EX200 Red Hat Certified System Administrator RHCSA – The Red Hat Linux file system explained and basic functions Part 3

7. Automating tasks with cron part 2. Creating a cron job and bash script

Welcome, everybody, to this tutorial. Today, I am actually going to create a cron job where we will define when it will actually run. So we will say, okay, either every minute or every hour or, I don’t know, every day or something of a kind. It doesn’t really matter. What is important is that we will create create a cron job. In addition to the cron job, I will create a bash script which will run along it. Now, I won’t actually describe what the bash script is, how to make it, et cetera. Now, we will cover that in the regular expression part of the course, and there I will cover such things in far greater detail. But for the time being, I just needed a sample to actually utilize it in order to run this thing, in order to create some sort of a meaningful cron job that can perform an action which we can actually see. Also, I will schedule it to run every minute so that we can see the results, as opposed to scheduling it to run every hour, which would be far more realistic.

But I will show you how to schedule them on an hourly basis, for example, or a monthly basis or something of a kind. It’s quite simple. Now, let me just go ahead and exit this file. I’m in insert mode. So quit and force quit. Do not save anything. Let’s go ahead and clear the screen. Now, one of the first things that we need to do, aside from creating a cron job, we need to create a script that’s going to run. So I’m going to do LS. Okay, I have already tried this out, have encountered some minor trivial problems, but all is well. Let’s go RF. And this is also a neat way to delete things, but also quite dangerous, because once I do this, it’s gone permanently. As I’ve described it before, it will be very difficult for you to recover anything. And I have also deleted it with Wildcard at the end. So pretty much anything that contains not contains, but any file that begins with this and ends with pretty much anything will get deleted there.

Anyway, I’m going to go ahead and skip the explanation of that. As I have already explained it. I wouldn’t want to bore you or anything of a kind. Now we’re going to go ahead now we’re going to do LS and go into where did I put that folder for myself? Oh, yes, there we go. Scripts LS. And I already have a script here. It’s called first sh. Let’s just see what is located in First Sh. I’ll use cat. Excellent. So we have this command here in my bash script. The extension for it is Sh.

Don’t worry about it. If you don’t figure it out straight away, or if it seems a bit confusing, we will simply use this script as something as an example, as something that we can utilize. Now, but the script itself is not relevant. In fact, this, what the script does is unimportant, except for this last part. All that it does is create new files in a specified folder. That is it, nothing else. I just needed to demonstrate how you can schedule a crown job that will run in a repeating fashion. Anyway, I’m going to go ahead and type in Vim so we can get a bit of a better view of this. Where is it first? Excellent.

So there it is. This is all that you would need to write in your file. All that you need to do is basically, literally type in Vim. Name it whatever you want. Just remember to put Sh at the end for reference sake. The name of mine is basically first Sh and this is all that you need to write. I have written here, that’s a Bash script. And down below I have written a command. I could have changed any number of commands here, I could have created loops, et cetera. But no, I just want to stick to something simple for the time being in terms of Bash scripting because we will do that later and I don’t know, as far as I know, you don’t have understanding of Bash or of scripting in Linux. So I decided to stick with something fairly simple until I get the chance to explain it in great detail, where you will become proficient with Bash. Anyway, you have the command touch which we have dealt with before.

I have written it here and then I have specified where I want my file to be created. Aside from that, I have named my file New file or minus sign, whichever way you want to look at it. And then I have an interesting command here. It’s Date. This dollar sign, open parentheses and closed parentheses means, as I’ve explained before, that this year will become a value which will be placed here, which will replace this whole thing, which will replace this whole expression. That is simple enough to understand. But date is another command which we have not used thus far and I would like to show it to you now. So let’s go ahead and see how date works and what it actually does. First, clear the screen, type in Date and by default it will print this out monday, 18 May, 1400 hours, 37 minutes and 27 seconds.

