Category: Linux

1. Show list of all tasks

Display all cron jobs for the current user:

crontab -l

2. Add a new task

Edit your crontab file:

crontab -e

Add the following line at the end of the file to run a command every day at midnight:

0 0 * * * php /full/path/to/index.php

Replace /full/path/to/index.php with the actual path to your script.

3. Edit a task

Run:

crontab -e

Then modify the desired line in the editor.

4. Delete a task

Also via:

crontab -e

Just delete the task line and save changes.

To delete all cron jobs at once:

crontab -r

Warning: This removes all crontab entries for the user without confirmation.

Useful Examples

Every 5 minutes:

*/5 * * * * php /home/user/script.php

Every day at 6:30 AM:

30 6 * * * php /home/user/script.php

Every Monday at 7:00 AM:

0 7 * * 1 php /home/user/script.php

Log output to file:

0 0 * * * php /home/user/index.php >> /home/user/logs/cron.log 2>&1

>> appends output; 2>&1 combines stderr and stdout.

Check if cron is running

systemctl status cron

If stopped, start it:

sudo systemctl start cron

Other tips

Edit crontab for another user:

sudo crontab -u username -e

System-wide cron jobs:

• /etc/crontab
• /etc/cron.daily/
• /etc/cron.hourly/
• /etc/cron.weekly/
• /etc/cron.monthly/

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Source: https://aiblocklab.com/en/articles/linux-server-tutorials/working-with-crontab-in-linux

In today’s digital world, ensuring the confidentiality of your sensitive data is more important than ever. Whether you’re backing up documents, transferring files, or protecting personal information, encrypting your files is a critical security step. In this article, we’ll explore how to securely encrypt and decrypt any file using the Linux terminal — leveraging robust, open-source tools like gpg and openssl.

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🔧 Symmetric Encryption with gpg

The simplest and most effective method for encrypting files locally is symmetric encryption, where the same password is used for both encryption and decryption.

🔒 Encrypt a File with a Password

gpg -c myfile.txt

You’ll be prompted to enter a passphrase. The resulting file will be myfile.txt.gpg.

🔓 Decrypt the File

gpg -o myfile_decrypted.txt -d myfile.txt.gpg

You’ll be prompted for the same passphrase to decrypt the file.

✅ Best For: Local file protection or sharing with someone you trust enough to share a password with.

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🔐 Asymmetric Encryption (Using Key Pairs)

For secure communication or file exchange between users, asymmetric encryption is recommended. This method uses a pair of keys: a public key for encryption and a private key for decryption.

Step 1: Generate a Key Pair

gpg --full-generate-key

Choose:

• Key type: RSA and RSA
• Key size: 4096 bits (recommended)
• Expiration: Set if desired
• User information: Name and email

Step 2: List Available Keys

gpg --list-keys

Step 3: Encrypt a File for a Specific User

gpg -e -r "User Name or Email" myfile.txt

This command will create myfile.txt.gpg, encrypted with the recipient’s public key.

Step 4: Decrypt the File

gpg -d myfile.txt.gpg > myfile_decrypted.txt

The recipient’s private key must be available on the system to decrypt.

✅ Best For: Secure communication and file exchange between multiple parties.

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🔧 Alternative: Using openssl

For those who prefer OpenSSL, here’s how to use AES-256 encryption:

🔒 Encrypt

openssl enc -aes-256-cbc -salt -in myfile.txt -out myfile.txt.enc

🔓 Decrypt

openssl enc -d -aes-256-cbc -in myfile.txt.enc -out myfile_decrypted.txt

You’ll be asked to provide the same password for decryption.

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🛡️ Security Tips

• Use strong, unique passphrases — at least 12+ characters with a mix of letters, numbers, and symbols.
• Avoid storing passphrases in plaintext.
• When sharing files, prefer asymmetric encryption over symmetric if possible.
• Back up your private keys in a secure location.

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📦 Conclusion

Encrypting files via the Linux terminal is both powerful and straightforward with tools like gpg and openssl. Whether you’re an individual protecting personal data or a business sharing confidential documents, these methods help ensure your information stays safe.

To check open ports on the server use Nmap program. To install Nmap use command:

sudo apt install nmap

This command check server IP for open ports:

sudo nmap -sC -sV -v IP_ADDRESS

For fast scan all ports with Nmap use command:

sudo nmap -sS -v IP_ADDRESS -p-

Scan directories on host:

ffuf -w ~/SecLists/Discovery/Web-Content/directory-lists-lowercase-2.3-small.txt:FUZZ -u "http://IP_ADDRESS/FUZZ" -ic -c -e .php

Scan Subdomains:

ffuf -w ~/SecLists/Discovery/Web-Content/directory-lists-lowercase-2.3-small.txt:FUZZ -u "http://DOMAIN.COM/" -H 'Host: FUZZ.DOMAIN.COM' -fw SIZE

-fw – filter words 522

-fs – filter size SIZE

The error “The repository is not updated and the previous index files will be used. GPG error: https://nginx.org/packages/mainline/debian bullseye InRelease: The following signatures were invalid” suggests that the GPG key for the Nginx repository is either missing, expired, or invalid. Here’s how to fix it in Debian 11 GNU Linux (Bullseye):

