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 com-
mands 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(1) 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
An active line in a crontab will be either an environment
setting or a cron command. An environment setting is of
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 set-
tings 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
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-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 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 daylight 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 0-31
month 0-12 (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
Ranges of numbers are allowed. Ranges are two numbers
separated with a hyphen. The specified range is inclu-
sive. 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. Fol-
lowing 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 execu-
tion 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 partic-
ular 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 exam-
``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"
When specifying day of week, both day 0 and day 7 will be
considered Sunday. BSD and ATT seem to disagree about
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
Names of months or days of the week can be specified by
Environment variables can be set in the crontab. In BSD
or ATT, the environment handed to child processes is basi-
cally 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
Paul Vixie <email@example.com>
24 January 1994 1