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Transcript from the "cron" Lesson
[00:00:00]
>> Jem Young: How do we do periodic tasks? This is a general problem when you're running servers, you want to do things periodically like clean up your log files, delete empty files, things like that. In our instance we want to make sure our certificate gets renewed regularly, without us thinking about it.
[00:00:15] So you're thinking, unattended upgrades when you're on that path, that's its own thing. It runs on its own, it runs its own daemon, we don't worry about that. In general, when we run periodic task we want to use a cron job. Cron, wait, I don't know what cron stand for, but let's look, see if it stands for anything.
[00:00:35] Cron,
>> Jem Young: No, a daemon to execute scheduled commands, that's Vixie cron, what if they created it? Maybe, cuz some of these like chown or chmod kinda makes sense, chsh for change shell, cron doesn't really fit anything.
>> Speaker 2: Chronologial if it had an h in there.
>> Jem Young: Maybe, maybe it says Vixie cron, person.
[00:00:59]
>> Speaker 3: Originally coded by Paul Vixie in 1987.
>> Speaker 3: But, it's a cron.
>> Jem Young: [LAUGH] Well.
>> Speaker 3: You know what, I found it. The origin of the name cron is from the Greek word for time. Awesome, So.
>> Jem Young: That's pretty cool. Maybe it is chronological but spelled differently. Anyways, [LAUGH]
[00:01:20]
>> Speaker 3: [LAUGH]
>> Jem Young: A bit of trivia, so a cron job is just a job that runs periodically and it has a, it looks like a strange syntax, but when you break it out it's not that strange. So it's minutes to hour, day of month, month, day of week, and then the job you wanna execute.
[00:01:40] It's strange cuz you see a cron job and you're like, what does this mean? The star is a wild card, it means anytime, so if I had star or if I had 01**** it's gonna run every minute of everyday of every month, every hour and things like that so on and so forth.
[00:01:55] But a useful tool is crontab.guru, that's really nice, it's very helpful.
>> Jem Young: So you don't have to think like, what does this mean? Four or five on a Monday.
>> Jem Young: Day of month, Monday. Cron has a seemingly strange syntax but as crontab.guru shows us, it's actually fairly, it makes sense.
[00:02:25] It's strange to us, but when you think about how you would structure how to build out a syntax that regularly do jobs, this makes sense. It's kind of the beauty of Unix that, cuz people have been doing this for a long time. They've kinda down it down as simple as possible.
[00:02:38] So let's make a cron job.
>> Jem Young: Yeah, I did make a cron job. This is gonna echo, it's party time at four o'clock on every Friday of the month. Just to make sure people aren't logged in and working too hard. I'm not really gonna do that, but you could.
[00:02:57]
>> Jem Young: And if you wanna say it's very, very specific at a certain time on a day a month every year, it's gonna execute some task. So you kinda get the gist of cron jobs, it's a little funky, but it's not too bad.