crontab - tables for driving cron
A crontab file contains instructions to the cron(8) daemon
of the general
form: ``at these times on these dates run this command''.
There may be a
system crontab (/etc/crontab) and each user may have their
own crontab
(/var/cron/tabs/<user>). Commands in any given crontab will
be executed
either as the user who owns the crontab or, in the case of
the system
crontab, as the user specified in the command line. Uucp
and News will
usually each have their own crontab, eliminating the need
for explicitly
running su(1) as part of a cron(8) command.
While a crontab is a text file, it is not intended to be directly edited.
Creation, modification, and removal of a crontab should be
done using
crontab(1).
Blank lines, 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(8) 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 is either an environment variable setting or
a cron(8) command.
Environment Variable Settings
Environment variable settings create the environment any
command in the
crontab is run in. An environment variable 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 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.
Note: on BSD systems the LOGNAME variable is sometimes
called USER. On
OpenBSD, cron(8) will set both USER and LOGNAME to the same
value.
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 for pseudo-users that lack an alias that
would otherwise
redirect the mail to a real person.
cron Commands
The format of a cron(8) command is very much the V7 standard, with a number
of upward-compatible extensions. Lines in the system
crontab have
six fixed fields plus a command in the form:
minute hour day-of-month month day-of-week user
command
While lines in a user crontab have five fixed fields plus a
command in
the form:
minute hour day-of-month month day-of-week command
Fields are separated by blanks or tabs. The command may be
one or more
fields long. The allowed values for the fields are:
field allowed values
----- --------------
minute * or 0-59
hour * or 0-23
day-of-month * or 1-31
month * or 1-12 or a name (see below)
day-of-week * or 0-7 or a name (0 or 7 is Sunday)
user a valid username
command text
Lists are allowed. A list is a set of numbers (or ranges)
separated by
commas. Examples: ``1,2,5,9'', ``0-4,8-12''.
Ranges of numbers are allowed. Ranges are two numbers separated with a
hyphen. The specified range is inclusive. For example,
8-11 for an hour
entry specifies execution at hours 8, 9, 10 and 11.
Step values can be used in conjunction with ranges. Following a range
with /number specifies skips of number through the range.
For example,
``0-23/2'' can be used in the hour 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''.
An asterisk (`*') is short form for a range of all allowed
values.
Names can be used in 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 command field (the rest of the line) is 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 crontab. Percent signs (`%') in the command, unless escaped with a
backslash (`'), will be changed into newline characters, and
all data
after the first `%' will be sent to the command as standard
input.
Commands are executed by cron(8) when the minute, hour, and
month fields
match the current time, and when at least one of the two day
fields (day-
of-month or day-of-week, see Note below) match the current
time. cron(8)
examines crontab entries once every minute.
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
(i.e.,
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 cron(8) startup.
@yearly Run every January 1, "0 0 1 1 *".
@annually (same as @yearly).
@monthly Run the first day of every month, "0 0 1 *
*".
@weekly Run every Sunday, "0 0 * * 0".
@daily Run every midnight, "0 0 * * *".
@midnight (same as @daily).
@hourly Run every hour, on the hour, "0 * * * *".
# 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"
crontab(1), cron(8)
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 coexist 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''.
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.
Paul Vixie <[email protected]>
OpenBSD 3.6 June 8, 1999
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