man.bsd.lv manual page server

DESCRIPTION

Blank lines and 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 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 will be either an environment setting or a cron command. An environment setting is of the form,

    name = value

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 may be overridden by settings in the crontab; LOGNAME may not.

(Another note: the LOGNAME variable is sometimes called USER on BSD systems... on these systems, USER will be set also.)

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 if you decide on mail(1) instead of sendmail(1) as your mailer when you install cron -- mail(1) doesn't do aliasing, and UUCP usually doesn't read its mail.

In order to provide finer control over when jobs execute, users can also set the environment variables CRON_TZ and CRON_WITHIN. 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.

The CRON_WITHIN variable should indicate the number of seconds within a job's scheduled time that it should still be run. 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.

The format of a cron command is very much the V7 standard, with a number of upward-compatible extensions. Each line has five time and date fields, followed by a user name if this is the system crontab file, followed by a command. Commands are executed by cron(8) when the minute, hour, and month of year 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 (see “Note” below). cron(8) examines cron entries once every minute. The time and date fields are:

A field may be 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.

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.

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 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”.

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 doesn't matter). Ranges or lists of names are not allowed.

If the crontab file is the system crontab /etc/crontab, then the next ( “sixth”) field contains the username to run the command as.

The “sixth” field (or the “seventh” one for /etc/crontab) (the rest of the line) specifies the command to be run. The entire command portion of the line, up to a newline or percent signs (‘%’), will be executed by sh(1) or by the shell specified in the SHELL variable of the cronfile. Percent signs (‘%’) in the command, unless escaped with 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 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:

# 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"
? ?2-4 1,15 * * echo "random between 2am-4:59am on the 1st and 15th"