newline or % character, 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.
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
# mail any output to `paul', no matter whose crontab this is
# 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"
/etc/crontab System crontab file
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 per-
son 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).
Paul Vixie <firstname.lastname@example.org>
4th Berkeley Distribution 24 January 1994 CRONTAB(5)