Let’s go ahead and run it again. You see that the amount of seconds changes and if I keep on running it, you see the amount of seconds keeps getting iterated until eventually minutes and hours are iterated as well. Let’s go ahead and clear this now. Why is this date so important? Why have I placed it there in that file? Well, I would like to tag every file with the exact time of its creation and the date of its creation. This is very important, as network administrators do very often tend to attach dates and times of creation to the file names. Several reasons for that. First of all, it makes the file names unique no matter what. Well, maybe not no matter what, but it’s highly unlikely. Well, actually it’s impossible for your files to have the same exact well, I would say unlikely or impossible. One of the two. I’m sure there is nothing that is not possible in this universe. So I guess you can find some way to make the file names exactly the same even though you are tagging them with date. Anyway, that would be one of the reasons to make the file names unique. But also another reason is that you have a clear overview of things. You know exactly when you made every single backup.

When the backup was made, not when, not the last edit date, which you can see from LS, but rather instead the creation date of the file, which is extremely convenient for you as you want. All that you need to do is LS and then you get the creation dates of all your files and if you have named them in any meaningful manner, you will immediately know what is what, when has what been created. For that purpose. We need to create better output of the date command because this output to attach basically to do this echo test, open parentheses date. I hate this keyboard. I really do. Press Enter. This would be highly inconvenient. Do you see what has happened here? I have echoed the word test and dot and after dot I have attached the date to it.

So it’s now test and you have this first problem. The first problem would of course be that you have spaces in between these things and that is definitely not convenient for file names. Not a good idea at all. Second problem is that we have too much information here. The names of the files would be too long. We would get some unnecessary things. For example, this month can be easily be represented with a number or something of a kind. We don’t need it to say that it was created on Monday and we don’t need this a year. Okay, maybe a year is actually not a bad idea.

For this purpose we are going to go ahead and type in date help. What would we like? Well, I would like the time. So I would get the hour, the minute and the second. Well, I don’t need the second. But for the sake of this tutorial, I’m going to go ahead and use all three. But a good idea is to stick to the hour without the minutes. However, feel free to do what I’ve done now. It will not cause harm. So percentage t as an argument will indeed give us the time. So you will see, it gives the hours, it gives the minutes and it gives the seconds. This is how you would define hours, minutes and seconds separately. And you could also exclude some of them without any bigger difficulty. Another thing that I would like to have is the date. The date is percentage T. Let me just see if I can find it. Should be around here somewhere. Come on, please. There we go. So percentage T is time.

And that’s going to give us the date pretty much same as oh, sorry, not T, it’s Ffff. I’m losing my mind here. It’s f am I? Sincere apologies for this. We need F. The percentage T is for time and F is actually for date. And there you go. Full date. Same as defining a year, a month and a day separately. Again, you can use this format and then omit either a month day here or something of a kind and in such a way, gain a better filter for your date output, for the output of the date command. Let’s go ahead and type in date just to see how this works. Percentage d OK, why isn’t this working? Invalidate. Okay, I know I need a plus. Let’s go back to date help. If we scroll up to the top somewhere around here, at the top, there we go. Usage. So it’s giving us the syntax. And you see in front of the format, you have to have a plus sign. And that is what I was missing. That is what’s lacking in this one. Let’s go ahead and clear the screen and we’re going to do date space percentage D again.

I am a genius. I am a true genius, I swear. Plus enter. And there we go. Somewhere. There we go. Excellent. So here it says 1400 hours, 44 minutes and 8 seconds. That’s what we get with this filter. Let’s try adding more to this filter. Let’s try percentage. This is the same way that you would, for example, say, our minute or second or something of a kind. You just keep on add plus and then just keep on adding percentage signs along with their according along with the letters that correspond to the values that you desire. Let’s add something else. Let’s add f full time. So we got the full time. It says 2015, fifth of the 18th. But there is a problem. Well, it’s not a technical problem, but it’s an aesthetical problem. And therefore it’s not very convenient for a human to actually read this. You see, 56 actually combines with a year. So we need to separate that.

Let’s go ahead and separate that with a minus sign. That seems most convenient thing to do. And there we go. Now it is completely separated. There we go. We have the I hope that you understand what I’m trying to achieve with this date command. I’m just trying to tag all the file names that will be created with the date of their creation. And that is what the Bash script is all about that I have made for the purposes of this tutorial. You see, this is exactly what I have added here. If we try this without the Bash script or without the cron jobs, if we just try to do it like this, I can type in touch test. Come on, dollar sign, parentheses, open, and then type in date space. Plus you’re going to get a percentage to it. You’re going to say D and then Hyphen in between.

