Wednesday, December 16, 2009

Cron: Understanding crontab deeply?

A crontab file contains instructions to the cron daemon of the general
form: `run this command at this time on this date'. Each user has
their own crontab, and commands in any given crontab will be executed as
the user who owns the crontab. Uucp and News will usually have their own
crontabs, eliminating the need for explicitly running su as part of a
cron command.

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

where the spaces around the equal-sign (=) are optional, and any subse-
quent 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. The name string may
also be placed in quote (single or double, but matching) to preserve
leading, traling or inner blanks.

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 may be overrid-
den by settings in the crontab; LOGNAME may not.

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

In addition to LOGNAME, HOME, and SHELL, cron 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 /bin/mail instead of /usr/lib/sendmail
as your mailer when you install cron -- /bin/mail doesn't do aliasing,
and UUCP usually doesn't read its mail.

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 (with optional `:' and
`/' suffixes) if this is the system crontab file, followed
by a command. Commands are executed by cron 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 examines cron entries once 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 Sun, or use names)

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.

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 `/' 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 mat-
ter). Ranges or lists of names are not allowed.

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 % char-
acter, will be executed by /bin/sh 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.

The command can optionally be prefixed by `@AppleNotOnBattery ' to tell
cron not to run the command when functioning on battery power. For exam-
ple, the `sixth' field when using this option would appear something
like `@AppleNotOnBattery /usr/bin/touch /tmp/foo'

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 (ie, 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

string meaning
------ -------
@reboot Run once, at startup.
@yearly Run once a year, "0 0 1 1 *".
@annually (sames as @yearly)
@monthly Run once a month, "0 0 1 * *".
@weekly Run once a week, "0 0 * * 0".
@daily Run once a day, "0 0 * * *".
@midnight (same as @daily)
@hourly Run once an hour, "0 * * * *".


# use /bin/sh to run commands, overriding the default set by cron
# mail any output to `paul', no matter whose crontab this is
# 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"

When specifying day of week, both day 0 and day 7 will be considered Sunday. BSD and ATT seem to disagree about this.

Lists and ranges are allowed to co-exist in the same field. "1-3,7-9" would be rejected by ATT or BSD cron -- they want to see "1-3" or "7,8,9" ONLY.

Ranges can include "steps", so "1-9/2" is the same as "1,3,5,7,9".

Names of months or days of the week can be specified by name. Environment variables can be set in the crontab. In BSD or ATT, the environment handed to child processes is basically the one from /etc/rc.

Command output is mailed to the crontab owner (BSD can't do this), can be mailed to a person other than the crontab owner (SysV can't do this), or the feature can be turned off and no mail will be sent at all (SysV can't do this either).

All of the `@' commands that can appear in place of the first five fields are extensions.

If you're in one of the 70-odd countries that observe Daylight Savings Time, jobs scheduled during the rollback or advance will be affected. In general, it's not a good idea to schedule jobs during this period.

"Wisdom is the power to put our time and our knowledge to the proper use" - Thomas J. Watson