Provided by: cronie_1.7.1-1_amd64 
NAME
crontab - files used to schedule the execution of programs
DESCRIPTION
A crontab file contains instructions for the cron(8) daemon in the following simplified manner: "run this
command at this time on this date". Each user can define their own crontab. Commands defined in any
given crontab are executed under the user who owns that particular crontab. Uucp and News usually have
their own crontabs, eliminating the need for explicitly running su(1) as part of a cron command.
Blank lines, leading spaces, and tabs are ignored. Lines whose first non-white space character is a
pound-sign (#) are comments, and are not processed. Note that comments are not allowed on the same line
as cron commands, since they are considered a 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 setting or a cron command. An environment setting
is of the form:
name = value
where the white spaces around the equal-sign (=) are optional, and any subsequent non-leading white
spaces in value is a 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 white spaces.
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 can be
overridden by settings in the crontab; LOGNAME can not.
(Note: the LOGNAME variable is sometimes called USER on BSD systems and is also automatically set).
In addition to LOGNAME, HOME, and SHELL, cron(8) looks at the MAILTO variable if a mail needs to be send
as a result of running any commands in that particular crontab. If MAILTO is defined (and non-empty),
mail is sent to the specified address. If MAILTO is defined but empty (MAILTO=""), no mail is sent.
Otherwise, mail is sent to the owner of the crontab. This option is useful if you decide to use
/bin/mail instead of /usr/lib/sendmail as your mailer. Note that /bin/mail does not provide aliasing and
UUCP usually does not read its mail. If MAILFROM is defined (and non-empty), it is used as the envelope
sender address, otherwise, ``root'' is used.
(Note: Both MAILFROM and MAILTO variables are expanded, so setting them as in the following example works
as expected: MAILFROM=cron-$USER@cron.com ($USER is replaced by the system user) )
By default, cron sends a mail using the 'Content-Type:' header of 'text/plain' with the 'charset='
parameter set to the 'charmap/codeset' of the locale in which crond(8) is started up, i.e., either the
default system locale, if no LC_* environment variables are set, or the locale specified by the LC_*
environment variables (see locale(7)). Different character encodings can be used for mailing cron job
outputs by setting the CONTENT_TYPE and CONTENT_TRANSFER_ENCODING variables in a crontab to the correct
values of the mail headers of those names.
The CRON_TZ variable specifies the time zone specific for the cron table. The user should enter a time
according to the specified time zone into the table. The time used for writing into a log file is taken
from the local time zone, where the daemon is running.
The MLS_LEVEL environment variable provides support for multiple per-job SELinux security contexts in the
same crontab. By default, cron jobs execute with the default SELinux security context of the user that
created the crontab file. When using multiple security levels and roles, this may not be sufficient,
because the same user may be running in different roles or in different security levels. For more
information about roles and SELinux MLS/MCS, see selinux(8) and the crontab example mentioned later on in
this text. You can set the MLS_LEVEL variable to the SELinux security context string specifying the
particular SELinux security context in which you want jobs to be run. crond will then set the execution
context of those jobs that meet the specifications of the particular security context. For more
information, see crontab(1) -s option.
The RANDOM_DELAY variable allows delaying job startups by random amount of minutes with upper limit
specified by the variable. The random scaling factor is determined during the cron daemon startup so it
remains constant for the whole run time of the daemon.
The format of a cron command is similar to the V7 standard, with a number of upward-compatible
extensions. Each line has five time-and-date fields followed by a username (if this is the system
crontab file), and followed by a command. Commands are executed by cron(8) when the 'minute', 'hour',
and 'month of the 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 the "missing hours" during the daylight savings
time conversion, will never match, causing jobs scheduled during the "missing times" not to be run.
Similarly, times that occur more than once (again, during the daylight savings time conversion) will
cause matching jobs to be run twice.
cron(8) examines cron entries 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 Sunday, or use names)
A field may contain 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. The
first number must be less than or equal to the second one.
Randomization of the execution time within a range can be used. A random number within a range specified
as two numbers separated with a tilde is picked. The specified range is inclusive. For example, 6~15
for a 'minutes' entry picks a random minute within 6 to 15 range. The random number is picked when
crontab file is parsed. The first number must be less than or equal to the second one. You might omit
one or both of the numbers specifying the range. For example, ~ for a 'minutes' entry picks a random
minute within 0 to 59 range.