Again, a percentage sign and then F and then close the bracket, press Enter. Okay, it didn’t actually output it to the screen, but if we do LS. You see, that test is now the file test. This is the name of the file. Look at it. It’s dot. And to be honest, most network administrators would usually, forgo the time part would forgo the hours, the minutes and seconds. They would mainly go ahead and take the date, as that is rather convenient. But I don’t know, just putting it for the sake of this tutorial so you can see other things. Again, it’s completely relevant. You can put in whatever is convenient for your purpose. You can put only hours, you can put only years, months, whatever you want, whatever is convenient, use it. You see, just adding these filters is not that hard. Just go to the Help menu, percentage sign and then the corresponding letter to whatever filter you wish. Very simple. If you have any difficulties with it, of course, I am always in the discussion section.

Okay, so now that we know what we are doing with this date, let’s delete this, as I no longer need it. Clear. And we know that the basscript has indeed been created. However, upon creating the Bash script, it will not be an executable by default. And that is something that you need to take into consideration. So if I type in ch mod minus x first sh. These are the sort of permissions that you will have upon your creation of this Bash file, of this Bash script. You’ll see, nobody will have any execution privileges of whatsoever, and that can be problematic. I’m going to go ahead, for the sake of convenience, give execution privileges to everybody. But I have taught you how to play around with permissions in the previous tutorials. See if you can use this as an exercise to give individual groups individual file permissions. For example, try giving only the users, only the order of the file execute permissions and nobody else. Try giving it only to the group. Or try giving it to the world.

Chmod plus x, not random guy. But first sh. Excellent. Also make sure that the script is owned by your current user, or at least by a regular user. As you can see from what I highlighted here, it is owned by Random Guy, the user, and it is owned by the group under the name of Random Guy. Okay then, now that we have this sorted out, let’s go ahead and clear this. And finally, let us go into the part that we have all been waiting for, which is now we’re going to create a Cron job for a specific user. There are system wide Cron jobs and there are user Cron jobs. For the time being, we’re going to stick to the user Chron jobs whereby where we will go ahead and type in. What else shall we type in here? Random guy and space. We can use L to list the current work, to list the current Cron jobs that exist nonexist. Or we can use E to actually enter and start editing the Cron jobs. Now, I am running a bit short on time here and I will continue in the follow up tutorial where we will create these chronic jobs and where we will see what we can actually do with them from a live example, so to say. In any case, I would do farewell and I hope to see you in the followup tutorial.

8. Automating tasks with cron part 3. Commenting out a line and making notes

Welcome back, everybody. Let’s just continue. So we have opened a Chron tab in order to edit it and schedule a task. Just in case you are tuning in now or you have made a break or something of a kind in between the tutorials. Let me just go ahead and clear the screen. I have used this command in order to create cron jobs for a specific user. That user is random guy and to create a contact. And to edit a Chron tab for that user, use E. That’s it. You enter it. Just press enter. That’s it. Now, when you open up your Chron tab files, you will not have this where it says min, Our, Dom, Mo, N, Dow and CMD. This I have created for convenience purposes, but also to teach you something. You see this bar sign? I call it a bar sign, but I’m sure that there is a proper name somewhere in the net for it. Feel free to correct me in the discussion section. I will be more than willing to accept any sort of criticism. I always value all the input that you can give me. But anyway, you would basically use the bar sign in order to comment out a line. What is it? Let’s go into Insert Mode in order to comment out this particular line. What does it mean to comment out a line? What does that mean?

Well, you want to make notes for yourselves. You want to make it a bit easier. Imagine if we didn’t have this up above. We would have to just buy memory, type in every single position needed. So we would have to buy memory, know where the minute or the hour, day of the month is, or the month is and where the command goes. Well, the command goes last. That’s easy to remember. But all of them, over time you will remember them and you won’t need this. But it’s a good help that you can make for yourself. Just go ahead, enter Insert Mode and type this. But I have typed in at the top. I’m sure that you know how to do that by now in order to make your lives a little bit easier and save a little bit of nerves as well as nerves are in short supply these days for pretty much any programmer, any system admin. If you are in this line of work, you will encounter a shortage of nerves. I guarantee it to you. But bear with me. Anyway, now we are going to now that we have created a script, I want to run that script.

