The previous articles have explained storage connections using volume and bind mount. This article will describe how to share a folder so that containers on other hosts can access it.

Problem

How to share a data folder between container hosts?

Solution

In this article, I use 3 servers where 1 server runs an NFS server with an IP of 192.168.56.12, and 2 servers run Docker with IPs 192.168.56.2 (docker1) and 192.168.56.102 (docker2). You can go to this article about NFS, and I use the /var/nfs folder as a data folder for all containers. After installing NFS on the server, type the following commands to configure NFS in the NFS server:

sudo mkdir /var/nfs
sudo chmod -R 777 /var/nfs
sudo echo "/var/nfs 192.168.56.0/24(rw,sync,no_subtree_check,no_root_squash)" | sudo tee /etc/exports
sudo exportfs -r
sudo touch /var/nfs/test.txt
sudo bash -c 'echo "This is from NFS server" > /var/nfs/test.txt'

On 2 other Docker hosts, type the command below to make a volume Docker:

docker volume create --driver local \
  --opt type=nfs \
  --opt o=addr=192.168.56.12,rw,nfsvers=4,noatime,nodev,nosuid \
  --opt device=:/var/nfs \
  nfs_volume

After that, type the command below on those 2 Docker hosts to run the container connected to your NFS server:

docker run --rm -it -u root --workdir /root \
  --mount source=nfs_volume,target=/root \
  alpine ash

The image below is an example of when a container from docker1 host (192.168.56.2) accesses the NFS server:

The image below is an example of when a container from the docker2 host (192.168.56.102) accesses the NFS server:

As you can see in the images above, all containers can access the NFS server and can change the files on the NFS server.

Note

On the internet, some developers make a Docker plugin to access NFS servers from containers, such as plugins docker-volume-netshare, nfs- volume-plugin, nfsvol, and so on. I have tried the first 3 plugins, but I always failed when accessing the NFS server using the plugins. But, there is a Docker plugin called docker-volume-sshfs that can access a folder, but the connection does not use NFS; but uses SSH, so you don’t need to install and configure NFS. As long as the folder can still be accessed using SSH, then this Docker plugin can still be used. For example, I create /home/sysadmin/data  as a data folder in IP 192.168.56.12, so I use the commands below to create the folder:

mkdir /home/sysadmin/data
cd /home/sysadmin/data
echo "This is from server" > test.txt

On the 2 Docker hosts, use the command below to install the Docker plugin:

docker plugin install --grant-all-permissions vieux/sshfs DEBUG=1
docker plugin ls

Use the command below to create a volume in Docker:

docker volume create \
-d vieux/sshfs \
-o sshcmd=sysadmin@192.168.56.12:/home/sysadmin/data \
-o port=22 \
-o password=qwerty \
ssh_volume 

After that, use the command below to run the container to connect to the folder:

docker run -it --rm \
--workdir /root  \
-v ssh_volume:/root  \
alpine sh

The image below is an example of when a container from docker1 host (192.168.56.2) accesses the data folder:

The image below is an example of when a container from docker2 host (192.168.56.102) accesses the data folder:

As you can see in the images above, all containers can access the data folder and can change the files in the folder.

References

youtube.dimas-maryanto.com
docs.docker.com

By default, when you connect to an Ubuntu server using SSH, Ubuntu will display a welcome message. But sometimes, the welcome message is not needed or even annoying, so you want to disable the welcome message.

Problem

How to disable a welcome message in Ubuntu?

Solution

Usually, the welcome message looks like Ubuntu displays information about the Ubuntu server, as shown in the image below:

There are 2 methods to disable the welcome message:

1. Remove the execute

You have to know that the welcome messages are generated by the files residing in /etc/update-motd.d/. So, use the command below to disable the welcome message in Ubuntu:

sudo chmod -x /etc/update-motd.d/*

After you run the above command, every time you access the Ubuntu server, the Ubuntu server does not display a welcome message anymore, but only displays the last login on this server as shown in the image below:

2. Create a file

The second method is to create an empty file known as .hushlogin in your $HOME directory by using the command below:

touch ~/.hushlogin

It should be after you do the above command, every time you access the server, the Ubuntu server does not display a welcome message or last login at all, as shown in the image below:

Note

If you want to return the default welcome message after you run one of the 2 methods above, then use the command below if you are using the first method:

sudo chmod +x /etc/update-motd.d/*

And use the command below if you are using the second method:

rm ~/.hushlogin

The default welcome message in Ubuntu should appear every time you access the Ubuntu server.

