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[No audio]

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Processes and jobs. Before we get into

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this lesson, I want you to understand

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what exactly each term means in Linux

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environment. Well, first we have

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applications, which are also referred to

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as service. An application or a service

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is like a program that runs in your

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computer. And then application could be

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anything like in Windows, you have

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Microsoft Word, PowerPoint, and Linux,

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you have programs or applications like

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NTP, NFS, Apache, and whole bunch of

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different programs. So that is an

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application and service that runs on

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your computer. Then you have a script.

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What exactly a script is? A script is

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something that is written in a file, and

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then packaged it to in a way that it

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will execute. So any application that is

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running in your computer, for example,

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Apache, you would have to run that as a

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script. And that will run in the

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background. So that's actually a script.

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And also all those different commands

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that we run in our Linux environment,

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all of them are also referred to as

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scripts. Then we have a process. What is

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a process? When you run an application,

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or when you store up an application, it

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actually generates process, with it's

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process ID. Now processes could be one

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associated to that application, or it

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could be multiple processes. Then we

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have daemon. The daemon is, I could tell

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you when I compare it with a process, is

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something that continuously runs in the

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background, or it doesn't stops. So when

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you run a daemon, which is also a

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process, but it keeps on running in the

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background, and it keeps listening to

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the incoming traffic or outgoing

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traffic. Then there is a thread or

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threads. Every process could have

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multiple threads associated with it. An

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application that is running in your

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background, let's say NFS. When you run

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that application in your Linux machine,

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it could have many multiple threads. If

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a machine or remote machine tried to

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connect to you through NFS, it will

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generate one thread for it, and it will

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generate a second thread or third thread

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for other computers that are attaching

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to it. Then we have a job. A job is

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something that is created by scheduler,

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like a work order to run those

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applications and services. In process

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and service commands, we have a first

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command is systemctl. This command is

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now used in Redhat 7 and it is

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being replaced. Sorry, if the old

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command that was running an earlier

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version of Red Hat or CentOS service.

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service is now being replaced with a

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newer command systemctl. Then we have a

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ps command. This command allows you to

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see what are the processes running in

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your Linux system. You could use

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different options with that, to find out

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exactly the process that you are looking

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for. Then you have a top command. Now

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when you run the top command, you will

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see all your processes running in your

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system, depending on which process you

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want to see first, based on its load,

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and it also tells you some your memory

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information CPU information that is

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being used by that process. Then we have

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a kill command. The kill Command kills

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the process, it kills by the process

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name or kills by the process ID.

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So when you start an application, again

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going back to our example of Apache,

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when you run an application HTTP, which

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associated with Apache, it has a process

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ID to it. You could stop that process by

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running systemctl stop httpd, or you

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could simply kill followed by the

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process name or process ID. We'll cover

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kill command later in this lesson. Then

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we have crontab. crontab command is used

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to schedule these applications or

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these process or these services for you

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in your system. You will run crontab

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with minus e to edit it, and there are

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different options that we'll cover with

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the crontab. So when that crontab when

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that process or application is scheduled

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in crontab, that becomes a job. And the

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last command that we'll cover is at. at

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command is just like crontab. The only

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difference between at and crontab is, at

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is scheduled as a one time basis or as an ad hoc process.

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[No audio]

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We'll look into our Linux

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machine, and we'll go over the processes

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and some of those commands. Let's log

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in, clear our screen, cd, check where we

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are, who we are, and what is the hostname.

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Good. So do clear. All these commands

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that we will be running, most of these

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commands need root privileges. So we

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could either become root on this

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terminal or we could use a PuTTY terminal,

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whichever the terminal that you prefer.

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For this, we will stay on the console so become root.

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[No audio]

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Okay, let's clear this out. Now systemctl

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is the command to start an

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application or stop an application. It

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also is the command that is used to list

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or sorry to enable an application to

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start or stop at the boot time, meaning

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every time your computer starts up,

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you need certain applications to run. So

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to start an application, let's say I

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wanted to start an application called

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NTP. NTP, is a daemon that runs all the

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time, which stands for Network Time

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Protocol, which syncs your time to other clocks.

