crontab

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CRONTAB(5) CRONTAB(5)

NAME
crontab - tables for driving cron (ISC Cron V4.1)

DESCRIPTION
A crontab file contains instructions to the cron(8) daemon of the gen-
eral form: ‘‘run this command at this time on this date’’. Each user
has their own crontab, and commands in any given crontab will be exe-
cuted 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. Simi-
larly, comments are not allowed on the same line as environment vari-
able 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 sub-
sequent 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 num-
ber 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 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-
existent times, such as "missing hours" during daylight savings con-
version, 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
----- --------------
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 per-
mitted 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 char-
acters, 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 cur-
rent 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"

FILES
/etc/crontab System crontab file