Cache memory is a type of data storage used to store frequently accessed information for faster response time, and it’s used to improve system performance. In Linux, the kernel uses the buff/cache memory to improve system performance by caching frequently accessed data (disk blocks, inodes, etc.) and buffering I/O operations. So if you have an application that is often used and accessible to many people, you will see that the memory cache on the server will often be used. But sometimes, because of necessity, you have to clean the cache memory.

Problem

How to clear cache memory on Linux?

Solution

 Before you delete the cache memory on Linux, you must know some of the terms related to the cache memory:

• Buffer: stores disk blocks that have been recently accessed or modified.
• PageCache: It improves file I/O performance by storing often-used file data in RAM.
• Dentries: help speed up file name searches by storing cached directory entries.
• Inodes: store important metadata about files and directories, they are also separate from the content or names.

So if you want to delete cache memory, use the format below:

sync; echo 1-3 > /proc/sys/vm/drop_caches

The sync command to clear the buffer and the drop_caches file controls which type of cached data should be cleared and the values are as follows:

• 1 – Clears only the page cache.
• 2 – Clears dentries and inodes.
• 3 – Clears page cache, dentries, and inodes.

So if you want to delete cache memory, use the format below:

sync; echo 3 > /proc/sys/vm/drop_caches 

And the cache memory on your Linux server will be deleted as shown below:

Note

It is not dangerous if you run the command to delete the memory cache on Linux but this will cause an increase in I/O disk because of the request that is usually handled by the memory cache because the memory cache is cleaned so that the request will immediately proceed to the hard disk, causing an increase I/O disk. Also on the application side will cause a decrease in performance because the application does not receive a response from the memory cache, but must receive a response from the hard disk, whose response time is slower than from the cache memory. 

References

By default, if you do a copy command or move a file in Linux CLI, no progress bar shows how long the commands will take to complete. I think the users will feel bored because they do not know how long the commands will take to complete.

Problem

How to show the progress bar in Linux’s cp and mv commands?

Solution

The image below shows no progress bar when you copy a file in Linux CLI:

To display progress bars on cp and mv commands on Linux, you need a tool made by a GitHub user named jarun, who modified the source code of Florian Zwicke. But before you can install this tool, you must install the required packages:

RockyLinux/AlmaLinux/CentOS

yum install -y tar curl patch make coreutils gcc libattr* libpcap* perl libacl*

Debian/Ubuntu

apt-get install -y tar curl patch make coreutils gcc libattr* libpcap* perl libacl*

OpenSUSE

zypper install -y tar curl patch make coreutils gcc libattr* libpcap* perl libacl*

After that, install the tool, and it is recommended to use a normal user to install the tool, but if you want to use the root user  to install this tool, run the command below:

export FORCE_UNSAFE_CONFIGURE=1

Install the tool on your server by running the command below:

curl https://raw.githubusercontent.com/jarun/advcpmv/master/install.sh --create-dirs -o ./advcpmv/install.sh && (cd advcpmv && sh install.sh)

After installation completes, it created two new commands under the advcpmv folder. You need to replace your original cp and mv commands with these two new commands to get the progress bar while copying files:

sudo mv ./advcpmv/advcp /usr/local/bin/cp
sudo mv ./advcpmv/advmv /usr/local/bin/mv

After that, run the commands below:

echo "alias cp='/usr/local/bin/cp -g'" >> ~/.bashrc
echo "alias mv='/usr/local/bin/mv -g'" >> ~/.bashrc
source ~/.bashrc

Try to copy a file in your Linux CLI, and there should be a progress bar when you copy the file:

Likewise, if you are going to move a file on Linux CLI, there will be a notification as below:

Note

You should know that if you want to use this tool on a different user from the user where this tool is installed on the server (for example, you installed this tool using another user but you want to use the tool using another user), you need to use the command below using your user so that this tool can be used on that user:

echo "alias cp='/usr/local/bin/cp -g'" >> ~/.bashrc 
echo "'alias mv='/usr/local/bin/mv -g'" >> ~/.bashrc 
source ~/.bashrc

References

tecmint.com
ostechnix.com
stackoverflow.com

As a Sysadmin, accessing a Linux server is a normal daily activity. But sometimes we accidentally make mistakes rebooting or shutting down the production server, causing the server to be inaccessible. Therefore, we need a tool to confirm if someone reboots or shuts down a Linux server.

Problem

How to protect the Linux server from an accidental reboot or shutdown?

