Linux Shell Commands || shell commands in Linux

Introduction to Shell Commands: Unlocking the Power of the Command Line

Imagine your computer as a vast library, filled with information and tools waiting to be accessed. But instead of using a graphical interface like a librarian, you have the power to interact with this library using text-based commands. That’s where shell commands come in.

What is a Shell?

In the world of computing, a shell is a program that acts as an interface between the user and the operating system. It interprets commands entered by the user and executes them. Think of it as your gateway to controlling and manipulating your computer’s resources and functionalities.

What are Shell Commands?

Shell commands are text-based instructions that you give to the shell to perform various tasks. These tasks can range from simple file management to complex system administration tasks. Each command has its own specific syntax and functionality, allowing you to precisely control what you want your computer to do.

Why Use Shell Commands?

1. Efficiency: Once you get the hang of it, using shell commands can be much faster than navigating through graphical interfaces. With just a few keystrokes, you can accomplish tasks that might take several clicks with a mouse.
2. Powerful Automation: Shell commands can be combined and automated using scripts, allowing you to perform repetitive tasks or complex operations with ease.
3. Accessibility: The command line interface is available on almost every Unix-like system, including Linux and macOS. Knowing how to use shell commands gives you the ability to work with these systems efficiently, whether you’re a developer, system administrator, or power user.

Getting Started with Shell Commands

To start using shell commands, you’ll need to open a terminal or command prompt on your computer. From there, you can type commands and press Enter to execute them. It’s like having a conversation with your computer in its own language.

Common Shell Commands

Some common shell commands you’ll encounter include ls (list directory contents), cd (change directory), pwd (print working directory), mkdir (make directory), rm (remove files or directories), cp (copy files or directories), and grep (search text for a pattern). These are just a few examples of the many commands available to you.

Conclusion

Shell commands are a powerful tool for interacting with your computer and performing a wide range of tasks efficiently. By mastering the command line interface and learning various shell commands, you can unlock new levels of productivity and control over your computing environment. So, don’t be afraid to dive in and start exploring the world of shell commands!

Shell commands in Linux :

who command :

Syntax:

$ who [options]

Definition: The who command in Linux displays information about users who are currently logged into the system. It shows details like usernames, terminal line numbers, and login times.

Example:

$ who

This command will display a list of users currently logged in, along with their login details such as username, terminal line, and login time.

Command Name: whoami

Syntax:

$ whoami

Definition: The whoami command is like asking the computer, “Who am I?” It tells you the username of the currently logged-in user.

Example: If you type whoami in the terminal and press Enter, the computer will respond with your username. For example:

$ whoami john

This means that the currently logged-in user is “john”.

Command Name: mkdir

Syntax:

$ mkdir directory_name

Definition: The mkdir command in Linux is used to create a new directory (folder) with the specified name. It’s like telling your computer to make a new folder for you to organize your files.

Example: If you want to create a folder named “Documents” in your current directory, you would type:

$ mkdir Documents

After executing this command, a new folder named “Documents” will be created in your current directory.

Command Name: cd

Syntax:

$ cd directory_name

Definition: The cd command stands for “change directory.” It’s like telling your computer to go to a different folder. You use it when you want to move from your current location in the file system to another location.

Example: If you want to go to a folder named “Documents,” you would type:

$ cd Documents

And then press Enter. Now, you’re inside the “Documents” folder, just like walking into a different room in your house.

Command Name: ls

Syntax:

$ ls [options] [directory]

Definition: The ls command stands for “list” and is used to display the contents of a directory. It shows you a list of files and subdirectories in the specified directory, or if no directory is specified, in the current directory.

Example: Suppose you want to see what files are in your current directory.

You simply type ls and press Enter.

The shell then displays a list of all the files and folders in that directory. If you want to see the contents of a specific directory, you type ls directory_name instead.

Command Name: rmdir

Syntax:

$ rmdir directory_name

Definition: The rmdir command in Linux is used to remove empty directories. It’s like throwing away an empty folder from your desk.

Example: Let’s say you have an empty folder named “documents” on your desktop that you want to delete. You would type:

$ rmdir documents

If the folder is indeed empty, the command will delete it. But if there are any files or subdirectories inside, it won’t work, and you’d need to remove those first or use a different command.