References

askubuntu.com
cyberciti.biz

By default, if a server containing Docker reboots, all Docker containers on that server will shut down. If the server has a lot of containers, you will have to manually turn them on, which is highly exhausting.

Problem

How to run a container automatically after the server reboots?

Solution

In Docker, the restart policy specifies when and how the Docker container will be automatically restarted in the event of a system failure, shutdown, or reboot. There are 4 types of restart policies that you can use:

Restart Policy in Docker

By default, Docker uses the no option for the restart policy so that the container won’t turn on when the server boots. There are several methods to automatically turn on containers after the server reboots:

1. If your container is still not running

Use the format below if you run a container and you want it to turn on automatically after restarting the server:

docker run -d ‐-restart restart_option your_docker_image

If your container name is a webserver that uses the nginx image and you want the container to run automatically after the server reboots, you can use the command below:

docker run -d --restart always --name webserver nginx

Once the container is running, try restarting the server, and it should continue to run automatically after the server restart, like in the image below:

Now, try to stop the container and restart the server, and then the container should still run automatically after the server restarts, like in the image below:

2. If your container is running

If a container is running but has not used the restart option, you can use the format below so that the container can run automatically after the server restarts:

docker update --restart unless-stopped container_name_or_id

For example, you see a memcached container that uses a redis image is already running on the server, but has not used the restart option. Use the command below if you want the container to keep running after the server restarts, but if the container previously stopped, then at the time of the server restart, the container still won’t run:

docker update --restart unless-stopped memcached

Now, try restarting the server while the container is running, and then the container should still run automatically if the server reboots, like in the image below:

Now, try to stop the container and then restart the server; the container should still not run after the server restarts, like in the image below:

3. Using crontab

The third method of using crontab is to enter the following formats into crontab:

@reboot /usr/bin/docker start container_name_or_id

For example, you want to run the webserver container automatically after restarting the server, so insert the script below into the crontab :

@reboot /usr/bin/docker start webserver

If a container uses a crontab to keep it running after restarting the server, it will continue to run after restarting the server, even if it uses the unless-stopped option and the container is not running before the server is restarted.

Note

If you want a container to run automatically after restarting the server, it is important to note the distinction between using the always and unless-stopped parameters. If the container is a very important application, such as a webserver or database, use the always option for the container, but use the unless-stopped option if the container is a non-essential application.

References

docs.docker.com
stackoverflow.com
betterstack.com
baeldung.com

Have you ever been doing a job using a terminal that runs the SSH protocol, whether it’s using a putty or a terminal in Linux, that takes a long time, for example syncing or downloading a file, but has to suddenly stop because the internet network is disconnected or your laptop suddenly shuts down? It is very disappointing because you have to repeat from the beginning of the work, or even more dangerous if you are updating the database, for example, doing an alter table in a database, but suddenly your internet connection is lost. Therefore, you need multiple sessions in one terminal so that if your internet connection or laptop goes down, the process will still run. There are several tools to run multiple sessions, but this article will explain the use of the screen tool.

Problem

How to run multiple sessions using the screen tool?

Solution

Screen or GNU Screen is a terminal application developed by the GNU project in 1987 that is used to create multiplex several virtual consoles, allowing a user to access multiple separate login sessions inside a single terminal window, or detach and reattach sessions from a terminal and keeping them running in the background even if your internet network or laptop goes down.

