The crontab Utility (Demo)
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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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and in today's lesson we're going to be
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discussing the crontab Utility.
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Upon completion of this lesson you are going to be able
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to explain why crontab is used,
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we're going to learn how the cron table is set up,
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and then at the end, we're going to demo
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creating some crontab entries.
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As we mentioned in the previous lesson,
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cron table files are just one of
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the places that the cron daemon looks
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to find scheduled jobs and
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cron tables are stored in the user directory.
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They can either be in /var/spool/cron/username
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in CentOS or if you're talking about Ubuntu,
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you're going to find that cron table for the user
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in /var/spool/cron/crontabs and then the username.
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You might be asking why use these directories
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>> instead of the interval cron directories that
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>> we talked about in the previous lesson?
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Why not just use /etc/hourly and /etc/cron.daily?
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Well, the cron tables in the crontab command are
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used by users as well to schedule their jobs,
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and they don't usually have
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access to those /etc directories.
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Sometimes they have jobs
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>> that just don't fall into one of
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>> those intervals that are used by
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cron in those interval directories.
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You should remember, if you're doing this in real life,
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the cron job definition,
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the job definition in the layout can always be
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found in /etc/crontab in any distribution.
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But for the purposes of the Linux plus exam,
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the column meanings, the layout,
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the fields, they are tested on,
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and so you could use a mnemonic
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if it helps you, it helps me.
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I use "My Happy Day May Dawn" and that means minute,
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hour, day of month,
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month, and day of week.
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Generally, in order to create
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a cron entry or create a cron table,
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user can use the command crontab -e,
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which is e for edit,
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and then the crontab is created by the user using
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the job definition format
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that we have over here on the right.
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But let's go ahead and see this in action by
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popping some tabs and having demo time.
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Here we are in our CentOS system to do our demo today.
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Like I said, when you're doing this in real life,
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>> you can always look in /etc/crontab,
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>> and then you can see this job definition.
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I'm actually just going to cheat
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a little bit and copy this,
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and this is what I generally
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suggest or recommend that people do as
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well when they're trying to do
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this on their own systems or in real life,
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is just copy that definition and
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then when you go into create a crontab,
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which we can deal with crontab -e,
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you can just hit ''Insert''
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and then paste that guy in and you're good to go.
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We're going to go ahead and just save
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>> and close out of this
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>> because let's think about what we want to do.
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>> For the purpose of this cron in this cron table entry,
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we'll just run on every minute an echo that
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sends a message into
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a temporary file or a file on our /tmp directory.
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Let's first grab the absolute path to the echo command,
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and remember we can do that by typing ''Which''
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and then the name of the command we
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want to know which is ''Echo.''
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The reason that we need to do this,
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and I'm just going to copy this as well,
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the reason that we want to do this is because sometimes
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cron doesn't have the same path as you or I do,
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and so it doesn't necessarily know
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where to find that echo command.
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But if we copy this absolute path to it,
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then cron will know where to find
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echo to run the command that we wanted it run,
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because it doesn't need to look for it,
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it actually just knows absolute path to it.
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Now that we know where echo is,
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let's go ahead and create our cron table
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or go back into our cron table rather,
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with crontab -e,
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and now what we can do is we can start
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creating our entry.
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I'm going to hit ''o'' to start on a new line,
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and we're going to have this run on the every minute
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of every hour of every day of every month,
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in every day of every week.
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The next thing we need to do is specify username.
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But in this case, this is
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my user crontab, I'm logged in as me,
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so I don't need to specify the username,
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it's just going to, by default,
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run it as me.
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What I'm going to have it do is run this echo command,
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I'm going to have it run /usr/bin/echo,
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and now what I'm going to actually have it
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echo or print is
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''This is running every minute'',
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and then what I'll do is I will do a
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double greater then sign to have
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this append to the file /tmp/minutetest.
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Then if I go ahead and hit ''Escape
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: WQ,'' this will save this command,
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and now what I can do,
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is I can do a less on
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/tmp/minutetest and
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I can see that it is going to print the output.
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This is running every minute,
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and if we were to follow this,
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if we were to do a tail -f on this file,
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we would see that it's going to go ahead and enter
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a new line into this file every time it
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runs and it will keep
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adding that to this file
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because it appends as we go along.
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But we can also see
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the crontab entry for my user root by doing
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an ls -al on /var/spool/cron/.
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We're looking for the user rob,
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not the user root, apologies.
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If we look at this, we can
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see that we don't have permissions,
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we need to do it with sudo.
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Let's go ahead and do sudo
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less on /var/spool/cron/rob,
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and now we can see the contents of my user file.
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We can see that this has
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the whole of the stuff that I configured
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>> in my user cron.
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>> This is helpful if you're trying to troubleshoot
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a user's crontab, for instance.
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The reason that we have that bsudo is because
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only root can read other people's crontabs.
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You have to either become the user as
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a system administrator or you have to
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go in as root to look at this.
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But we can see the contents of
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this file just by going in as
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sudo or elevating privileges
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to become root, to see this file.
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But this is silly, so let's go ahead.
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We can actually remove a crontab
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entry and before we do that,
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we can see that this is still running
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every minute, so that's dumb.
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We can remove a crontab by doing crontab -r,
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which removes the crontab.
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Now we can do a sudo ls -al on /var/spool/cron/,
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and we'll see that there's no entry for
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my user anymore, so it's been removed.
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We've reached the end of this lesson.
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In this lesson we covered why crontab is used in Linux,
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and we talked about the layout and
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the way a cron table is structured,
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and then finally, we created
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some cron table entries during the demo time.
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Thanks so much for being here and I look
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forward to seeing you in the next lesson.
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