Command: pwd

• Syntax: $ pwd
• Definition: pwd stands for “print working directory.” It tells you the name of the directory (folder) you’re currently in. It’s like asking your computer, “Where am I right now?”
• Example: Suppose you’re exploring your computer’s files using the terminal. You type $ pwd and press Enter. The terminal shows you something like /home/username/Documents, which means you’re currently in the “Documents” folder inside the “username” folder in your home directory.

Command Name: cp

Syntax:

$ cp source_file destination_file

Definition: The cp command in Linux is like making a copy of a file or directory. It duplicates files or directories from one place to another.

Example: Let’s say you have a file named “report.txt” in your Documents folder, and you want to make a copy of it in your Backup folder. You’d type:

$ cp Documents/report.txt Backup/report_copy.txt

This command would create a copy of “report.txt” in the Backup folder with the name “report_copy.txt”.

Command Name: clear

Syntax:

$clear

Definition: The clear command is used to clear the terminal screen, giving you a fresh workspace without any previous commands or outputs cluttering the screen.

Example:

$ clear

After typing this command and pressing Enter, the terminal screen will be cleared, providing you with a clean slate to continue your work or enter new commands.

Command Name: echo

Syntax:

$ echo [text]

Definition: The echo command is like speaking out loud in the terminal. It simply repeats whatever text you give it back to you. It’s handy for printing messages, variables, or anything else you want to see on the screen.

Example: Suppose you want to print the message “Hello, World!” on the screen. You’d type:

$ echo Hello, World!

And when you press Enter, the terminal will show:

Hello, World!

It’s that simple! You can use echo to display any text you want in the terminal.

Command Name: date

Syntax:

$ date

Definition: The date command in Linux is used to display the current date and time according to the system’s clock.

Example: If you want to know what the current date and time are, you just type date into the terminal and hit Enter. The system will then show you something like this:

Tue May 4 12:30:45 UTC 2024

This tells you the current day of the week, month, date, time, timezone (UTC), and year.

Command Name: cal

Syntax:

$ cal [month] [year]

Definition: The cal command in Linux displays a calendar for the specified month and year. If no arguments are provided, it shows the current month’s calendar.

Example:

1. To display the calendar for the current month:

$ cal

This will show the calendar for the current month, with today’s date highlighted.

2. To display the calendar for a specific month and year, for example, May 2024:

$ cal 5 2024

This will show the calendar for May 2024.

The cal command is handy for quickly checking dates and planning events right from the command line.

Command Name: grep

Syntax:

$ grep ‘string to be search’ file_name

Definition: The grep command is like a detective that helps you search for specific words or patterns in files. It scans through a file and prints lines that match the pattern you provide.

Example: Let’s say you have a file named notes.txt and you want to find all lines containing the word “important.” You would type:

$ grep 'important' notes.txt

This command would search notes.txt for any lines that contain the word “important” and print those lines to the terminal.

Command Name: chmod

Syntax:

$ chmod permissions file_name

Definition: chmod is a command in Linux used to change the permissions (read, write, execute) of a file or directory. Permissions control who can read, write, or execute a file. This command helps you control who can do what with your files.

Example: Suppose you have a file named “document.txt” that you want to make readable, writable, and executable by the file owner, but only readable by everyone else. You can use chmod like this:

$ chmod u=rwx,g=rx,o=r document.txt

This command gives the file owner (u) read, write, and execute permissions (rwx), the group (g) read and execute permissions (rx), and others (o) read permission (r). After running this command, the permissions of “document.txt” will be set accordingly.

Command Name: cat Command

Syntax:

$ cat [options] [file]

Definition: The cat command in Linux is like a friendly cat that loves to display the contents of files on your screen. It stands for “concatenate” but its main job is to display text files, or to combine copies of one or more files into a single output.

Example: Let’s say you have a file named “example.txt” with some text in it. To display the contents of this file, simply type:

$ cat example.txt

Press Enter, and the cat command will obediently show you the text inside “example.txt” on your screen. If you have multiple files, you can also concatenate them by listing their names after the cat command, like this:

$ cat file1.txt file2.txt

This will display the contents of “file1.txt” followed by the contents of “file2.txt”. So, just like a cat showing off its toys, the cat command proudly presents your text files for you to see.