1. Remove Old Nginx GPG Keys

Old or conflicting keys can cause issues, so remove them first:

sudo rm -rf /etc/apt/keyrings/nginx*
sudo rm -rf /usr/share/keyrings/nginx-archive-keyring.gpg

2. Add the Correct Nginx GPG Key

Run the following command to download and add the updated signing key:

curl -fsSL https://nginx.org/keys/nginx_signing.key | sudo gpg --dearmor -o /usr/share/keyrings/nginx-archive-keyring.gpg

Verify that the key was added:

gpg --show-keys /usr/share/keyrings/nginx-archive-keyring.gpg

3. Update the Repository Configuration

Ensure your /etc/apt/sources.list.d/nginx.list contains the correct repository entry:

echo "deb [signed-by=/usr/share/keyrings/nginx-archive-keyring.gpg] https://nginx.org/packages/mainline/debian bullseye nginx" | sudo tee /etc/apt/sources.list.d/nginx.list

4. Update Package Lists

Now, refresh the package lists:

sudo apt update

If the update runs without errors, you can proceed with installation:

sudo apt install nginx

To fix error “The repository is not updated and the previous index files will be used. GPG error: https://packages.sury.org/apache2 bullseye InRelease: The following signatures were invalid” use commands:

curl -sSlo /usr/share/keyrings/sury-keyring.gpg https://packages.sury.org/php/apt.gpg
curl -sSlo /usr/share/keyrings/apache2-keyring.gpg https://packages.sury.org/apache2/apt.gpg

Resizing Linux Virtual Machine Disks

This article will show how to grow a partition on your Linux partition to fill the entire disk using growpart. This is useful if you have resized a virtual machine disk, or moved to a larger disk on your desktop or laptop Linux system.

Step 1: Listing and Identifying Storage Devices

To expand the filesystem on your resized disk, you’ll need to first locate the disk using the lsblk command, execute it by running:

lsblk

And you will see output similar to the following listing storage devices and the partitions on them:

Above, you can see that disk sda has a size of 50 gigabytes, but that the volume containing the root partition (sda3) is only 29.5 gigabytes – there is unused space on the storage device.

There may be multiple entries in the output from lsblk – you’ll need to identify the disk you have resized by the size and utilization – it should be apparent which disk has unused space. Usually on single-disk machines, the first and only storage device will be named sda.

You will also need to know the name of the partition your wish to grow – in this case sda3 – usually identified by it having the root mount point of /.

Step 2: Installing growpart

growpart is a utility that makes it super easy to grow a partition. It’s part of the cloud-guest-utils package. Note that while this package is intended to work on cloud-hosted virtual machines, the growpart utility also works just fine on physical machines.

On Debian and Ubuntu, run:

sudo apt install cloud-guest-utils

On Arch, run:

pacman install cloud-guest-utils

On RedHat, run:

yum install cloud-utils-growpart -y

Step 3: Grow your Partition

Once growpart is available, growing a partition to use the entire remaining disk space is as simple as running:

sudo growpart /dev/sda 3

You’ll need to specify the correct partition name above, replaceing sda 3 (note the space! the device and partition number are separated when using growpart) if necessary. growpart is executed with no additional parameters – if the size parameter is not specified, it will default to the available size of the partition.

Now that the partition has been expanded, the file system must be also using resize2fs:

sudo resize2fs /dev/sda3

Note that the space has disappeared in the device path again.

You will see output similar to:

Confirming the change. Once this final step is done, reboot:

sudo reboot

If using systemd-resolved as your DNS resolver (i.e. the hosts line of your /etc/nsswitch.conf file includes the word resolve and/or /etc/resolv.conf contains the line nameserver 127.0.0.53), then this command will flush its cache:

sudo systemd-resolve --flush-caches

A newer version of this command seems to be:

sudo resolvectl flush-caches

You can use FFmpeg (a free command-line tool for Mac, Linux and Windows) to encode WMV to MP4. Here is an example syntax:

ffmpeg -i input.wmv -c:v libx264 -crf 23 -c:a aac -q:a 100 output.mp4

This will encode the video to H.264 video and AAC audio, using the default quality. To change the quality for the video, use a different CRF value, where lower means better, e.g. 20 or 18. For audio, 100% is the default quality. Increase the value for better quality.

For the AppleTV specifically, this is what Apple says it supports:

H.264 video up to 1080p, 30 frames per second, High or Main Profile level 4.0 or lower, Baseline profile level 3.0 or lower with AAC-LC audio up to 160 kbit/s per channel, 48 kHz, stereo audio in .m4v, .mp4, and .mov file formats

So, you could use the following command to force the 30 Hz frame rate and High profile:

ffmpeg -i input.wmv -c:v libx264 -crf 23 -profile:v high -r 30 -c:a aac -q:a 100 -ar 48000 output.mp4

To use L2TP VPN connections on Linux Mint you need to install this packages:

sudo apt-get install network-manager-l2tp network-manager-l2tp-gnome