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 for every other hour (the alternative in the V7 standard is "0,2,4,6,8,10,12,
14,16,18,20,22"). Step values are also permitted after an asterisk, so if specifying a job to be run
every two hours, you can use "*/2". Please note that steps are evaluated just within the field they are
applied to. For example "*/23" in hours field means to execute the job on the hour 0 and the hour 23
within a calendar day. See "NOTES" below for a workaround.
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 does not matter). Ranges and lists of names are allowed. Examples:
"mon,wed,fri", "jan-mar".
If the UID of the owner is 0 (root), the first character of a crontab entry can be "-" character. This
will prevent cron from writing a syslog message about the command being executed.
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 a "%" character, will be executed by /bin/sh or by the shell specified in
the SHELL variable of the cronfile. A "%" character 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.
Note: The day of a command's execution can be specified in the following two fields — 'day of month', and
'day of week'. If both fields are restricted (i.e., do not contain the "*" character), 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.
A crontab file syntax can be tested before an install using the -T option. See crontab(1) for details.
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
#
CRON_TZ=Japan
# 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"
Jobs in /etc/cron.d/
The jobs in cron.d and /etc/crontab are system jobs, which are used usually for more than one user, thus,
additionally the username is needed. MAILTO on the first line is optional.
EXAMPLE OF A JOB IN /etc/cron.d/job
#login as root
#create job with preferred editor (e.g. vim)
MAILTO=root
* * * * * root touch /tmp/file
NOTES
As noted above, skip values only operate within the time period they're attached to. For example,
specifying "0/35" for the minute field of a crontab entry won't cause that entry to be executed every 35
minutes; instead, it will be executed twice every hour, at 0 and 35 minutes past. For more fine-grained
control you can do something like this:
* * * * * if [ $(expr ( $(date +%s) / 60 ) % 58) = 0 ]; then echo this runs every 58 minutes; fi
0 * * * * if [ $(expr ( $(date +%s) / 3600 ) % 23) = 0 ]; then echo this runs every 23 hours on the hour; fi
Adjust as needed if your date(1) command does not accept "+%s" as the format string specifier to output
the current UNIX timestamp.
SELinux with multi level security (MLS)
In a crontab, it is important to specify a security level by crontab -s or specifying the required level
on the first line of the crontab. Each level is specified in /etc/selinux/targeted/seusers. When using
crontab in the MLS mode, it is especially important to:
- check/change the actual role,
- set correct role for directory, which is used for input/output.
EXAMPLE FOR SELINUX MLS
# login as root
newrole -r sysadm_r
mkdir /tmp/SystemHigh
chcon -l SystemHigh /tmp/SystemHigh
crontab -e
# write in crontab file
MLS_LEVEL=SystemHigh
0-59 * * * * id -Z > /tmp/SystemHigh/crontest
FILES
/etc/crontab main system crontab file. /var/spool/cron/crontabs a directory for storing crontabs defined
by users. /etc/cron.d/ a directory for storing system crontabs.
SEE ALSO
cron(8), crontab(1)
EXTENSIONS
These special time specification "nicknames" which replace the 5 initial time and date fields, and are
prefixed with the '@' character, are supported:
@reboot : Run once after reboot.
@yearly : Run once a year, ie. "0 0 1 1 *".
@annually : Run once a year, ie. "0 0 1 1 *".
@monthly : Run once a month, ie. "0 0 1 * *".
@weekly : Run once a week, ie. "0 0 * * 0".
@daily : Run once a day, ie. "0 0 * * *".
@hourly : Run once an hour, ie. "0 * * * *".
CAVEATS
crontab files have to be regular files or symlinks to regular files, they must not be executable or
writable for anyone else but the owner. This requirement can be overridden by using the -p option on the
crond command line. If inotify support is in use, changes in the symlinked crontabs are not
automatically noticed by the cron daemon. The cron daemon must receive a SIGHUP signal to reload the
crontabs. This is a limitation of the inotify API.
cron requires that each entry in a crontab end in a newline character. If the last entry in a crontab is
missing a newline (i.e. terminated by EOF), cron will consider the crontab (at least partially) broken.
A warning will be written to syslog.
AUTHOR
Paul Vixie
cronie 2012-11-22 CRONTAB(5)