1728: "Cron Mail"

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pogrmman
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1728: "Cron Mail"

Postby pogrmman » Fri Sep 02, 2016 12:18 pm UTC

Image

Alt text: Take THAT, piece of 1980s-era infrastructure I've inexplicably maintained on my systems for 15 years despite never really learning how it works.

Why do I use cron? I've never found out how to use it all that well either (I can add a task, but that's about it)...
It's almost like tar -- it's there, I can (kind of) use it, but I don't really know how.

Also, I read the title as "Corn Mail" and was really confused at first.
Last edited by pogrmman on Mon Sep 05, 2016 11:52 am UTC, edited 1 time in total.

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Re: 1782: "Cron Mail"

Postby FOARP » Fri Sep 02, 2016 12:25 pm UTC

To quote Barney Gumble from the Simpsons: "Hahahahaha . . . . I don't get it".

EDIT: though I'm sure all will become clear soon with XKCD explained.

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Re: 1782: "Cron Mail"

Postby pogrmman » Fri Sep 02, 2016 12:58 pm UTC

Cron is basically a job sechedulinn system for UNIX-like systems. You can set tasks to repeat, say, once a week or whatever.

It also tends to produce a lot of messages that you are supposed to read -- I think I have a job scheduled to clear them out weekly. I never read them.

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Re: 1782: "Cron Mail"

Postby orthogon » Fri Sep 02, 2016 1:11 pm UTC

AIUI, cron actually emails you the entirety of stdout (and possibly stderr) produced by the job, so if the scheduled job is verbose, you'll get massive e-mails. If it produces zero output the mail won't get sent. You can make it so by redirecting stdout and stderr to /dev/null. So it's not really that cron is trying to tell you something, it's the job that you scheduled that's trying to communicate. Cron is just the messenger, and you know what they say about them.
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Re: 1782: "Cron Mail"

Postby MrDude_1 » Fri Sep 02, 2016 1:16 pm UTC

Yeah, this is the first time in a long time XKCD actually stumped me. I suppose because its a Linux/Unix based joke.

So is he just putting the mail message back to the system object that sent it?

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Re: 1782: "Cron Mail"

Postby bachaddict » Fri Sep 02, 2016 1:24 pm UTC

Is he overwriting the list of cron jobs with the next email it sends? I agree with Ponytail in the last panel.
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Re: 1782: "Cron Mail"

Postby gpq » Fri Sep 02, 2016 2:33 pm UTC

bachaddict wrote:Is he overwriting the list of cron jobs with the next email it sends?


Yes.

Edit: Well, he's actually overwriting the system-wide crontab - which I don't think his own jobs should be able to do, unless he's logged in as root? Each user can also have their personal crontab file, which is what would cause mail sent to his user.

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Re: 1782: "Cron Mail"

Postby cellocgw » Fri Sep 02, 2016 3:34 pm UTC

All the same, I don't quite see how cronmail managed to get a posted comic from 4 or 5 months in the future :twisted:
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Re: 1782: "Cron Mail"

Postby Rombobjörn » Fri Sep 02, 2016 3:40 pm UTC

pogrmman wrote:It also tends to produce a lot of messages that you are supposed to read -- I think I have a job scheduled to clear them out weekly. I never read them.

That's definitely not the right solution.

If you don't care that your cron job isn't working, then remove that cron job so it stops wasting resources.

If the cron job does something useful but also spews garbage, then fix it to silence the garbage and only output important error messages.

If you really want to silence even the error messages, then redirect all output from that one cron job to /dev/null, so that other cron jobs can still report errors.

And to Ponytail: Instead of messing with MAILTO, consider enabling your local mailbox in your email client. Competent email programs can pick up your mail from /var/mail.

gpq wrote:Well, he's actually overwriting the system-wide crontab - which I don't think his own jobs should be able to do, unless he's logged in as root? Each user can also have their personal crontab file, which is what would cause mail sent to his user.

I suppose writing to /etc/crontab might work if the mail is from one of root's cron jobs. I would guess that it will append to the file rather than overwriting it. Either way, I suppose he'll start getting errors about a malformed crontab, although those probably won't be sent by email but end up in the system log – which he probably doesn't read either.

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Re: 1782: "Cron Mail"

Postby pogrmman » Fri Sep 02, 2016 4:19 pm UTC

I know what I did is the exact wrong solution. I got tired of reading everything every few days, and I have a lot of cron jobs, and rather then spend time resetting all of them to redirect output to /dev/null, I decided it was easier to just add another hub to clear them out...

Also, I kind of stopped maintaining that system around 6 months to a year ago because I got too busy and got a new computer (it's a Gentoo system, so I'm kind of dreading the process I'll have to go to to get it back up to date -- there are probably a new portage version, changes to config file syntax and more (I booted it up around a month ago to see how much work I had to do)).

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Re: 1782: "Cron Mail"

Postby keithl » Fri Sep 02, 2016 5:06 pm UTC

Randall wants more efficient tools to manage cron output, and hopes to get useful suggestions from this forum. So do I. Sysadmin gurus, start explaining!