Let’s say I want to run it every minute, okay? That would make no sense of whatsoever in the real world in most situations. But now I don’t want to schedule it to run every hour or every month or something of a kind. Why? Well, I really don’t feel like waiting for a month until the command gets executed. And I’m sure that neither do you. For convenience sake, I’m just going to say that it should run every minute. But I will also show you, either in this tutorial or in the follow up tutorials, the times, the time arcations that you can assign to every one of these positions and basically schedule it to run every month or something like that. Because, Tarballs, I don’t know, you would run on a weekly basis or a monthly basis or something of a kind.

Now, I have shown you the scopes as well, but the time scopes that you can use, for example, per minutes from one to 60. But I haven’t shown you asterisk signs. You can type in an Asterisk sign, and that means run every minute. Asterisk sign under the hour, run every hour, run every day of the month, run every month, run every day of the week. And you can’t put an Asterisk sign for a command. You actually need to type in a command. Let’s say LS here, so every minute LS command will get executed. Pointless consumption of systems resources. But you get the idea. For example, a more meaningful statement would be if I was to change the day of the month into one. So what I would say is I don’t really care about minutes, I don’t really care about hours, but I want you to do this on the first day of the month and execute a certain command, execute a certain script or something like that. Of course, I would need to change the other ones a bit around as well, but that we’re going to do a bit later on. Let’s just leave it as it is now, so that we get the idea of what the cron job does. Next thing that we need to do is run our script.

Now, we know that our script is called First Sh. Let’s say that we forgot the path. Let’s say we forgot where it is because you would have to specify, obviously, a full path to the script here, and then you would be able to run it. How do we find it? Well, simple enough. Just use the locate command. Don’t forget to run Update DB. Otherwise you won’t be able to find it until next restart of the computer. But you can just manually type in Update DB and you will be able to type Update DB as root, of course. So locate space. What shall be here?

Well, we know that it’s called First Sh. You can use various combinations with this, like with Wildcards or something of a kind. I’ve shown that in the previous tutorials. For the time being, we just want to find this from Vim. We don’t want to exit Vim. It’s not convenient for us. We want to do it from them and be able to find it. And there you go. We have indeed found it. Now, you can actually copy paste this, but I’m just going to press Enter and type it in myself, which is Enter Insert Mode tabhole randomguypt first. Sh. Excellent. So now we have that sex. It in cert mode. I just want to verify, is it Script or script? No problem. Let’s run the command. Locate again. And it’s actually scripts. You see if this would not have worked if I didn’t actually check this. Go back to the insert mode scripts.

Excellent. There we go. If I have made some other typos, this is not going to work. But if I have it, it should work flawlessly. So let us do this. Write and quit. Actually, I want to ask me any questions. If it has any problems with it, I wanted to let me know. Press Enter. There you go. It has saved it. It says Chron tab. Installing new chron tab. Very well. Let’s go back a step. Let’s do LS. As you can see, the file name, and there is no file in this folder named New. But if we type in date, it’s 29 minutes and 40 seconds. Like 20 more. And 40 there. On 30 minutes and 30 minutes and 40 seconds, this should run. And a new file with a date stamp with a timestamp should appear here.

And then you will be able to see it. You will be able to see that the Chron Tab has actually executed manual, that it has executed a task, a certain script. That script can be anything you want it to be, ranging from backups to complex batch scripts to anything you can imagine. As far as your imagination goes, that is what you can run with it without any bigger difficulty. So let us go ahead and see date 30 37. Okay. LS. No, it’s precise. It’s dead on time, which is nice if my calculations are correct. Is it? Come on, don’t do this to me. I can swear I have mistyped the name. Let’s go. Head back into the Chron Tabu random guy. E enter. Let’s make sure that I didn’t make any typos here first. So locate and then I’m going to show you how you can. Also, another way to troubleshoot things to view the log files. So type in locate and what we need is first. Sh.