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[No audio]

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So systemctl restart ntpd.

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The d we are putting in because it's

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telling it it's a daemon. You hit enter,

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and you get your prompt back. The reason

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you get your prompt back is because it

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did start it that NTP service. Now how

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do we check if that service is running?

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You do ps and the option that I use my

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favorite is to have I open or dash ef,

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you could use other options. ps minus ef

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you hit enter. When you hit Enter,

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you're going to see all bunch of

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processes that are running in your

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system. Do you really want to look into

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all these processes? No, you are looking

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for only one process which you just

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started. So you do ps minus ef, pipe it,

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grep it, ntpd, and it will tell you

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yes, the process is running or the

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application is running. That's right

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here. You could also use the same

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command systemctl status command to

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view if the process is running. There it

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tells you right here, the Network Time

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Protocol, that's a service. And it's

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loaded and it's active and it's running.

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And it's running since the time and it

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gives you a lot of other details.

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Now what if you want it to stop this

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process, you could do systemctl stop

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ntpd, and it stopped. See if it still

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exist, ps minus ef grep ntpd, it does

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not exist. If it does not exist, then why

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we are getting this line. Well, if you

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pay very close attention, this is

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telling you that the command that you

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just ran ps minus ef, when you run

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this, the Linux system, the operating

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system is so fast and so sure, that it

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gets you the same command that you

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running. You see it's giving you this

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grep, that I ran this grep, and if and

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this grep is also considered as a

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process itself, within itself. So that's

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why it's giving you. You always going to

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get this grep whenever you run this

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grep command. So this is about ps systemctl command,

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and if you wanted to enable

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an application, you could just run

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systemctl enable ntpd, and this will

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enable. See it says create a symbolic link

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is create a symbolic link in the etc

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configuration file. So now every time my

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computer, my Linux machine is going to

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start, it will start the NTP daemon with

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it Another example of different process

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is sendmail or rsyslog. Let's see

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rsyslog is another application or

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process that runs which collects your

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logs. So let's see if it's running. Yes,

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you see it is running. And if you're

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wanting to check the status, you could

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also do systemctl status rsyslog.

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[No audio]

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And it will tell you it is active and it

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is running. So that's how you find out

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about all the processes. Now, what if

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you want to kill that process?

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So let's take one step back, and we'll

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run top command. top command is one of

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the favorite command for many system

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administrators, because this will tell

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you about the system resources, or also

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one of the favorite question by an

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interviewer, who was going to interview

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you, they will ask you tell me about how

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do you monitor the system? This is where

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you could tell them you will run the top

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command, and the top command will give

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you their total number of total tasks, 171

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is running, 169 is sleeping. What are the

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CPU, usage, is it too high, is it too

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low, what is the memory usage, what is

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the swap space, and then right here it

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is listing every process that is running

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in our Linux environment, with the

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process ID, who the user is? And what is

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the memory being used? What is the CPU

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being used? How long is it running for?

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and what is the command associated with that process? So this

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has so much information that I cannot

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cover in this lecture. So what I will

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encourage you to do is, do man on it or

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read a lot more about it in different

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documentation, I will also include some

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documentation that will explain top

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command in detail. You could also feel

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free to Google it, you could view top,

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but you should spend some time quite a

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bit time to look into the output of this

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command. Now what if you want to get out

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of it, just simply type mpt Once you type

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Q, you will get your prompt back. Now

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we'll go to the command kill. If you

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remember, we use ps minus ef to check

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which processes are running, grep, and I

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wanted to do ntpd. So NTP is not

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running. Let's start it first. So we'll

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do systemctl restart or start, doesn't

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matter, ntpd. It has started, let's check

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again, ps minus ef grep ntpd. It is

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running. Now I wanted to kill this

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process. There are two ways to do it.