Command Name: cat (create new file)

Syntax: $ cat > filename
Definition: Cat is a command used to create and display the content of files. It can also be used to concatenate and display the content of multiple files. When used with the > symbol, it allows you to create a new file and add content to it.
Example:

$ cat > newfile.txt This is a new file created using the cat command. Press Ctrl + D to save and exit.

This command will create a new file named newfile.txt and add the specified text to it. Pressing Ctrl + D will save the changes and exit the cat command.

Command Name: cat (combine file)

Syntax:

$ cat [file1] [file2] ...

Definition: The cat command in Linux is short for “concatenate.” It is used to display the contents of one or more text files on the terminal.

Example: Suppose you have two text files named “file1.txt” and “file2.txt” containing some text. To display the contents of both files on the terminal, you would use the following command:

$ cat file1.txt file2.txt

This will print the contents of “file1.txt” followed by the contents of “file2.txt” on the terminal.

$ cat [file1] [file2] ... > [combined_file]

Definition: To combine text files in Linux, you can use the cat command along with the output redirection symbol (>). This command takes the contents of multiple text files and writes them into a single file.

Example: Let’s say you have two text files named “report1.txt” and “report2.txt” that you want to combine into a single file named “combined_report.txt”. You would use the following command:

Command Name: cat (append file)

Syntax: $ cat >> filename
Definition: The cat command in Linux is used to concatenate files and display the content of files on the terminal. When used with the >> operator followed by a filename, it is used to append text to an existing file.
Example: Suppose you have a file named notes.txt and you want to add some additional text to it. You would use the following command:

$ cat >> notes.txt

After entering this command, you can start typing the text you want to append to the file. Once you’re done, you can press Ctrl + D to save the changes and exit. This will append the text to the end of the notes.txt file.

$ cat report1.txt report2.txt > combined_report.txt

This command will combine the contents of “report1.txt” and “report2.txt” and save them into a new file called “combined_report.txt”.

Command: find

Syntax:

$ find [directory] [options]

Definition: The find command in Linux is used to search for files and directories within a specified directory hierarchy based on various criteria such as file name, size, permissions, and more.

Example: Suppose you want to find all text files in the “Documents” directory. You would use the following command:

$ find Documents -name "*.txt"

This command searches the “Documents” directory and its subdirectories for files with the .txt extension and lists them.

Command Name: locate

Syntax:

$ locate [options] filename

Definition: The locate command helps you find the path of files and directories on your system. It searches through a pre-built database of all the files and directories on your computer, making it very fast.

Example: Suppose you’re looking for a file named “notes.txt” on your computer. You would type:

$ locate notes.txt

The command will then search the database and show you the path to the file if it exists. For instance, it might return:

/home/user/Documents/notes.txt

This means that the file “notes.txt” is located in the “Documents” folder under your home directory.

Command Name: sort

Syntax:

$ sort [options] [file]

Definition: The sort command in Linux arranges the lines of text in a file alphabetically or numerically. It takes input from a file, sorts it according to specified criteria, and outputs the sorted lines.

Example: Suppose you have a file named “grades.txt” containing the following grades:

Bobby: 80 Alice: 95 Charlie: 75

To sort these grades alphabetically by student name, you’d use the sort command like this:

$ sort grades.txt

The output would be:

Alice: 95 Bobby: 80 Charlie: 75

By default, sort arranges lines alphabetically based on the first character. To sort numerically, you’d use the -n option. For example:

$ sort -n grades.txt

This would sort the grades based on the numerical values:

Charlie: 75 Bobby: 80 Alice: 95

That’s the essence of the sort command—simple yet powerful for arranging text data in a desired order.

Command Name: cmp

Syntax: $ cmp [OPTION]... FILE1 FILE2

Definition: The cmp command is used to compare two files byte by byte. It checks if the contents of the two files are identical or if there are any differences between them.