There we go. Let’s copy this. Enter insert mode. Delete all of this. I know that it seems a bit tedious of a job, but this is the sure way to make sure that you do not have any typos or anything of a kind. You wouldn’t believe how much time I have allowed to go down the drain because of typos on some serious jobs or something of a kind. Anyway. LS and service. We’re going to use the service command. This is also a neat trick where you can restart individual services on the system. So you need to type in Cron d restart. We’re going to be dealing with this in more detail, just using it here in order to make it work. Please okay, so we now have the date. It’s 50, so 33, 33, 50. It should work. And there we go. As soon as I have restarted the cron job, I think it was created or something of a kind, it doesn’t really matter, but either my time calculations are wrong. The computer is far more precise than me, so don’t take my word for it. Take the computer’s word and there you go.

The file indeed has been created and you can see the format of the file name. So it says new file and then you have the time and then after the time you have the date. Easy fix, no problems. I’m just reminding everybody that I can actually edit this stuff out without a bigger difficulty. That’s probably be the easiest thing to actually just cut it out. But I do consider that it is important I do encounter errors along the way and that I do troubleshoot them here so that you may see that it’s actually a really normal thing to happen to anyone. I don’t know people, anyone doesn’t really matter how skilled or unskilled you might be encountering errors, encountering problems is a common thing, nothing more. Another way you could have seen that there was an error or something of a kind is check the log files with a tail command. So tailspace f and go into VAR logcron.

There you go. You can actually see that we have the user. Okay, so we have the timestamp of course, but aside from that we also have the command. So CMD and this is the command that we are running. And this is usually where you can see that things are off if you have misspelled something or that something is wrong. In one of my tests I’ve actually missed I random and I couldn’t figure it out what was wrong and I’ve just swapped A and N which was hilarious. Another place to look for problems is log messages. And there you go. You also have a list of things that have happened here as well. Just something to keep in mind where you can go and look for problems if you go into VAR. I have shown this before, but let’s just go ahead to VAR Log and do LS.

You can see that there are all sorts of log files here. Before I’ve just mentioned them, but now we’ve actually opened them and TailF allows you to follow them in real time. So as things happen, they will be written to the log file and you will be able to see them. Let’s go ahead and clear the screen and wrap the tutorial up here. I really wish you a ton load of luck with this. I hope that you do encounter errors. I know that that sounds a bit wrong for me, but really that is the best way to learn to encounter a problem. Solve it. Encounter next problem solve it. Encounter next problem solve it. And so on and so forth. I know it can be tedious, but it’s worth it in the end. Well, it’s worth it long before the end. Because as you keep on solving problems, each problem that you solve, the next one you will solve faster and faster and faster and faster and we’d ease. That is how it works for me, in any case. So I bid you all farewell. And once again a tumbled flock.

9. Automating tasks with cron part 4. More about crontabs

Welcome back, everybody. Today I’m going to go ahead and continue on with the chron tabs. Now, look, if I go ahead and type chron tab u. Random guy. E random guys. Nice random guy. So now, as I said, we can specify minutes and hours and whatever we want. So let’s go into the insert mode. We can say on the 12th hour, I want you to do something, say, 30th minutes, 12th hour, day of the month, I don’t know, it doesn’t matter. Do it on the 12th tower on every day of the month. But you can also say, okay, just do it on the first day of the month. And then you can say, I don’t know. Do it every month and the day of the week. You can specify it, but let’s say I don’t know. It doesn’t really matter. We can leave it as it is. Let’s not speculate anyway. And now the script will have a completely different repeatance interval aside from the dots. However, we cannot wait for this one to happen, as it is too long. But I hope you get the idea. Aside from this, there is something far similar that you can use, far simpler that you can use, actually, that would be simply this.

Look, I’m going to go ahead and delete all these timers here and go ahead and forget about them for the time being, for the duration of this particular tutorial. Don’t forget about them completely. I can type in this at reboot and every time the machine reboots, this will run at monthly. I’m praying that I spelled that correctly. At monthly, meaning that this script will run once a month. Especially useful for backups if you have tar commands that we’ve learned previously, and if you just put them instead where it says home random guy scripts first sh. You will have a monthly backup, which is very useful. I mean, you don’t need to bother with all these things.

