CRONTAB(5) 1990 CRONTAB(5)
NAME
crontab - tables for driving cron
DESCRIPTION
A crontab file contains instructions to the crond(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 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
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 crond(8) daemon from the /etc/passwd line of the
crontab's owner: USER, HOME, and SHELL. HOME and SHELL
may be overridden by settings in the crontab; USER may
not.
(Note: for UUCP, always set SHELL=/bin/sh, or crond(8)
will cheerfully try to execute your commands using
/usr/lib/uucp/uucico.)
(Another note: the USER variable is sometimes called
LOGNAME or worse on System V... on these systems, LOGNAME
will be set rather than USER.)
In addition to USER, HOME, and SHELL, crond(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
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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 command.
Commands are executed by crond(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).
crond(8) examines cron entries once every minute. The
time and date fields are:
field allowed values
----- --------------
minute 0-59
hour 0-23
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 matches the
current time.
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'').
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
the user's login shell 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
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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 `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"
SEE ALSO
crond(8), crontab(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, paul@vixie.sf.ca.us
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