Working in IT, there will be a day when you will need to purge a file on one of your systems. As an administrator, managing file sizes and content is crucial for maintaining the system performance and stability you require. Regardless of what you call it – emptying, clearing, wiping, purging; There are various reasons why you might want to clear a file without actually deleting it:
Log Management: Logs can grow excessively large, consuming valuable disk space. Clearing logs without deleting them ensures continuity in logging.
Data Reset: Some applications might require periodic resets while keeping the file structure intact.
Error Resolution: Clearing files with erroneous or corrupted data can be a quick way to restore normal operations without affecting the file’s existence or permissions.
Here are nine methods to empty a file from the command line:
Using thetruncate Command: The truncate command can be used to resize files. Setting the size to zero effectively clears the file.
truncate -s 0 file.txt
This command is straightforward and efficient for emptying a file while preserving its metadata.
Using the echo Command: The echo command can output an empty string to a file, thereby clearing its contents.
echo -n > file.txt
The -n option ensures that no newline character is added, leaving the file empty.
Using Vim Editor: Vim, a powerful text editor, can also be used to clear a file. Open the file with vim. In Vim, type the following command to delete all lines:
vim file.txt
:1,$d
This command deletes all lines from the first to the last line in the file.
Using the dd Command: The dd command is useful for low-level data manipulation and can clear a file by reading from /dev/null.
dd if=/dev/null of=file.txt
This reads from /dev/null and writes to file.txt, making it empty.
Using the cp Command with /dev/null: The cp command can replace the file’s contents with the empty contents of /dev/null.
cp /dev/null file.txt
This is an efficient way to clear a file while maintaining its attributes.
Using the > Operator: The simplest method involves using the redirection operator to truncate the file.
> file.txt
This method is quick and commonly used for clearing file contents.
Using the cat Command: By redirecting the contents of /dev/null to the file, you can clear its contents.
cat /dev/null > file.txt
This is another straightforward method to empty a file.
Using the : (Colon) Command: The colon (:) is a built-in shell command that does nothing but return a true exit status. When combined with the redirection operator, it can clear a file.
: > file.txt
This command is both simple and efficient for emptying files.
Using the sed Command: The sed command can delete all lines in a file.
sed -i d file.txt
The -i option tells sed to edit the file in place, and the d command deletes all lines.
Conclusion
Emptying files without deleting them is a common administrative task in Linux. Each of these methods allows you to clear file contents while preserving the file itself, along with its permissions and ownership. Whether you are managing log files, resetting data, or addressing errors, these commands provide efficient ways to handle files without removing them. The choice of method simply depends on your specific needs and the tools you are comfortable with. Hopefully this helps you somewhere in your day-to-day linux administration.
Okay, it’s time to install Git so you can play with some pull and merge requests for some projects you are working on. This set of instructions should work on your system regardless if you are running CentOS 7, 8, or 9.
The first thing to do is elevate…
sudo su
Then update your system.
dnf update
Install Git
dnf install git
Check the installed version
git --version
Just like that, you are ready to “Git” yourself back to coding something grand!
Docker is an operating system virtualization tool that allows us to run applications as containers. In simplest terms, that means you are virtualizing only the application, and not creating an entire virtual machine as you would traditionally do in hypervisors like VMware, Hyper-V, or Nutanix.
Okay, that’s cool… How do we install Docker so we can start to test workloads on it? Well, let me show you how to install Docker on a virtual machine running CentOS 9 Stream. **While I have not tested to confirm, this Docker installation method should be identical on CentOS 8 Stream, as well as for CentOS 7.x
Let us begin by shifting to Sudo mode by running this command first…
sudo su
Then the first thing to do is remove PodMan as it conflicts with Docker.
Let us view what we installed by running these two commands.
rpm -q docker-ce
docker version
Congratulations! You now have Docker installed on your machine.
You’ll probably want to install Docker Compose on your machine too so you can build and run a docker image. You can install it with this simple command.
Here is how to add the open source antivirus tool ClamAV to the CentOS machine and configure it automatically run a virus scan on newly uploaded files. ClamAV detects all forms of malware including Trojan horses, viruses, and worms, and it operates on all major file types including Windows, Linux, and Mac files, compressed files, executables, image files, Flash, PDF, and many others. ClamAV’s Freshclam daemon automatically updates its malware signature database at scheduled intervals.