You can also type in hourly and some others as well to get the idea of how this actually gets to work. I mean, you don’t even need to specify those timers there. There is one more daily that we can use here, which is also quite in handy. They got hourly, daily, monthly, yearly. And instead of actually typing in all these values and numbers, you can just type in these things with add signs. That works like a charm. And it’s quite easy to understand, much easier than the numbers. I mean, don’t you think? Okay, then let’s leave this file for the time being. Let us abandon it completely. I want you to quit, and I don’t want you to write anything. Don’t care. Clear the screen. Now, these are user specific cron jobs.

There is also something that is called a system cron job, which you can find its configuration file you can find in Etsy. And there we’re going to see more than just a configuration file. So let’s go. Vim, what do we got? Now we are in the Chron tab and we can take a look at what is in here. Now, if we modify a Chron tab here, this is typically for route only. It’s going to be a system wide. And here we have pretty much the same things which I have explained previously. You have minutes, you have hours, you have days of the month, day of the week, et cetera. And everything is explained. And even this very nice detail here. Give me insert. It says that Sunday is equal to zero or seven.

So it’s not only from zero to six, but you can also use a seven, although it says or. So it’s either a zero or a seven. But the zero to six is a valid range. As you see. It says here that the range is from zero to six indeed. Or you can even type in the days. You can type in Sunday, Monday, Tuesday, Wednesday with these shortcuts day of the month. But you can also type in the days of the month in this fashion, which can be quite useful, I assure you. It’s much easier than the numbers. It’s something that is far closer to us as human beings, to an extent, that is. Anyway, if I was to give exactly the same command as I have given it, if I was to apply exactly the same syntax, exactly the same lines as I have applied them to the Cron jobs of user Random Guy, I would get the same effect.

But it would be a system wide thing. Another thing that I would like to show you is oh no, I don’t need the dot now, I need to quit. Is that we have these things. Just let me find them. No, hold on. Let’s see. And then I’m going to do a less graph chron. You have Chron Deny, chron daily, Cron hourly, chron monthly chron weekly. You have the Chron tab for the system wide Cron jobs for daily occurrences, hourly, monthly occurrences, weekly occurrences and so on. And we also have deny. There should be one more file here which you can create. It’s cron allow. But here we have cron. Deny. And if we just type this in, I don’t think anything is going to be in here. I don’t remember putting anything in Cron Deny. Excellent. So it’s empty. If I typed in Random Guy, this particular user would not be able to run any Cron jobs of whatsoever. So you can allow or deny Cron jobs to users. You just need another file called Cron Allow. Just like this one is cron. Deny. And you simply list usernames one username per align to either allow or deny Cron jobs for that particular user.

So let’s go ahead and quit and we can actually as well clear the screen without any of your difficulties. Now, that would be it as far as the Chrome jobs are concerned and cron in general. One more thing to mention is that Cron is a daemon. So it does run in the background. You can restart it with the service command just in case you’re uncertain whether something’s going to work or not. But it should work without the restart as well. It does work. There was probably something else wrong with that command that I was issuing it. But in any case, what you need to know is that the Cron job that runs in the background here, let me actually show it to you. PS, ox grep. Cron. So we got two of them.

One is Grep, which we are not interested in. We are interested in the first one that I have selected here. You can see that it is running, and it is running indeed. It checks for these things every minute. 1 minute is the minimum interval refresh rate. I have pulled out that term. I’ve scrapped it out of my head. Don’t hold my word to it, but to the term itself. But this is absolutely correct. Every 30 minutes it conducts a check. Every minute it conducts a check. 1 minute is that time frame where it conducts a check. Anyway, as I was saying, this was it as far as the Cron jobs are concerned. Now, we’re going to go ahead and jump into user management.

That’s quite interesting, as it does directly allow you to see the usefulness of those particular commands. Of course, all the commands that I’ve taught you thus far and all the things that I’ve taught you thus far, they will be of use to you as a system administrator. Most of them are a must have skill, but user management is if you do not know how to manage users, and by that I mean create, delete, assign groups, permissions, et cetera, for users. Well, you won’t be able to do most of the things that an administrator would do on the server. It is an absolute must have that you need to know. Anyway, that awaits us in the next chapter. For the time being, we are done with Cron jobs and we will eventually go into regular expressions as well, where you will see bash, scripting and so on and so forth. Until then, I bid you all farewell and I wish you a ton load of luck.

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