CRONTAB(5) | File Formats Manual | CRONTAB(5) |
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
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:
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. 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-n
option is an attempt to cure potentially
copious volumes of mail coming from
cron(8).-q
command-s
commandCommands 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 * * * *). |
CRON_TZ
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
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
crontab
.LOGNAME
crontab
.MAILTO
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
crontab
.USER
crontab
.# 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"
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:
-n
.-q
.@
’ commands that can
appear in place of the first five fields.crontab
was written by
Paul Vixie
<vixie@isc.org>.
February 26, 2022 | NetBSD 10.99 |