Process_Management_Essentials_Devops

  ​​ ​​ ​​ ​​ ​​ ​​ ​​ ​​​​ Process​​ management​​ essentials for Devops

What you will Learn:

  • Difference between service and process

  • Exploring Processes​​ (ps -a, ps -au, ps -aux)

  • kill the process

  • netstat (for listening services)

  • 0.0.0.0 (listen on all interfaces)

  • Edit nginx configuration file

Difference between service and process

In the previous tutorial, we saw that nginx​​ can be​​ run as a​​ background and foreground​​ 'service'.​​ 

Now, what is the difference between service and process?

All services running will have one or more processes associated​​ with them. So, irrespective of, whether​​ service​​ is running in the foreground or​​ background, it will have a process.

Exploring Processes

Let us get back to our nginx webserver.​​ Make sure that nginx service is running. Now, to look for​​ all the​​ processes that are running, we can use ‘ps aux’ command.​​ You can look at the​​ manual of ‘ps’ and see what a/u/x switches mean

a ->​​ used to​​ list all processes​​ with a terminal

So notice the below output,​​ ps a​​ shows all terminal processes​​ currently running for a user.​​ TTY column shows to which terminal the process is attached to.​​ PID​​ column shows​​ the process id​​ (every process will have a process id associated with it)

The output of ‘ps​​ au’ shows some additional details.​​ You can see USER information​​ (under which the process is running), %CPU utilized, %Memory utilized, Virtual memory utilized (VSZ), Actual RAM usage​​ or physical memory usage​​ (RSS)​​ etc

Next ‘ps​​ aux’ will show all the processes that are currently running (for all the users including root user), see below.​​ 

All the services are running without a terminal (TTY) viz the services are running as background processes

Now ‘bash’ is a process and you can see that the ‘bash’ process is running on a terminal pts/0​​ 

Similarly, the command ‘ps aux’ that we executed just now too is running on a terminal​​ 

Let us grep the​​ nginx service, notice that the 2 PIDs​​ are running on background (without a terminal), the 3rd​​ PID (grep nginx command) is running on a terminal. Please note that even ‘grep’ is a process and hence the command ‘grep nginx’ too contains ‘nginx’ and hence got printed in the output

If you want to exclude the grep process from output, you can pipe another command ‘grep -v grep’. Now if you notice below, we no more see the grep process in output

Also, PID 4409 is child process of 4408. Now, since the nginx processes are running, we can see the webserver running

kill the process

Now, if you try to kill the parent process id, you will get below error

The reason is, the owner of parent process is root user

So let us run it as sudo

Refresh the age, notice that service is now down

Also see below, none of the nginx processes are running

Let us start the service again

netstat

We can use ‘netstat -tulnp’ command to find out the ports the various services are listening to.​​ Nginx listens on port number 80 because it’s an http web server

Similarly​​ ‘ssh’​​ process​​ is listening​​ on port 22, see below

0.0.0.0 (listen on all interfaces)

See below,​​ 0.0.0.0 means the nginx service is listening on all network interfaces

When we execute ‘ip a’ command, we can see 2 network interfaces:

  • loopback interface 127.0.0.1​​ (ubuntu localhost)

  • Ethernet network interface 192.168.0.106​​ (bridged from​​ our host computer)

So 0.0.0.0 means, whatever networking interface the system has, this nginx service should listen on all of them.​​ So the service should respond if you try to access it using 127.0.0.1 ip address or 192.168.0.106 ip address.

The loopback interface​​ 127.0.0.1​​ is valid​​ ONLY​​ on same system (here, ubuntu). It has no value outside of the system. So as you​​ notice below, we are trying to access the nginx service on ubuntu localhost using 127.0.0.1 ip address and we are getting the response

However see below. As expected, we are NOT getting a response​​ when​​ we​​ try to​​ access​​ the service on​​ 127.0.0.1​​ outside ubuntu system (from​​ our windows host machine)

However, Ethernet n/w interface ip address​​ 192.168.0.106​​ can be accessed on ubuntu as well as on windows host, see below


Edit nginx configuration file

To understand the above concept better, let us​​ open​​ the nginx config file​​ in edit mode. The config file location is:
/etc/nginx/sites-enabled/default

Hit Enter, the editor window opens. The highlighted line​​ seen below​​ means that the nginx service is listening at​​ default​​ port number 80 (for all the network interfaces) ​​ 

Bring the cursor to the line​​ 

Press key ‘i’ to insert the text, type # to comment the above line

Press escape key​​ to enter into read-only mode.

Bring the cursor to the below line

Next using the right arrow key, move the cursor and bring it on 80

Press key ‘i’ to enter into write mode. Type​​ 127.0.0.1:

So we are telling nginx to listen to only loopback interface 127.0.0.1

Save the file​​ by performing below-

Esc key followed by colon : followed by wq​​ and​​ Hit Enter

The file contents​​ are saved as you can see below

We can execute ‘sudo nginx -t’ to check if there is any syntax error

Let us restart nginx ‘sudo systemctl restart nginx

Now when we execute ‘sudo netstat -tulnp | grep nginx, we see that nginx service is listening at loopback interface 127.0.0.1 instead of 0.0.0.0

So what it means is that, nginx will no more listen on Ethernet ip address 192.168.0.106. It will only listen to loopback

See below

Also see below that nginx is not listening on ethernet

So this is how we can change the config file if needed. Let us edit the file and bring it back to original form.

Open the file again in editor

To uncomment the below line, using down arrow key, bring the cursor on # and type​​ x​​ to delete #

Using arrow key, bring the cursor one by one on​​ 127.0.0.1:​​ and type​​ x​​ to delete the numbers

Save the file​​ and make sure that changes are saved

Run below command to ensure syntax is perfect

Restart nginx

The below command now shows that nginx is again listening on all n/w interfaces​​ 

Refresh browser, see below

Similarly

 

Thank you for reading!

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