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One is if you do systemctl and do stop,

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this command or you could simply do

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kill. kill will kill the process,

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regardless of whether it's going to go

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down gracefully or not. So which process

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we're going to kill right here. This is the process ID.

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[No audio]

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So kill simply 12027, the

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process ID, hit enter. Now let's verify

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if it killed it.

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Yes, see, it's gone.

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Because we are killing the process ID.

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The same way whichever process that you

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have, you could just do kill, ps minus

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ef, and then do more, because you want to

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see all the processes. There are tons of

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processes that are running through your system.

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Any process that is hung you could

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kill it. And how do you know it is hung,

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you could type top command and you will

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see here under memory or CPU if any of

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the process is using, let's say 99% of

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CPU or 99% of memory, that's definitely

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one of the your candidates to use the

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kill command to kill it to free up the

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resources. System administrators use

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this command many times to keep your

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system up and running and healthy. Next

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we will learn crontab. Let's exit out, and

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we will become ourselves. crontab allows

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you to schedule our scripts or our

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commands whichever way you want to

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schedule. So let's say I wanted to

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schedule to create a file that has some

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kind of output at a certain time. First,

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let's check the date. My date is

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Tuesday, March 13 2019. This clock is

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one hour off, I want to set this clock

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first, so become root, set this clock.

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[Author typing]

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And I'll do date minus s to set. And it will

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be, I think I have it set here. Yes, it

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will be 13, march

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[No audio]

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2018. Did the hour,

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[No audio]

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the hour,

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[No audio]

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time, the second,

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hit enter. And then you do date and it

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sets the correct date. Now let's go back

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to our self whoami, and we are self,

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clear the screen, now let's do crontab.

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[Author typing]

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crontab is a command self, you have to do

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crontab minus e to edit the content, and

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specify whatever that you are trying to

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schedule. Hit enter, the first, once you run

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sorry, once you run crontab minus e,

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it will bring you to the vi editor mode,

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the same way as if you run vi followed

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by the file name. So you are in there,

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hit i to insert. The first one is the

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minute. So we want to schedule something

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at 22, second one hour, 13, then

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the day of the month, I'll put every day

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which is *, month is three which is

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March day of the week. I'll do every

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day. Okay, so we scheduled a time. Now

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what do we want to do? We wanted to do

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echo, This is my first cron tab entry.

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And I wanted to put that to a file. crontab-file,

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or crontab-entry. So if

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you notice the file time has quickly

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changed, it already become 122. So I'm

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just going to change 23. Hit escape, write,

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quit, and hit enter. Now the time is 21.

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They still have 43. So let me just do

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quickly,2, so this will be don't lose time.

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date, 53. Let's see. Do we have the

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file created yet? Yeah, this is the

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first one I had it before. That's fine.

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date 22. Perfect. So let's see, did you

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create it? There you go myfirstfile.

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[No audio]

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Sorry, not myfirstfile, ls -ltr,

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it will be a crontab-entry, because we

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are doing crontab -l, it created

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crontab-entry. And the crontab-entry is

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cat crontab-entry. This is my

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first crontab. So it did create it. So

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but it created itself. So I schedule it,

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that's why I created it. To do a quick

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comparison of our commands that we

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learned with our Windows environment is

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the first one systemctl. When you are in

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your in your Windows environment, right

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click on your Start Task Manager. You

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see all these applications, these are

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the application you can stop and start

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using the systemctl in Linux. These are

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the processes that run, that are

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associated with all those processes. And

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many of them are daemon processes like

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service host host process for Windows.

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Many of these run as the process. These

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are the services that you could shut

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down with systemctl and the performance,

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remember we ran the top command. The top

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command tells you about your CPU, your

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memory. Exactly this is what you're

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looking at with the Windows Task

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Manager. This one gives you the network

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usage and of course the User itself. So

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this is a quick comparison between

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Windows and Linux. And I hope you will

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run all these commands one by one to

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learn more about them.

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[No audio]