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Re: restart cron in Solaris


From: Ted Zlatanov
Subject: Re: restart cron in Solaris
Date: Mon, 10 Dec 2001 12:37:22 -0500
User-agent: Gnus/5.090004 (Oort Gnus v0.04) Emacs/21.1 (i386-redhat-linux-gnu)

Darren Dunham <address@hidden> writes:

>> Is this safe? Seems dangerous to edit /var/spool/cron/crontabs/root
>> directly, cron is placing the real file here, what if cron overwrites the
>> edits?
>
> 'cron' doesn't move/write crontab files, 'crontab' does.  
>
> Yes, there's a race condition here, but you should decide how bad it
> could be.  I would imagine that it's pretty safe.  If you're talking
> about root's crontab, then if you know that it's maintained by cfengine,
> you shouldn't be editing it anyway.

1) I don't have the source to crontab for Solaris, but fcron's
   fcrontab.c seems to handle this correctly, first reading the entire
   file into memory and then writing it out to disk, whether the file
   is from disk or from STDIN.  Interestingly, the fcrontab.c copy()
   function would overwrite the original crontab if it were used, but
   it's not used to install the crontab as far as I can see.  I think
   the file is always prebuffered because otherwise you'd have to make
   two functions, one for STDIN and one for regular files.

2) I've been using this method (invoking crontab on the cron tab file
   itself) for a while now on Linux and Solaris without any problems.
   I suppose it could be hacked with a precopy:

"/bin/cp $(root_crontab) /tmp/XYZ; $(crontab) /tmp/XYZ"

   or by copying the crontab from /etc/cfengine/crontabs and editing
   that file, but the direct-from-cron-spool method is more elegant
   and pretty safe in my experience.

3) I'd only use this method where cfengine was the sole maintainer of
   the file, of course...

Ted




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