Crontab – files used to schedule the execution of programs
A crontab file contains instructions for the cron 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 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 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 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.
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 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. 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.
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 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.
Lists are allowed. A list is a set of numbers (or ranges) separated by commas. Examples: “1,2,5,9”, “0-4,8-12”.
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
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
- /etc/crontab main system crontab file.
- /var/spool/cron/ a directory for storing crontabs defined by users.
- /etc/cron.d/ a directory for storing system crontabs.
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 * * * *".