First edit freshclam.conf and configure your options.
vi /etc/freshclam.conf
Freshclam updates your malware database, so you want it to run frequently to get updated malware signatures. Run it manually post-installation to download your first set of malware signatures:
freshclam
Next, edit scan.conf.
vi /etc/clamd.d/scan.conf
Uncomment this line
LocalSocket /run/clamd.scan/clamd.sock
When you’re finished you must enable the clamd service file and start clamd:
Verify PHP is installed and the version. You can see I was able to install PHP v8.1.4
sudo php -v
Open the php.ini config file and set your timezone. You will need to uncomment the line for date.timezone and set it to your timezone of choice and set it to your timezone of choice. .
Secure or instance of Maria DB by running the ‘mariadb_secure_installation‘ command.
sudo mariadb-secure-installation
Enter your root credentials when prompted. For the next two prompts, if you have your root account protected correctly, it will tell you so and you can follow the recommendation to enter ‘n’ for them.
For the next four prompts, enter ‘Y’ for them.
Check your MariaDB and what version it is running this command below or login into the database and check as shown in the image below.
So I’m going to walk thru installing Nextcloud on CentOS 7. Your mileage will vary if you attempt to use this as a guide to install NextCloud on CentOS 8 (which is EOL) or CentOS Stream 8/9 as it is not intended for those versions of CentOS.
Nextcloud is an open-source self-hosted sync and file sharing server that was forked from OwnCloud. It is written in PHP and JavaScript and supports multiple databases like MySQL, PostgreSQL, SQLite, and Oracle Database.
Before we get started, we will need to make sure we are set up with a LAMP stack. LAMP stands for Linux, Apache, MySQL, PHP. It’s bascially setting us up as a web server. And since we are going to be a webserver, we should also add Let’s Encrypt for SSL on our machine.
First step is to update your system.
yum -y update
Install PHP
To install PHP 8, you will need to add the EPEL and Remi repositories to your machine. You should also import the repo’s signing key.
Verify PHP is installed and the version. You can see I was able to install PHP v8.0.17
php -v
Open the php.ini config file and set your timezone. You will need to uncomment the line for date.timezone and set it to your timezone of choice.
vi /etc/php.ini
date.timezone = Pacific/Honolulu
Raise PHP’s memory limit
sed -i '/^memory_limit =/s/=.*/= 512M/' /etc/php.ini
Install Apache
Install Apache on your machine.
yum -y install httpd mod_ssl
Start Apache and enable the Apache service at boot.
systemctl start httpd
systemctl enable httpd
Install MariaDB
Add the MariaDB repository to your machine
cat <<EOF | sudo tee /etc/yum.repos.d/MariaDB.repo
[mariadb]
name = MariaDB
baseurl = http://yum.mariadb.org/10.6/centos7-amd64
gpgkey=https://yum.mariadb.org/RPM-GPG-KEY-MariaDB
gpgcheck=1
EOF
Clean the yum cache
yum makecache fast
Install MariaDB 10.6
yum -y install MariaDB-server MariaDB-client
Start and enable MariaDB service:
systemctl start mariadb
systemctl enable mariadb
Secure or instance of Maria DB by running the ‘mariadb_secure_installation‘ command.
mariadb-secure-installation
Enter your root credentials when prompted. For the next two prompts, if you have your root account protected correctly, it will tell you so and you can follow the recommendation to enter ‘n’ for them.
For the next four prompts, enter ‘Y’ for them.
Check your MariaDB and what version it is running this command below or login into the database and check as shown in the image below.
mysql -V
Create the Database and the user account for NextCloud using the commands below.
Take note of what you set for: <nextcloud_db> : This will be the name of your NextCloud database. <nextcloud_user> : This will be the NextCloud user. <nextcloud_pw> : This is a strong password that you have created for your ‘nextcloud_user’.
mysql -u root -p
create database <nextcloud_db>;
create user '<nextclouduser>'@'localhost' identified BY '<nextcloud_pw>';
grant all privileges on <nextcloud_db>.* to '<nextclouduser>'@'localhost';
flush privileges;
\q
Give Apache access to MariaDB
setsebool -P httpd_can_network_connect_db 1
Let us go ahead and reboot the system before we proceed with installing NextCloud.
init 6
Installing NextCloud
Download the packages needed to download and unzip NextCloud
yum -y install wget unzip
Next, download the latest stable release of NextCloud to your system.
