at daemon (Demo)

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Time
21 hours 25 minutes
Difficulty
Intermediate
CEU/CPE
21
Video Transcription
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>> Hey, Cybrarians, and welcome back to
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the Linux plus course here at Cybrary.
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I'm your instructor, Rob Goelz.
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In today's lesson, we're going to be
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discussing the at command
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>> and hanging out with that ats.
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>> Upon completion of today's lesson,
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you're going to be able to understand when
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the at command would be used,
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we're going to learn about the at command syntax,
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and then we're going to run the at command
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to schedule jobs during our demo.
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The at command allows you to schedule a job to run at
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a specified time using a natural language syntax.
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It's helpful to run at when you want to run
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a non-recurring job because you
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can just kick it off really quickly.
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The at daemon or atd runs
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in the background of the system.
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An atd checks the directory of var/spool/at
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when we're talking about RPM-based systems CentOS.
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It looks in there for jobs submitted and it checks
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every 60 seconds by default.
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Now the basic syntax we see with at is at,
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>> and then an optional file name and time.
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>> at does except the file parameter using
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the -f option before the time,
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and then it will run the script at the time specified.
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For example, if we say at -f script.sh
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>> now plus one minute,
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>> it will run that script in one minute from now.
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Without specifying a file, at defaults
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>> into an interactive syntax,
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>> it drops into a shell,
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and then what we can do is enter
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the command or script to run and hit Enter,
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and then on the next line,
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>> we hit Control D to schedule the job.
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>> Now, when we're scheduling these jobs,
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the time specified can be fixed,
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meaning you can say something like at noon,
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at midnight, or 8:05,
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or it can be relative, at now, at 10 minutes from now.
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Now, regardless of how we submit the job,
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if you do it via a file or just interactively,
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all jobs submitted wind up in a queue
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and in CentOS, that queue is found at var/spool/at.
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Job entries in the queue
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>> can be seen with the command atq as well.
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>> Each job in a each queue receives a number.
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>> If you decide, oh, no,
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I don't want to run this command,
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you can actually go and remove
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the job by using the atrm command,
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at A-T remove and then the job number.
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Let's go ahead and take a look at
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this with some demo time.
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Here we are in our demo environment,
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and let's first run a command interactively.
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We'll say at now plus three minutes.
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What it's going to do is it's going to drop us
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into a shell and we can just run,
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let's just run a echo.
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This is an at test,
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and we're going to pen this to temp.
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We'll create a file here called attest,
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and we hit Enter and then Control D,
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and we can see that that is set up,
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we can hit atq,
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and we can see that the job is number 2.
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We could do a sudo ls on /var/spool/at,
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and inside of here,
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we can actually see that there is an entry.
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If we try and look at this,
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we can do a sudo ls on var/spool/at
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and this weird number.
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We could do a less on it.
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We're going to see that this is carrying a lot of
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information about the user environment.
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But if we go down to the very bottom,
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we can see what it's actually doing is
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just the echo that we submitted here.
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This echo, this is an attest to
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temp attest. There's no limit.
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This is the Control D that we ran at the end of it.
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Now, if we wait a little bit,
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we should be able to see this output in
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less /temp/attest,
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and we see that, that is run.
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We see a display that this is an at test.
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Then, we'll just run that one time with at D.
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>> It's just used to run jobs on-demand.
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>> We could schedule something out in advance,
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>> like I said, by using a specified fixed time.
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>> But we can also do it just using
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the time that we just specified,
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using that command where we're just specifying
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your relative time three minutes from now.
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But in this lesson, we covered the purpose of the at
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command and how the atd daemon works.
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We also talked about the at command syntax.
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Then finally we submitted a job to the at queue
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>> with the at command and looked at it in the queue.
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>> Thank you so much for being here,
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and I look forward to seeing you in the next lesson.
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