View Source: crontab(1) - Waikato Linux Users Group

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!!NAME
crontab - maintain crontab files for individual users (V3)

!!SYNOPSIS
crontab [[ -u user ] file
crontab [[ -u user ] { -l | -r | -e }

!!DESCRIPTION
''crontab'' is the program used to install, deinstall or list the tables used to drive the cron(8) daemon in Vixie Cron.  Each user can have their own crontab, and though these are files in /var/spool/cron/crontabs, they are not intended to be edited directly.

If the ''/etc/cron.allow'' file exists, then you must be listed therein in order to be allowed to use this command.  If the ''/etc/cron.allow'' file does not exist but the ''/etc/cron.deny'' file does exist, then you must __not__ be listed in the ''/etc/cron.deny'' file in order to use this command.  If neither of these files exists, then depending on site-dependent configuration parameters, only the super user will be allowed to use this command, or all users will be able to use this command. For standard Debian systems, all users may use this command.

If the ''-u'' option is given, it specifies the name of the user whose crontab is to be tweaked.  If this option is not given, ''crontab'' examines "your" crontab, i.e., the crontab of the person executing the command.  Note that su(1) can confuse ''crontab'' and that if you are running inside of su(1) you should always use the ''-u'' option for safety's sake.

The first form of this command is used to install a new crontab from some named file or standard input if the pseudo-filename "-" is given.

The ''-l'' option causes the current crontab to be displayed on standard output. See the note under  __DEBIAN SPECIFIC__ below.

The ''-r'' option causes the current crontab to be removed.

The ''-e'' option is used to edit the current crontab using the editor specified by the VISUAL or EDITOR environment variables. The specified editor __must__ edit the file in place; any editor that unlinks the file and recreates it cannot be used. After you exit from the editor, the modified crontab will be installed automatically.

!!DEBIAN SPECIFIC
The "out-of-the-box" behaviour for ''crontab -l'' is to display the three line "DO NOT EDIT THIS FILE" header  that is placed at the beginning of the crontab when it is installed. The problem is that it makes the sequence

crontab -l | crontab -

non-idempotent -- you keep adding copies of the header. This causes pain to scripts that use sed to edit a crontab. Therefore, the default behaviour of the  __-l__ option has been changed to not output such header. You may obtain the original behaviour by setting the environment variable  __CRONTAB_NOHEADER__ to 'N', which will cause the ''crontab -l'' command to emit the extraneous header.

!!SEE ALSO
crontab(5), cron(8)

!!FILES

/etc/cron.allow %%%
/etc/cron.deny %%%



!!STANDARDS
The ''crontab'' command conforms to IEEE Std1003.2-1992 ("POSIX").  This new command syntax differs from previous versions of Vixie Cron, as well as from the classic SVR3 syntax.



!!DIAGNOSTICS
A fairly informative usage message appears if you run it with a bad command line.



!!BUGS
Although cron requires that each entry in a crontab end in a newline character, the neither the crontab command nor the cron daemon will detect this error. Instead, the crontab will appear load normally. However, the command will never run. The best choice is to ensure that your crontab has a blank line at the end.



!!AUTHOR

Paul Vixie <paul@vix.com>

!!LOCAL NOTES

I recently had a problem where a computer was running cronjobs at the wrong time, according to system time.  I searched around a bit and found that cronjobs are dependent on the hardware clock's time and as such, even though the system time looked right and my crontab was correce, jobs were running at the wrong time.  A quick look at the adjtimex and hwclock man pages fixed that -- !DrewStephens
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Last edited on Monday, February 14, 2005 12:17:52 am by CraigBox
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