Create a data directory to store files that get uploaded to NextCloud. If you use a symlink, this can be any type of path to a NAS, SAN, or NFS. Give Apache permiss
Give the Apache user and group ownership of the NextCloud folder.
chown apache:apache -R /var/www/html/nextcloud
The next step will create an Apache VirtualHost configuration file.
vi /etc/httpd/conf.d/nextcloud.conf
Copy and paste the following code block into the file. Note: Make sure to update the “ServerName” and “ServerAdmin” settings to suit your environment. The “ServerName” is its FQDN, so remember to setup your DNS entry for it, if necessary.
<VirtualHost *:80>
ServerName nextcloud.pwwf.com
ServerAdmin nextcloud.admin@pwwf.com
DocumentRoot /var/www/html/nextcloud
<directory /var/www/html/nextcloud>
Require all granted
AllowOverride All
Options FollowSymLinks MultiViews
SetEnv HOME /var/www/html/nextcloud
SetEnv HTTP_HOME /var/www/html/nextcloud
</directory>
</VirtualHost>
Configure SELinux
Install the SEMange package.
yum -y install policycoreutils-python
Add the context rules to allow NextCloud to write data into its directories.
semanage fcontext -a -t httpd_sys_rw_content_t '/var/www/html/nextcloud/data'
semanage fcontext -a -t httpd_sys_rw_content_t "/var/www/html(/.*)?"
semanage fcontext -a -t httpd_sys_rw_content_t '/var/www/html/nextcloud/config(/.*)?'
semanage fcontext -a -t httpd_sys_rw_content_t '/var/www/html/nextcloud/apps(/.*)?'
semanage fcontext -a -t httpd_sys_rw_content_t '/var/www/html/nextcloud/3rdparty(/.*)?'
semanage fcontext -a -t httpd_sys_rw_content_t '/var/www/html/nextcloud/.htaccess'
semanage fcontext -a -t httpd_sys_rw_content_t '/var/www/html/nextcloud/.user.ini'
restorecon -Rv /var/www/html
Open your web browser of choice and enter either the server name URL you entered in the ‘nextcloud.conf’ file, or alternatively you could use the IP address of your machine, to access the NextCloud Web GUI.
example – http://nextcloud.pwwf.com/ http://10.1.2.169/
The first fields are for creating an admin account for your NextCloud instance. Set it to anything you wish, just don’t forget those credentials.
Then select “MySQL/MariaDB” and configure the database fields with the information we used earlier when we set up the database in MariaDB.
Then click on the “Install” button at the very bottom of the page.
Once the install completes, your dashboard will be ready to use. In your browser, go to: http://<ServerName>/nextcloud/index.php/apps/dashboard
Having HTTP access is great… but I think that we would like to have some security. There are plenty of paid services out there to get an SSL from. But for this post let us add SSL encryption using the FREE resource that is Let’s Encrypt so that we can utilize HTTPS without any additional cost.
The first thing we need to do is install certbot.
yum -y install epel-release certbot
Next we will need to request our SSL certificate for this machine.
Note: If certbot is not working for you, you will need to figure out whatever issue it is having before proceeding. If you cannot resolve it, the rest of this article will not benefit you. Unfortunately, troubleshooting certbot is outside the scope of this article.
After the SSL certificate has successfully been generated, it is time to edit your Apache config file for NextCloud, again.
vi /etc/httpd/conf.d/nextcloud.conf
Make your configuration file look like what I have below. Note: Make sure to update the “ServerName” and “ServerAdmin” settings to suit your environment.
To remove the “index.php” from every URL, open the Nextcloud config file.
vi /var/www/html/nextcloud/config/config.php
Depending on how your config file is setup, you will add one of the following entries below based on how your URL is configured. If you get this wrong, don’t worry, you will see an “Internal Server Error” message instead of your NextCloud page and will have to come back into this file and change it.
If your line for “overwrite.cli.url” looks like this
Now go back to your browser and in the address bar, enter your pretty url without the ‘index.php’ in it… In my case, it will be “https://nextcloud.pwwf.com/”
Proxy override
I was having an issue with the UI inside NextCloud. I could view folders and files, but I could not create new folders or files. After some troubleshooting recreating the NextCloud server and testing before adding the SSL certificate and also after adding the certificate, as well as testing bypassing the proxy I was able to confirm that the proxy was indeed causing me my headaches. This should help you if you are behind a proxy…
vi /var/www/html/nextcloud/config/config.php
Under your line for “overwrite.cli.url” add this entry.