Alt explanation 1: Since the "explainxkcd.com" page for this comic is also difficult to understand, Randall is hoping this comic will motivate the creation of the website "explainexplainxkcd.com".

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Re: 1782: "Cron Mail"

Postby ucim » Fri Sep 02, 2016 5:21 pm UTC

keithl wrote:Randall is hoping this comic will motivate the creation of the website "explainexplainxkcd.com".
Don't be silly. He's looking for us to solve the general problem of explaining arbitrary explanations. Bonus points for doing it in a standard way.

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Re: 1782: "Cron Mail"

Postby EugeneStyles » Fri Sep 02, 2016 5:43 pm UTC

ucim wrote:
keithl wrote:Randall is hoping this comic will motivate the creation of the website "explainexplainxkcd.com".
Don't be silly. He's looking for us to solve the general problem of explaining arbitrary explanations. Bonus points for doing it in a standard way.


Well, now I'm thinking about starting a website called "explainarbitrarysite.com". (Hmm... looks like explain.com is for sale...) Enter site name, then it responds with something like, "political jibber jabber," "niche website for small splinter group of obscure hobby," "a wizard did it," or "it's a joke".

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Re: 1782: "Cron Mail"

Postby pogrmman » Fri Sep 02, 2016 5:53 pm UTC

EugeneStyles wrote:
ucim wrote:
keithl wrote:Randall is hoping this comic will motivate the creation of the website "explainexplainxkcd.com".
Don't be silly. He's looking for us to solve the general problem of explaining arbitrary explanations. Bonus points for doing it in a standard way.


Well, now I'm thinking about starting a website called "explainarbitrarysite.com". (Hmm... looks like explain.com is for sale...) Enter site name, then it responds with something like, "political jibber jabber," "niche website for small splinter group of obscure hobby," "a wizard did it," or "it's a joke".



I wonder if you could use machine learning for that. For training, you'd come up with classifications, and train it using a random selection of websites. Or, you could do unsupervised learning -- the problem here becomes generating the witty output.

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Re: 1782: "Cron Mail"

Postby somitomi » Fri Sep 02, 2016 7:22 pm UTC

keithl wrote:Randall wants more efficient tools to manage cron output, and hopes to get useful suggestions from this forum. So do I. Sysadmin gurus, start explaining!

Alt explanation 1: Since the "explainxkcd.com" page for this comic is also difficult to understand, Randall is hoping this comic will motivate the creation of the website "explainexplainxkcd.com".

So if explainexplainxkcd.com explains xkcd explanations, what does explainxkcdxkcd.com do?
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Re: 1782: "Cron Mail"

Postby Copper Bezel » Fri Sep 02, 2016 7:33 pm UTC

Observational humor and graphs based on experience with explainxkcd.com, I should think.
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Re: 1782: "Cron Mail"

Postby Flumble » Fri Sep 02, 2016 11:29 pm UTC

gpq wrote:Edit: Well, he's actually overwriting the system-wide crontab - which I don't think his own jobs should be able to do, unless he's logged in as root? Each user can also have their personal crontab file, which is what would cause mail sent to his user.

So, does that mean you could turn this into a denial of service if you have a user account (with cron permissions) on a server?
Or does the real cron check/drop permissions for writing the logs?

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Re: 1782: "Cron Mail"

Postby bz-mof » Sat Sep 03, 2016 8:41 am UTC

I think the joke in this comix was not properl explained so far...

He is supposed to read all the messages cron generates for him. Because he thinks they are unimportant he sends them to cron (here: cron's config file), so cron has to read all the messages by itself. :!:

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Re: 1782: "Cron Mail"

Postby Brito » Sun Sep 04, 2016 10:05 pm UTC

Am I the only one that thought the strip would involve Conan's god Crom? (well, *now* I know it ends with a M, but I didn't yesterday...)

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Re: 1782: "Cron Mail"

Postby generalz » Mon Sep 05, 2016 7:51 am UTC

@op: you might want to change the comic number in the subject, it is 1728 and not 1782 :)

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Re: 1782: "Cron Mail"

Postby pogrmman » Mon Sep 05, 2016 11:53 am UTC

generalz wrote:@op: you might want to change the comic number in the subject, it is 1728 and not 1782 :)


Thanks for pointing that out.

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Re: 1782: "Cron Mail"

Postby Bounty » Wed Sep 14, 2016 5:56 pm UTC

cellocgw wrote:All the same, I don't quite see how cronmail managed to get a posted comic from 4 or 5 months in the future :twisted:
pogrmman wrote:
generalz wrote:@op: you might want to change the comic number in the subject, it is 1728 and not 1782 :)


Thanks for pointing that out.


And thank you for explaining that random comment up thread I hadn't had time to investigate yet!

Brito wrote:Am I the only one that thought the strip would involve Conan's god Crom? (well, *now* I know it ends with a M, but I didn't yesterday...)


So now we need a ponytail v. beret guy comic about how a Typo a new Linux Package resulted in the resurgence of a Hyborian Diety!
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