A. Install the screen

On some Linux distros, this tool is already installed by default, and to check it, use the command below:

screen --version

But if your Linux distro doesn’t have this tool, you can run the command below to install the screen:

RockyLinux/Almalinux/CentOS

yum install screen

Ubuntu/Debian

sudo apt update
sudo apt install screen

OpenSUSE

zypper install screen

B. Run the screen

Type the command below to run the screen:

screen

There will be a display like the image below:

Press the Space or Enter key to continue, and after that, you are in the application screen. On the screen, you are free to use the Linux commands you want to run, for example, if you want to run the command:

free -m

The command will display the size of the memory on the server. If you want to exit the screen but want the command to continue on the screen, press the Ctrl+ad key, then you will return to your default terminal. To view the running screen session, use the command below:

screen -ls

It will look like in the image below:

C. Reattach to the screen session

If you want to reattach the previous screen session, use the format below:

screen -r pid_screen

For example, my previous screen session used pid 116673.pts-1.docker, like in the image above. If I want to log in to the previous screen session, I type the command below:

screen -r 116673.pts-1.docker

And I can log into the screen session, and the size of the server memory will be displayed as I leave the screen session, like in the image below:

D. Naming a screen session

By default, the screen will give a pid in the form of a pid number followed by the terminal type and hostname, as shown in the image above (116673.pts-1.docker). However, it may be confusing to the user if the user uses more than one screen with different processes on each screen. Therefore, you can give a name to distinguish it using the format below:

screen -S screen_session_name

For example, if you want to use a screen to run the watch process, then you can use the command below:

screen -S watch_ram

If you display the screen session will look like the image below:

From the image above, the screen name will change according to what you wrote, and will no longer use the connection type format and hostname for naming a screen session.

E. Delete a screen session

There are 2 ways to delete a screen session: from inside the screen and from outside the screen.

1. From inside the screen

If it is from inside the screen, just type exit or press Ctrl+d for the session to end, the screen session will end, which is marked by the sentence [screen is terminating] on the screen, and the screen session will disappear as shown in the image below:

2. From outside the screen

If it is from outside the screen, there are 2 methods to turn off the screen session. The first method is to use the command in the format below:

screen -S screen_session -X quit

Let’s say you have a PID session 117360.watch_ram, and want to turn off the session from outside the screen. You can use the command:

screen -S 117360.watch_ram -X quit

And the screen session should be deleted, like in the image below:

The second method is to use the kill command using the format:

kill pid_screen_session

Let’s say you have a PID session 117675.watch_ram, and want to turn off the session from outside the screen. You can use the command:

kill 117675.watch_ram

And the screen session should be deleted, like in the image below:

Note

If you open a screen session on a terminal but suddenly your internet connection is lost, you can open a new terminal and connect to the previous screen session. But after you type the command screen -r pid_screen_session, there is a statement as below:

There is no screen to be resumed matching 1195600.pts-1.linuxmint.

It means that the screen session is already attached somewhere ‐- likely in another terminal, SSH session, or even an old/abandoned connection. You can forcefully take over the session using:

screen -D -r 1195600.pts-1.linuxmint

And you should be able to reattach to the screen session.

References

en.wikipedia.org
hostinger.com
geeksforgeeks.org
veeble.com
baeldung.com

The previous article explained how to display the total size of memory used by applications on Linux. This article will explain how to display the total CPU size used by an application on Linux.

Problem

How to display the total CPU size used by an application on Linux?

Solution

You can use the application commonly used to see processes that run on Linux, such as top and htop, because it will display the CPU usage. Still, I want to display the size of the CPU used by an application because the display of the two commands is still confusing. Here are some scripts that can be used to see the size of the CPU used by an application on Linux:

A. Displays CPU per one application

Run the command below to see the CPU size used by an application (this script will display the size of the CPU used by the Vivaldi browser):

echo "Firefox browser use $(ps -C firefox -o %cpu --no-headers 2>/dev/null \
| awk -v cores=$(nproc) '{sum+=$1} END {if (NR>0) printf "%.1f%% CPU (of %d cores)", \
sum, cores; else print "0% CPU (not running)"}')"

Then the memory used by Firefox will be displayed as shown in the picture below:

B. Display the CPU for more than one application

If you want to display the CPU size used by more than one application, use the command below (this script will display the CPU size used by the Vivaldi and Firefox browsers):

for app in firefox-bin vivaldi-bin; do   echo -n "$app: ";   ps -C "$app" -o %cpu --no-headers 2>/dev/null |     awk -v cores=$(nproc) '{
      sum+=$1
    } END {
      if (NR>0) printf "%.1f%% CPU (of %d cores)\n", sum, cores
      else print "0% CPU (not running)"
    }'; done

Then the CPU used by Vivaldi and Firefox browsers will be displayed as shown in the picture below:

C. Displays the 5 apps that use the most CPU

Run the script below to see the 5 Linux applications that use the most CPU:

{ 
  echo -e "Application Name       %CPU"
  ps -eo comm,%cpu --no-headers | awk '{cpu[$1]+=$2} END {for (p in cpu) printf "%-20s %6.2f\n", p, cpu[p]}' | sort -k2 -nr | head -n 5
}

The script above will show the 5 Linux applications that use the most CPU:

Note

If you want to know the simple Linux command to display the memory and CPU of an application, you can use the command below (for this case, I chose the Firefox application):

ps -p $(pgrep -d ',' firefox-bin) -o pid,user,%cpu,%mem,cmd

Then there will be a display as below:

References

stackoverflow.com
lindevs.com
tecmint.com

I want to see the total memory size used by an application on Linux.

Problem

How to display the total memory size used by an application on Linux?

Solution

I usually use top or htop to see the process that occurs in Linux. But I want to be more specific to see how much the total size of the memory used by an application on Linux is. After searching on the internet, 2 tools can be used:

A. Using the ps_mem Tool

The ps_mem tool is used to see the use of memory for a program made by Pixelb at Github. To install this tool, make sure you have a Python package on your Linux device. Use the commands below to install ps_mem:

sudo wget -qO /usr/local/bin/ps_mem https://raw.githubusercontent.com/pixelb/ps_mem/master/ps_mem.py
sudo chmod a+x /usr/local/bin/ps_mem

If your Linux device has python3 and its location at /usr/bin/python3 (to know the location of the python, use whereis python3), use the command below so that this tool can run on your device:

sudo ln -s /usr/bin/python3 /usr/bin/python

After that, type the command below whether the ps_mem tool is running or not:

ps_mem --version

To see the options used in this tool, use the command below:

ps_mem --help

In general, the format used to see the size of a memory used in a Linux application is as follows:

ps_mem -S -p $(pgrep application_name)

For example, if you want to see how much memory size is used in the Firefox application, then type the command below:

ps_mem -S -p $(pgrep firefox)

As seen in the picture above, the Firefox application uses memory of 548.2 MB. To see the total memory used per Linux user, you can use the command below:

for i in $(ps -e -o user= | sort | uniq); do
  printf '%-20s%10s\n' $i $(sudo ps_mem --total -p $(pgrep -d, -u $i))
done

B. Using the smem Tool

This tool provides detailed reports on memory usage per process and per user, as well as providing additional information such as USS (Unique Set Size) that is not available in traditional tools such as top or htop. Below is how to install the smem tool:

Ubuntu/Debian

sudo apt update
sudo apt install smem

RockyLinux/AlmaLinux/CentOS

sudo yum install smem 

OpenSUSE

sudo zypper install smem 

Type the command below to run a smem command:

smem

Then the display will appear as shown below:

The terms USS, PSS, and RSS can be briefly explained below:

• USS (Unique Set Size) = Memory used exclusively by the process.
• PSS (Proportional Set Size) = Shared memory divided among processes.
• RSS (Resident Set Size) = Total RAM used (including shared).