Solution

In the Debian/Ubuntu distribution, the molly-guard tool can be used to protect the Linux server from an accidental reboot or shutdown. Use the two commands below to install molly-guard:

sudo apt update
sudo apt-get install molly-guard

After that, try to reboot the server, and there should be a notification like the image below:

Someone who wants to reboot the server must write the server’s hostname. If the nameserver does not match the hostname on the server, the reboot process will not be continued, but if it matches the hostname on the server, the reboot process will be continued.

This is very useful if the sysadmin accidentally types the reboot command on the server. However, this tool not only protects the server from the reboot command, but also other commands such as the poweroff, shutdown, coldreboot, pm-hibernate, pm-suspend, and pm-suspend-hybrid commands.

Note

Keep in mind that this molly-guard tool can only work in the Debian/Ubuntu distribution and its derivatives, and this tool only works on SSH connections. If you access the Linux server without an SSH connection, for example, by directly connecting the keyboard to the Linux server, this tool will not work, so if you run the reboot command, the Linux server will immediately reboot.

References

manpages.ubuntu.com
launchpad.net
techbits.io

By default, if you open the vi application on Linux (especially using PuTTY), the color for comments is blue. However, sometimes this makes it very difficult for me to read the comments, especially if the background color of the terminal is black.

Problem

How to change the color of comments in the vi application?

Solution

Below is an image of a /etc/crontab file opened using Putty:

For me, it’s very difficult to read the comments in the Linux file if they’re blue like in the image above. So, if you want to change the color of comments in the vi application, for example, if you want to change the color of comments to yellow, then open the .vimrc file by:

vi ~/.vimrc

Type the script below into the file:

highlight Comment ctermfg=yellow

After that, open the file using the vi application, and the comments on the file should change to yellow as in the image below:

You can see that the comment color is changed to yellow after you configure the .vimrc file.

Note

Currently, the vi application can support up to 256 colors that can be used in the vi application. So if you want to use more colors supported by the vi application, in the .vimrc file type the script below:

set t_Co=256

Then you can choose the colors on this page, for example, you want to use purple for comments, then enter the script below in the .vimrc file:

set t_Co=256
highlight Comment ctermfg=93

Then the comments in the vi/vim application should be purple as in the image below:

References

spinspire.com
linode.com

Crontab, which stands for cron table, is used to run one or more scripts in Linux based on a specific time. Usually, if you want to change something in the crontab, you use the crontab -e command and then change the crontab. But I want to change crontab using a shell script.

Problem

How to change crontab using a bash script?

Solution

I create a bash script to execute something on my Linux server, and in my script, I want to change the crontab so the script will add, change, or remove the script in the crontab. Here are ways to change crontab using a script:

1. Add a script in crontab

For example, if you want to add a random.sh script which is in the /root/scripts folder in crontab and will run every 5 minutes, then use the command below:

(crontab -l 2>/dev/null || true; echo "*/5 * * * *		/root/scripts/random.sh") | crontab -

Or if you want to add the script to the crontab in another form of writing, then you can use the command below:

(crontab -l 2>/dev/null || true; echo "*/5 * * * *             cd /root/scripts;./random.sh") | crontab -

2. Change the script in crontab

If you want to change the file in crontab to once every 10 minutes (previously every 5 minutes) for the random.sh script in the /root/scripts folder, then use the command below:

crontab -l | sed 's/\*\/5 \* \* \* \*                \/root\/scripts\/random.sh/\*\/10 \* \* \* \*                \/root\/scripts\/random.sh/g' | crontab -

Or, you can execute the command below if your script uses another form of writing in crontab:

crontab -l | sed 's/\*\/5 \* \* \* \*             cd\ \/root\/scripts\;\.\/random.sh/\*\/10 \* \* \* \*             cd\ \/root\/scripts\;\.\/random.sh/g' | crontab -

3. Disable and enable the script

If you want to disable the random.sh script in crontab, then use the command below:

crontab -l | sed 's/\*\/10 \* \* \* \*             cd\ \/root\/scripts\;\.\/random.sh/\#\*\/10 \* \* \* \*             cd\ \/root\/scripts\;\.\/random.sh/g' | crontab -

But if you want to enable it, use the command below:

crontab -l | sed 's/\#\*\/10 \* \* \* \*             cd\ \/root\/scripts\;\.\/random.sh/\*\/10 \* \* \* \*             cd\ \/root\/scripts\;\.\/random.sh/g' | crontab -

4. Deleting the script in crontab

Use the command below if you want to delete the random.sh file in crontab:

crontab -l | sed '/\*\/5 \* \* \* \*         \/root\/scripts\/random.sh/d' | crontab -

Or, you can execute the command below if your script uses another form of writing in crontab:

crontab -l | sed '/\*\/5 \* \* \* \*             cd\ \/root\/scripts\;\.\/random.sh/d' | crontab -

Note

You have to pay attention to whether the script in the crontab uses spaces or tabs because it greatly affects whether the script that you run can change something in the crontab or not. You have to put a backslash(\) if you want to change or delete your script in crontab that uses symbols like an asterisk(*), forward slash(/), hash(#), space, and so on.

