85 lines
3.8 KiB
Plaintext
85 lines
3.8 KiB
Plaintext
$Id$
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Features of Vixie's cron relative to BSD 4.[23] and SysV crons:
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-- Environment variables can be set in each crontab. SHELL, USER,
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LOGNAME, and HOME are set from the user's passwd entry; all except
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USER can be changed in the crontab. PATH is especially useful to
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set there. TZ can be set, but cron ignores it other than passing
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it on through to the commands it runs. Format is
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variable=value
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Blanks surrounding the '=' will be eaten; other blanks in value are
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okay. Leading or trailing blanks can be preserved by quoting, single
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or double quotes are okay, just so they match.
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PATH=.:/bin:/usr/bin
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SHELL=/bin/sh
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FOOBAR = this is a long blanky example
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Above, FOOBAR would get "this is a long blanky example" as its value.
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SHELL and HOME will be used when it's time to run a command; if
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you don't set them, HOME defaults to your /etc/passwd entry
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and SHELL defaults to /bin/sh.
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MAILTO, if set to the login name of a user on your system, will be the
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person that cron mails the output of commands in that crontab. This is
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useful if you decide on BINMAIL when configuring cron.h, since binmail
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doesn't know anything about aliasing.
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-- Weekdays can be specified by name. Case is not significant, but only
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the first three letters should be specified.
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-- Months can likewise be specified by name. Three letters only.
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-- Ranges and lists can be mixed. Standard crons won't allow '1,3-5'.
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-- Ranges can specify 'step' values. '10-16/2' is like '10,12,14,16'.
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-- Sunday is both day 0 and day 7 -- apparently BSD and ATT disagree
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about this.
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-- Each user gets their own crontab file. This is a win over BSD 4.2,
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where only root has one, and over BSD 4.3, where they made the crontab
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format incompatible and although the commands can be run by non-root
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uid's, root is still the only one who can edit the crontab file. This
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feature mimics the SysV cron.
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-- The 'crontab' command is loosely compatible with SysV, but has more
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options which just generally make more sense. Running crontab with
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no arguments will print a cute little summary of the command syntax.
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-- Comments and blank lines are allowed in the crontab file. Comments
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must be on a line by themselves; leading whitespace is ignored, and
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a '#' introduces the comment.
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-- (big win) If the `crontab' command changes anything in any crontab,
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the 'cron' daemon will reload all the tables before running the
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next iteration. In some crons, you have to kill and restart the
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daemon whenever you change a crontab. In other crons, the crontab
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file is reread and reparsed every minute even if it didn't change.
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-- In order to support the automatic reload, the crontab files are not
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readable or writable except by 'crontab' or 'cron'. This is not a
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problem, since 'crontab' will let you do pretty much whatever you
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want to your own crontab, or if you are root, to anybody's crontab.
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-- If any output is generated by a command (on stdout OR stderr), it will
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be mailed to the owner of the crontab that contained the command (or
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MAILTO, see discussion of environment variables, above). The headers
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of the mail message will include the command that was run, and a
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complete list of the environment that was passed to it, which will
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contain (at least) the USER (LOGNAME on SysV), HOME, and SHELL.
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-- the dom/dow situation is odd. '* * 1,15 * Sun' will run on the
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first and fifteenth AND every Sunday; '* * * * Sun' will run *only*
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on Sundays; '* * 1,15 * *' will run *only* the 1st and 15th. this
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is why we keep 'e->dow_star' and 'e->dom_star'. I didn't think up
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this behaviour; it's how cron has always worked but the documentation
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hasn't been very clear. I have been told that some AT&T crons do not
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act this way and do the more reasonable thing, which is (IMHO) to "or"
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the various field-matches together. In that sense this cron may not
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be completely similar to some AT&T crons.
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