'overwriteprotocol' => 'https',
This will make sure that any requests, and replies, are done over HTTPS and now HTTP.
Max Upload
PHP is going to try to limit the file upload size that you can use. Since I know you are going to probably want to save/share some large files, let us update those limits to something more realistic.
vi /etc/php.ini
Search the file and update these values to your desired limit, I’m going to set it to 10GB.
While you can adjust these values to your environment, just remember to always make your “post_max_size” a little bit larger than your “upload_max_filesize”. This will keep you from having any issues when uploading a file that is the same size as your max upload limit.
Lastly, you will need to restart Apache.
systemctl restart httpd
Trash Cleanup
So NextCloud isn’t always great at cleaning up your deleted files. By design, it is set to hold on to your deleted items for 30 days, then it only forces a delete if you are running low on space. Since you’re probably sitting on at least a few terabytes of storage, those deleted files may never actually get deleted.
vi /var/www/html/nextcloud/config/config.php
Open your NextCloud config file.
Here is how you can control NextCloud’s behavior with these settings.
auto – default setting. keeps files and folders in the trash bin for 30 days and automatically deletes anytime after that if space is needed (note: files may not be deleted if space is not needed).
D, auto – keeps files and folders in the trash bin for D+ days, delete anytime if space needed (note: files may not be deleted if space is not needed)
auto, D – delete all files in the trash bin that are older than D days automatically, delete other files anytime if space needed
D1, D2 – keep files and folders in the trash bin for at least D1 days and delete when exceeds D2 days (note: files will not be deleted automatically if space is needed)
disabled – trash bin auto clean disabled, files and folders will be kept forever
To automatically delete the files after 30 days and allow NextCloud to purge them sooner if space is needed, you can add this line.
'trashbin_retention_obligation' => 'auto, 30',
To retain the files for 30 days and then absolutely purge them after 40 days, you would add this line.
'trashbin_retention_obligation' => '30, 40',
Install ClamAV
Here is how to add the open source antivirus tool ClamAV to the CentOS machine and configure it automatically run a virus scan on newly uploaded files. ClamAV detects all forms of malware including Trojan horses, viruses, and worms, and it operates on all major file types including Windows, Linux, and Mac files, compressed files, executables, image files, Flash, PDF, and many others. ClamAV’s Freshclam daemon automatically updates its malware signature database at scheduled intervals.
First edit freshclam.conf and configure your options.
vi /etc/freshclam.conf
Freshclam updates your malware database, so you want it to run frequently to get updated malware signatures. Run it manually post-installation to download your first set of malware signatures:
freshclam
Next, edit scan.conf.
vi /etc/clamd.d/scan.conf
Uncomment this line
LocalSocket /run/clamd.scan/clamd.sock
When you’re finished you must enable the clamd service file and start clamd:
With CentOS 8 now EOL, it is officially time to upgrade CentOS 8 virtual machines to CentOS 8 Stream. The good news is that it is even quicker and easier than the upgrade from CentOS 7 to CentOS 8 was.
First things first… Take a backup of your virtual machine, or at least a snapshot so that you have something you can revert back to if something goes wrong in this process.
Take a look at what release your CentOS machine is currently running.
cat /etc/centos-release
cat /etc/os-release
As you can see this machine is currently on CentOS 8.5.2111.
At this point, I’m going to enter “sudo su” on my VM and then enter my credentials, so that I can continue as ‘root’ and I don’t have to type “sudo” before every single command.
To begin, start by updating your system.
dnf -y update
The next step is to update your machine to the current CentOS Stream release package.
List and view all of the enabled repositories. You should see they are set to “CentOS Stream 8”.
sudo dnf repolist
Next, synchronize all of the installed packages on your machine.
Note: For situational awareness, this step will upgrade or downgrade packages to match the new CentOS Stream ABI/API and will apparently break 100% RHEL compatibility due to the ABI/API change. This is the perfect example of why you would want to take a full backup of the system before making any changes, just in case the ABI/API change breaks one of your applications running on the system.
dnf -y distro-sync
Reboot your system.
init 6
Confirm that we are now running on CentOS 8 Stream.
cat /etc/centos-release
cat /etc/os-release
We can now see that this machine is now running on CentOS Stream 8.