Example: Suppose we have two text files named file1.txt and file2.txt. To compare these files, you would type:

$ cmp file1.txt file2.txt

If the files are identical, cmp will produce no output. If there are differences, it will display the byte and line numbers where the first difference occurs.

Command Name: zip

Syntax:

$ zip [options] output_file input_files

Definition: The zip command in Linux is used to compress files and directories into a single zip archive. It bundles multiple files and folders together into a single file, making it easier to store, send, or manage them.

Example: Let’s say you have two files named file1.txt and file2.txt, and you want to zip them into a single archive named archive.zip. You would use the following command:

$ zip archive.zip file1.txt file2.txt

This command creates a zip file named archive.zip containing file1.txt and file2.txt.

Command Name: tar

Syntax:

$ tar options archive_name files_or_directories

Definition: The tar command in Linux is used to create, manipulate, and extract files from tar archives. A tar archive is like a box that holds multiple files and directories together. It’s commonly used for compressing and distributing collections of files.

Example: Suppose you have a folder named “documents” containing several text files and you want to create an archive named “my_documents.tar” containing all those files. You would use the following command:

$ tar -cvf my_documents.tar documents

Explanation of options:

• -c: Create a new archive.
• -v: Verbose mode (show the progress of the operation).
• -f: Specifies the name of the archive file.

After executing the command, a new tar archive named “my_documents.tar” will be created, containing all the files from the “documents” folder.

Command Name: ps

Syntax:

$ ps [options]

Definition: The ps command in Linux is like a detective that helps you investigate what processes are currently running on your system. It stands for “process status.” Processes are running instances of programs, and ps gives you information about them, such as their process IDs (PIDs), CPU and memory usage, and more.

Example: To see a list of processes currently running on your system, you can simply type:

$ ps

This will display a list of processes along with their details, such as PID, terminal, CPU usage, and command. If you want more detailed information, you can use options like -e to show all processes or -f to display a full-format listing. For example:

$ ps -ef

This will provide a comprehensive view of all processes, including their user, PID, CPU and memory usage, and the command they’re running.

Command Name: top

Syntax:

$ top

Definition: The top command in Linux allows you to view dynamic, real-time information about the processes running on your system. It displays a continuously updated list of processes, sorted by various criteria such as CPU usage, memory usage, and more.

Example:

$ top

When you enter this command in your terminal and press Enter, you’ll see a screen showing details about all the processes currently running on your system. This includes information like the process ID (PID), CPU and memory usage, user, and more. Press ‘q’ to exit the top command and return to the terminal prompt.

Command Name: kill

Syntax:

$ kill [options] <PID>

Definition: The kill command in Linux is used to terminate processes running on the system. It sends a signal to the specified process, instructing it to stop running. Each process on a Linux system has a unique identifier called a Process ID (PID), which is used with the kill command to identify which process to terminate.

Example:

$ kill 1234

This command will send a termination signal to the process with PID 1234, instructing it to stop running.

Command Name: netstat

Syntax: $ netstat [options]

Definition: The netstat command in Linux displays network connections, routing tables, interface statistics, masquerade connections, and multicast memberships. It provides information about the network status of the system.

Example:

$ netstat -a

This command displays all active connections and listening ports.

$ netstat -tuln

This command shows all listening TCP and UDP connections along with their port numbers and addresses.

$ netstat -r

This command displays the kernel routing table, which shows how packets will be forwarded.

$ netstat -i

This command provides a list of all network interfaces on the system along with their statistics.

Using netstat with different options helps you diagnose network issues, monitor network activity, and understand how your system is communicating over the network.

Command Name: df

Syntax:

$ df [options]

Definition: The df command, short for “disk free”, is used to display information about the disk space usage on your Linux system. It shows the amount of disk space used and available on all mounted filesystems.

Example:

$ df -h

This command displays disk space usage in a human-readable format. The output will show information like filesystem, size, used space, available space, and usage percentage, making it easier to understand the disk usage at a glance.

Command Name: logout

Syntax: $ logout
Definition: The logout command is used to log out of the current shell session. It terminates the current session and returns you to the login screen or closes the terminal window, depending on how you’re accessing the shell.
Example:

$ logout

When you enter this command and press Enter, your current shell session will end, and you’ll be logged out of your user account.