NAME
crontab
—
tables for driving cron
DESCRIPTION
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 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.
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:
While lines in a user crontab
have five
fixed fields plus a command in the form:
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 * * * *".
EXAMPLES
# 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
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 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.
AUTHORS
Paul Vixie ⟨vixie@isc.org⟩