After that, to display the size of memory used by a Linux application, such as the Vivaldi browser application, you can use the following script:

echo "Vivaldi using memory of $(smem -c "name pss" | grep -i vivaldi | awk -v avail_mem_kb=$(grep MemAvailable /proc/meminfo | awk '{print $2}') -v total_mem_kb=$(grep MemTotal /proc/meminfo | awk '{print $2}') '{sum+=$2} END {if(sum>0) {printf "%.2f MB (%.1f%% of total RAM, %.1f%% of available RAM)", sum/1024, (sum/total_mem_kb)*100, (sum/avail_mem_kb)*100} else {print "0 MB (0% of total RAM, 0% of available RAM)"}}')"

If you want to display the memory size used by more than one application, such as Vivaldi and Brave browser applications, copy the script below:

for app in vivaldi  brave; do    avail_mem_kb=$(grep MemAvailable /proc/meminfo | awk '{print $2}');   total_mem_kb=$(grep MemTotal /proc/meminfo | awk '{print $2}');   sum=$(smem -c "name pss" | grep -i "$app" | awk '{s+=$2} END {print s+0}');      LC_NUMERIC=C printf "%s using memory of %.2f MB (%.1f%% of total RAM, %.1f%% of available RAM)\n"     "$app" "$(echo "$sum/1024" | bc -l)" "$(echo "($sum/$total_mem_kb)*100" | bc -l)" "$(echo "($sum/$avail_mem_kb)*100" | bc -l)"; done

If you want to see the applications on your Linux device that use the most memory, copy the script below where the script will display the 5 Linux applications that use the most memory:

echo; printf "%-20s %12s %16s\n" "Application" "Memory (MB)" "% of Total RAM" && \
smem -c "name pss" | \
awk -v total_mem_kb=$(grep MemTotal /proc/meminfo | awk '{print $2}') \
    '{mem[$1]+=$2} END {for (proc in mem) { 
        if (mem[proc] > 0) {
            printf "%-20s %12.1f %15.1f%%\n", 
                proc,
                mem[proc]/1024, 
                (mem[proc]/total_mem_kb)*100
        }
    }}' | \
sort -k2 -rn | \
head -n 5

Note

If you compare the results of the two tools, you can see the results obtained are almost the same as in the picture below:

If you want to see the total size of memory used by an application on Linux easily then you can use the ps_mem tool. However, if you want to see a list of Linux applications that use the most memory, you can use the smem application.

References

stackoverflow.com
lindevs.com
tecmint.com

To see the total size of a folder in Linux usually sysadmin uses the command du -sh*. However, this command has a drawback because it only displays the size of all files and folders visible in the Linux folder but cannot display the size of the files and folders hidden in a Linux folder.

Problem

How to display hidden files and folder sizes on Linux?

Solution

I have a BACKUP folder with a total folder size of 1.4GB, but, when I go to the folder and run the command du -sh *, it turns out that the size of all the files and folders in the folder is only 261 MB as shown in the image below:

So there is a difference of 1.1 GB from the total size of the BACKUP folder. The difference is that the du -sh* command does not count the hidden files and folders in a Linux folder. To display hidden files and folder sizes on Linux, use the command below:

du -shc .[^.]* 2>/dev/null | sort -hr

and it will look like in the image below:

As seen in the image above, the command only displays hidden files and directories and shows a total of 1.1 GB of hidden files and folders. If you want to display the size of the entire file or folder, both hidden and visible, use the command below:

du -shc * .* 2>/dev/null | sort -hr

there will be a display like the image below:

You can see the total size of a file and folder, both visible and hidden, like the image above.

Note

You can view file sizes and directories on Linux by using an application named ncdu. To install it, enter the following command:

Ubuntu/Debian

sudo apt update
sudo apt ncdu

RockyLinux/AlmaLinux/CentOS

sudo dnf install epel-release
sudo dnf update
sudo dnf install ncdu

OpenSUSE

sudo zypper refresh
sudo zypper install ncdu

Next, choose the folder you wish to view to see the overall size of the files and folders included within. If you want to see the total of all folders in Linux, log in as a root user and type the command below:

ncdu -x /

then the application will scan and after the scanning process is complete it will display the total size of all folders in Linux as shown in the image below:

Please select the folder you want to enter and press the Enter key to enter the folder. I choose the BACKUP folder and push the Enter button, it will display like the image below:

The -x option in the above command means that he will ignore other mounted filesystems outside where the root filesystem is hosted. If you want to use more options in this application, please read the description using the command below:

man ncdu

References

askubuntu.com
unix.stackexchange.com
stackoverflow.com