Updated: 2022/Sep/29
Please read Privacy Policy. It's for your privacy.
CRONTAB(5) File Formats Manual CRONTAB(5)
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 and each user may have their own crontab. 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 on the
command line.
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
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.
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. For example, "1,2,5,9" or "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.
A field may begin with a question mark (`?'), which indicates a single
value randomly selected when the crontab file is read. If the field
contains only a question mark, the value is randomly selected from the
range of all possible values for the field. If the question mark
precedes a range, the value is randomly selected from the range. For
example, "? ?2-5 * * *" specifies that a task will be performed daily
between 2:00am and and 5:59am at a time randomly selected when the
crontab file is first read. As just one example, this feature can be
used to prevent a large number of hosts from contacting a server
simultaneously and overloading it by staggering the time at which a
download script is executed.
Step values can be used in conjunction with ranges (but not random ranges
which represent a single number). 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.
Steps are also permitted after an asterisk, so 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 may be modified as follows:
-n command
No mail is sent after a successful run. The execution output
will only be mailed if the command exits with a non-zero exit
code. The -n option is an attempt to cure potentially copious
volumes of mail coming from cron(8).
-q command
Execution will not be logged.
-s command
Only a single instance of command will be run concurrently.
Additional instances of command will not be scheduled until the
earlier one completes.
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), match the current time.
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 startup.
@yearly Run every January 1 (0 0 1 1 *).
@annually The 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 The same as @daily.
@hourly Run every hour, on the hour (0 * * * *).
ENVIRONMENT
CRON_TZ The CRON_TZ variable can be set to an alternate time zone in
order to affect when the job is run. Note that this only
affects the scheduling of the job, not the time zone that
the job perceives when it is run. If CRON_TZ is defined but
empty (CRON_TZ=""), jobs are scheduled with respect to the
local time zone.
CRON_WITHIN The CRON_WITHIN variable should indicate the number of
seconds within a job's scheduled time that it should still
be run. For example if a job is scheduled for 12:30pm and
CRON_WITHIN is 120 (2 minutes), then the job will not be run
if the system attempts to start it past 12:32pm. On a
heavily loaded system, or on a system that has just been
"woken up", jobs will sometimes start later than originally
intended, and by skipping non-critical jobs because of
delays, system load can be lightened. If CRON_WITHIN is
defined but empty (CRON_WITHIN=""), or set to some non-
positive value (0, a negative number, or a non-numeric
string), it is treated as if it was unset, that is the job
will always run, even if it is going to start at a time long
past its scheduled time.
HOME Set from the user's /etc/passwd entry. May be overridden by
settings in the crontab.
LOGNAME Set from the user's /etc/passwd entry. May not be
overridden by settings in the crontab.
MAILTO 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 is useful for pseudo-users that lack an
alias that would otherwise redirect the mail to a real
person.
SHELL Set to /bin/sh. May be overridden by settings in the
crontab.
USER Set from the user's /etc/passwd entry. May not be
overridden by settings in the crontab.
FILES
/etc/crontab System crontab.
/var/cron/tabs/<user> User crontab.
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
crontab(1), cron(8)
STANDARDS
The crontab file format is compliant with the IEEE Std 1003.1-2008
("POSIX.1") specification. The behaviours described below are all
extensions to that standard:
- The day-of-week field may use 7 to represent Sunday.
- Ranges may include "steps".
- Months or days of the week can be specified by name.
- Mailing after a successful run can be suppressed with -n.
- Logging can be suppressed with -q.
- Environment variables can be set in a crontab.
- Command output can be mailed to a person other than the crontab
owner, or the feature can be turned off and no mail will be sent at
all.
- All of the `@' commands that can appear in place of the first five
fields.
AUTHORS
crontab was written by Paul Vixie <vixie@isc.org>.
NetBSD 10.99 February 26, 2022 NetBSD 10.99