Linux is an operating system's kernel. You might have heard of UNIX. Well, Linux is a UNIX clone. But it was actually created by Linus Torvalds from Scratch. Linux is free and open-source, that means that you can simply change anything in Linux and redistribute it in your own name! There are several Linux Distributions.

• Ubuntu Linux
• Red Hat Enterprise Linux
• Linux Mint
• Debian
• Fedora

Linux is Mainly used in servers. About 90% of the internet is powered by Linux servers. This is because Linux is fast, secure, and free! The main problem of using Windows servers are their cost. This is solved by using Linux servers. The OS that runs in about 80% of the smartphones in the world, Android, is also made from the Linux kernel. Most of the viruses in the world run on Windows, but not on Linux!

A Linux command is a program or utility that runs on the CLI – a console that interacts with the system via texts and processes. It’s similar to the Command Prompt application in Windows.

Linux commands are executed on Terminal by pressing Enter at the end of the line. You can run commands to perform various tasks, from package installation to user management and file manipulation.

Here’s what a Linux command’s general syntax looks like:

CommandName [option(s)] [parameter(s)]

A command may contain an option or a parameter. In some cases, it can still run without them. These are the three most common parts of a command:

• CommandName is the rule that you want to perform.
• Option or flag modifies a command’s operation. To invoke it, use hyphens (–) or double hyphens (—).
• Parameter or argument specifies any necessary information for the command.

Keep in mind that all Linux commands are case-sensitive.

In the current folder, if you want to list the all the items you can use ls command

ls

To see other directories content, type ls followed by the desired path. For example, to view files in the Documents folder, enter:

ls /home/username/Documents

Here are some options you can use with the ls command:

• ls -R lists all the files in the subdirectories.
• ls -a shows hidden files in addition to the visible ones.
• ls -lh shows the file sizes in easily readable formats, such as MB, GB, and TB.

mkdir command is used to create a directory

mkdir dirname

-p or –parents create a directory between two existing folders. For example, mkdir -p Music/2020/Songs will make the new “2020” directory.

To navigate through the directories, use the cd command.

cd dirname

Here are some shortcuts to help you navigate:

• cd ~[username] goes to another user’s home directory.
• cd .. moves one directory up (move back).

Use the pwd command to find the path of your current working directory. Simply entering pwd will return the full current path.

pwd

Concatenate, or cat, is one of the most frequently used Linux commands. It lists, combines, and writes file content to the standard output. To run the cat command, type cat followed by the file name and its extension.

cat filename.txt

Here are other ways to use the cat command:

• cat > filename.txt creates a new file.
• cat filename1.txt filename2.txt > filename3.txt merges filename1.txt and filename2.txt and stores the output in filename3.txt.
• tac filename.txt displays content in reverse order.

The man command provides a user manual of any commands or utilities you can run in Terminal, including the name, description, and options.

It consists of nine sections:

• Executable programs or shell commands
• System calls
• Library calls
• Games
• Special files
• File formats and conventions
• System administration commands
• Kernel routines
• Miscellaneous
• To display the complete manual, enter:

man [command_name]

For example, you want to access the manual for the ls command:

man ls

tr is used to translate like lower to upper case vice versa.

cat old.txt | tr a-z A-Z > new.txt

The touch command allows you to create an empty file

touch Web.html

Use the cp command to copy files or directories and their content.

cp file.txt copy.txt

To copy an entire directory, pass the -R flag before typing the source directory, followed by the destination directory:

cp -R /home/username/Documents /home/username/Documents_backup

The primary use of the mv command is to move and rename files and directories. Additionally, it doesn’t produce an output upon execution.

Simply type mv followed by the filename and the destination directory. For example, you want to move filename.txt to the /home/username/Documents directory:

mv filename.txt /home/username/Documents.

You can also use the mv command to rename a file:

mv old_filename.txt new_filename.txt

The rm command is used to delete files within a directory. Make sure that the user performing this command has write permissions.

Remember the directory’s location as this will remove the file(s) and you can’t undo it.

Here’s the general syntax:

rm filename

To remove multiple files, enter the following command:

rm filename1 filename2 filename3

Here are some acceptable options you can add:

• -i prompts system confirmation before deleting a file.
• -f allows the system to remove without a confirmation.
• -r deletes files and directories recursively.

Short for superuser do, sudo is one of the most popular basic Linux commands that lets you perform tasks that require administrative or root permissions.

When using sudo, the system will prompt users to authenticate themselves with a password. Then, the Linux system will log a timestamp as a tracker. By default, every root user can run sudo commands for 15 minutes/session.

If you try to run sudo in the command line without authenticating yourself, the system will log the activity as a security event.

sudo

You can also add an option, such as:

• -k or –reset-timestamp invalidates the timestamp file.
• -g or –group=group runs commands as a specified group name or ID.
• -h or –host=host runs commands on the host.

df command is used to check systems space usage, the df command to report the system’s disk space usage

df file

These are some acceptable options to use:

• df -m displays information on the file system usage in MBs.
• df -k displays file system usage in KBs.
• df -T shows the file system type in a new column.

If you want to check how much space a file or a directory takes up, use the du command. You can run this command to identify which part of the system uses the storage excessively.

Remember, you must specify the directory path when using the du command. For example, to check /home/user/Documents enter:

du /home/user/Documents

Adding a flag to the du command will modify the operation, such as:

• -s offers the total size of a specified folder.
• -m provides folder and file information in MB
• k displays information in KB.
• -h informs the last modification date of the displayed folders and files.

The head command allows you to view the first ten lines of a text. Adding an option lets you change the number of lines shown. The head command is also used to output piped data to the CLI.

Here’s the general syntax:

head [option] [file]

For instance, you want to view the first ten lines of note.txt, located in the current directory:

head note.txt

Below are some options you can add:

• -n or –lines prints the first customized number of lines. For example, enter head -n 5 filename.txt to show the first five lines of filename.txt.
• -c or –bytes prints the first customized number of bytes of each file.
• -q or –quiet will not print headers specifying the file name.

The tail command displays the last ten lines of a file. It allows users to check whether a file has new data or to read error messages.

Here’s the general format:

tail [option] [file]

For example, you want to show the last ten lines of the colors.txt file:

tail -n colors.txt

Short for difference, the diff command compares two contents of a file line by line. After analyzing them, it will display the parts that do not match.

Programmers often use the diff command to alter a program instead of rewriting the entire source code.

Here’s the general format:

diff [option] file1 file2

For example, you want to compare two text files – note.txt and note_update.txt:

diff note.txt note_update.txt

Here are some acceptable options to add:

• -c displays the difference between two files in a context form.
• -u displays the output without redundant information.
• -i makes the diff command case insensitive.