References

techtarget.com
stackoverflow.com
webopedia.com

If you install a Linux server, you will usually install it with only one partition and not separate the other partitions. Problems will arise if one of these partitions uses a large enough hard disk, resulting in you running out of HDD space on your Linux server.

Problem

How to move the partition to a new partition in the Linux server?

Solution

In this article, I use the Ubuntu Server OS, and this article should be applied to any Linux distribution. Currently, the condition of the hard disk on my Ubuntu server is like the image below:

From the image above, the root partition only has a free HDD of 9 percent. After I checked, it turned out that the cause was the /var partition, which took up a lot of hard disk so I want to move the /var partition to the new partition.

Here are the steps to move the partition to a new partition in the Linux Server:

1. Add a new hard drive

I insert a new 10 GB HDD into my Linux server. After that, I check if the new HDD is detected by Linux using the command:

fdisk -l

From the image above, it can be seen that the new HDD was detected by Linux with a partition in sdb.

2. Create a new partition

Run the command below to create a new partition in Linux (Adjust to the hard disk partition detected on your Linux server after typing the fdisk -l command):

fdisk /dev/sdb

Press the n and p keys, then the number ,1 and enter 2x, then press the w button as seen in the image below:

Then create a filesystem from the new HDD, and I want to use ext4 for the filesystem of the new HDD using the command:

mkfs.ext4 /dev/sdb1

3. Create and mount a new folder

After that, create a new folder using the command:

mkdir /mnt/newvar

Then, mount the new partition to the new folder using the command:

mount /dev/sdb1 /mnt/newvar

4. Enter maintenance mode

Type the command below:

init 1

to enter the rescue mode:

After that, press the Enter button to enter maintenance mode.

5. Copy the folder

Go to the /var folder and copy all the files and folders in the folder into a new folder by typing the following commands:

cd /var
cp -ax * /mnt/newvar

6. Rename the folder

Once the copy process is complete, change the /var folder to the var.old folder and then create a new /var folder using the command:

cd /
mv var var.old
mkdir /var

7. Mount the new folder

Next, do umount on the /sdb1 partition by using the command:

umount /dev/sdb1

And mount the /sdb1 partition to the new /var folder using the command:

mount /dev/sdb1 /var

8. Change the fstab file

Change the /etc/fstab file by adding the following script to the file:

/dev/sdb1	/var	ext4	defaults	0 0

9. Restart the server

After that, restart the Linux server and make sure there is no problem when the Linux server reboots.

10. Delete the folder

If the Linux server has finished restarting, then you can delete the var.old folder so that the size of the hard disk of the root partition increases by using the command:

cd /
rm -rf var.old

Note

Reboot the server again to make sure there are no problems after you delete the var.old folder. You can use the steps above when you want to move another folder to a new partition in the Linux server.

References

blog.oshim.net
phoenixnap.com

The previous article explained how to install Docker on Linux. This article will explain how to manage a container in Docker.

Problem

How to manage a container in Docker?

Solution

To manage a container in Docker, you have to remember basic Docker commands. And here are the basic Docker commands:

1. Search for container images

To run containers in Docker, we need a Docker image. A Docker image is an immutable (unchangeable) file that contains the source code, libraries, dependencies, tools, and other files needed for an application to run. The place to store Docker images is known as the Docker registry, which by default uses the Docker Hub located at hub.docker.com. If you are looking for a container image in Docker, use the format below:

docker search container_name

For example, if you want to find an nginx image, then use the command below:

docker search nginx

2. Download the Docker image

To download the Docker image, use the following format:

docker image pull image_name:tag_version

where the tag_version is the version of the image, and if you don’t write the tag, it is considered that you want to install the latest version of the image. For example, if you want to download the newest version of the nginx image, use the command:

docker image pull nginx

But if you want to download nginx with a certain version, for example, version 1.27.2, then use the command:

docker image pull nginx:1.27.2

3. List the Docker image(s)

To display the Docker image that you have downloaded, use the command below:

docker image ls

Or you can use the command below:

docker images

4. Create a container

You can create the container using the format:

docker container create –name container_name image_name:tag

For example, if you want to create a container with the name webapp1, which contains the nginx application, then use the command below:

docker container create --name webapp1 nginx

When you use this command, Docker will first check whether the nginx image is on the server. If the image is not on the server, then Docker will download the nginx image, and after that, it will create an nginx container, and the image will remain on your server, as shown in the image below:

You can use an image to create multiple containers as long as the container names are different, as in the image below:

5. List the status of the container(s)

To display the container status, you can use the command:

docker ps

Maybe you are confused about why there is no container status displayed, even though you have made 2 containers before. Remember that the docker ps command only displays the ongoing container status. While the 2 containers you made had not been running, you just made a container. If you want to display all container statuses, use the command below:

docker ps -a

6. Turn on the container

To turn on a container, you can use the format:

docker container start container_id/container_name

Usually, I use container_name instead of container_id because it’s easier to memorize, so I run the following command:

docker container start webapp1

7. Pause the container

You can pause a container with the following format:

docker container pause container_id/container_name

So, you can use the command below to pause the container:

docker container pause webapp1

To resume the container, use the following format:

docker containers unpause container_id/container_name

You can use the command below to resume the container:

docker container unpause webapp1

8. Run a container with a single command

As explained above, if you want to run a container, you have to download the image first, create a container, and turn on the container (see numbers 2, 4, and 6). There is a command that can summarize the three commands above, using the format below:

docker run -d --name container_id/container_name image_name:tag

where the -d option is to run the container in the background. So if you want to run a container with the name webapp2, which contains the nginx application, then use the command below:

docker run -d --name webapp3 nginx

9. Display the size of Docker

To display how large Docker is installed on your server, use the command below:

docker system df

To display a Docker size in detail, use the command below:

docker system df -v

10. Display logs

To display logs of the running container to check something, follow the format below:

docker container logs container_id/container_name

So, run the command below to check the logs of your container:

docker container logs webapp1

If you want to display real-time logs of the container, give an option -f like in the below command:

docker container logs -f webapp1

Press Ctrl-C to exit the log.

11. Inspect the container

To display detailed information about a container, use the following format:

docker inspect container_name/container_id

So, if you want to see the detailed information about the container that you created before, use the command below:

docker inspect webapp1

If you only want to display specific items when running the inspect command, use the following format:

docker container inspect container_name/container_id -f '{{json .the item_you_want_to_display<.sub_item> }}' | python -m json.tool

So if you want to display only the network section when using the Docker inspect command, use the command below:

docker container inspect webapp1 -f '{{json .NetworkSettings.Networks }}' | python3 -m json.tool

12. Stop the container(s)

To stop the container, use the format below:

docker container stop container_id/container_name

For example, if I want to stop my container, then use the command below:

docker container stop webapp1

You can stop all the containers running with the below command:

docker stop webapp1 webapp2

Or use the below command to stop all the running containers:

docker kill $(docker ps -q)

13. Remove the container(s)

Before you remove the container, you have to stop the container first. To delete a container that’s already turned off, use the format below:

docker container rm container_id/container_name

Run the command below to remove the container:

docker container rm webapp1

By default, you can’t remove a container if the container is still running. You can use the command below to delete the container even if the container is still running, but it is not recommended:

docker container rm -f webapp2

If you have a lot of containers that are no longer used and you don’t want to delete them one by one, you can use the command below to delete all the unused containers:

docker rm $(docker ps -a -q)

You can also use the command below to delete all the stop containers:

docker container prune

14. Delete the image(s)

To delete the Docker image that you have already downloaded, use the format below:

docker image rm image_name

Run the image below if you want to delete the nginx image:

docker image rm nginx

However, you must know that you can’t delete the Docker image if the image is still running in the container. So you must remove the container first before you delete the image. If you want to delete multiple Docker images, use the following format:

docker image rm image_name1 image_name2 ...

So if you want to delete the nginx image and nginx:1.27.2 at once, then use the command below:

docker image rm nginx nginx:1.27.2

And if you want to delete all the images, you can use the command below:

docker rmi $(docker images -a -q)

Or, you can use the command below to remove the unused images:

docker image prune -a

Note

If you forget or don’t know what command to use in Docker, use the following command:

docker --help

After that, if you want to know the options in the Docker command, then use the following format:

docker command --help

For example, if you want to know the options of the run command in Docker, then type the command below:

docker run --help

You can shorten all the Docker container commands to just the docker command to shorten the typing time. For example, if you want to create a container, you can use the command:

docker create --name webapp6 nginx

References

geeksforgeeks.org
mygreatlearning.com
youtube.com
youtube.dimas-maryanto.com