Basics LINUX Commands

What is the Linux command:-

Linux commands are used to perform tasks in a Linux operating system. They are executed from the Linux terminal (or) command-line interface.

Some Category

Directory Commands

• pwd - Print working directory

• mkdir - Make a directory

• rmdir - Remove directory

• ls - List directory contents

File Commands

• touch - Create an empty file

• cat - Concatenate and display file content

• rm - Remove the file

• cp - Copy file

• mv - Move or rename the file

File Content Commands

• head - Display the first lines of a file

• tail - Display the last lines of a file

• more - View file page by page

• less - View file page by page with navigation

User Commands

• su - Switch to another user

• id - Print current user and group IDs

• useradd - Add new user

• passwd - Change user password

Filter Commands

• grep - Search file content

• cut - Cut out selected fields from the file

• sort - Sort lines of text files

• wc - Count lines, words and characters in files

Utility Commands

• find - Find files matching certain criteria

• locate - Find files by name

• date - Print or set system date and time

• df - Report file system disk space usage

Networking Commands

• ip - Assign an IP address to interface

• ping - Test connectivity to network host

• ssh - Log into remote machines

• traceroute - Trace route to network host

To view what's written in a file:-

• cat simply prints the contents of the files (your screen)

Example of command

Basic syntax:

  cat [filename]

To view the contents of a single file:

  cat test.txt

To view multiple files:

  cat test1.txt test2.txt

You can also redirect the output of cat to a file instead of displaying it:

  cat test.txt > output.txt

This will write the contents of test.txt to output.txt.

The cat command has some more options:

• -n: Display line numbers

• -s: Suppress repeated empty lines

• -e: Display end-of-line characters

• -T: Display tab characters as ^I

To change the access permissions of files:-

Viewing File Permissions

You can view permissions of a file or directory using the ls -l command:

  ls -l filename

-rwxr-xr--

The permissions are displayed in 10 characters:

• The first character indicates the file type (- for file, d for directory)

• The next three characters are the permissions for the owner/user

• The next three are for the group

• The last three are for others

Changing File Permissions

You can change file permissions using the chmod command.

Symbolic mode - Uses symbols like u (user), g (group), o (others), a (all) and +/- (add/remove) permission.

example, to add write permission for the group:

  chmod g+w filename

Absolute (octal) mode - Uses a three-digit octal number where each digit represents owner, group and other permissions respectively.

example, to make a file readable and writable by owner, readable by group and others:

  chmod 644 filename

Here 6 is for owner (read + write), 4 is for group (read only) and 4 is for others (read only).

You can also change permissions recursively for directories using the -R flag:

  chmod -R 755 dirname

This will apply the permissions to all files and subdirectories within dirname.

To check which commands you have run till now:-

The history Command

The history command lists the commands you have entered in the terminal/CLI.

  history

You can specify a number to limit the output:

  history 100  # Shows the last 100 commands

The history command stores your command history in the .bash_history file in your home directory.

You can view and edit this file directly to view all commands across terminal sessions:

  cat ~/.bash_history

  vim ~/.bash_history

Repeating Commands

The history command also allows you to easily repeat previous commands.

You can use !! to repeat the last command:

  !!

Or use a command number to repeat a specific command:

  !342   # Repeats command number 342

You can also search for a command and repeat the last one that matches:

  !ls    # Repeats the last command that starts with "ls"

  !?sudo # Repeats the last command that contains "sudo"

To remove a Directory/ Folder:-

Deleting Folders in Linux Command Line

There are two main commands used to delete folders in Linux

• rmdir

• rm

The rmdir command is used to delete empty folders, while rm can delete both empty and non-empty folders.

Using rmdir

The rmdir command simply removes empty folders. It has the syntax:

rmdir folder_name

For example, to delete an empty folder named test:

rmdir test

If the folder is not empty, rmdir will give an error:

rmdir: failed to remove 'test': Directory not empty

In this case, you'll need to use the rm command.

Using rm

The rm command can delete both empty and non-empty folders.

-d: Delete empty folders

-r or -R: Recursively delete folders and their contents

To delete an empty folder:

rm -d folder_name

To delete a non-empty folder:

rm -r folder_name

For example:

rm -r test

This will delete the test folder and all its files and subfolders.

You can also use rm with some other options:

• -f: Force removal without prompting

• -i: Prompt before deleting each file

• -v: Verbose mode

So a full command to delete a non-empty folder would be:

rm -r -f folder_name

The -f option is useful to avoid having to confirm deletion for each file.

To create a fruits.txt file and to view the content:-

Using the touch command

The simplest way is to use the touch command:

  touch fruits.txt

This will create an empty text file called fruits.txt.

Using the echo command

The echo command can be used to create a file with some content:

  echo "Apple" > fruits.txt
  echo "Orange" >> fruits.txt

The first echo will overwrite any existing file, while the second will append to the end.

Add content in devops.txt (One in each line) - Apple, Mango, Banana, Cherry, Kiwi, Orange, Guava:-

Adding Content to a Text File in Linux

You can add content to a text file and name it devops.txt using the vim editor in Linux. Steps:

1. Open the file in vim

vim devops.txt

1. Since the file is empty, you will see an empty buffer.

2. Start typing the content, one item per line

3. To write any thing I to get into insert mode.

apple
mango  
banana
cherry
kiwi
orange
guava

1. Press Esc to exit insert mode.

2. Save and close the file using:

:wq

1. You can verify the content was added using the cat command:

cat devops.txt

Output:

apple
mango   
banana
cherry
kiwi
orange  
guava

Show only the top three fruits from the file:-

Using Head

The head command displays the first few lines of a file. To display the top 3 lines

head -n 3 filename.txt

The -n option specifies the number of lines to display.

example:

$ head -n 3 lines.txt 
line 1
line 2
line 3

Using Tail and Head Together

We can also use tail and head together to display the top 3 lines

head -n 3 filename.txt | tail

This works because

• head -n 3 displays the first 3 lines

• Piping that output to tail will then display the last 3 lines, which in this case are also the top 3 lines.

example:

$ head -n 3 lines.txt | tail
line 1
line 2  
line 3

Using Sed

With sed you can display a specific line range. To display the top 3 lines

sed -n '1,3p' filename.txt

• The -n suppresses automatic printing of pattern space

• 1,3p prints lines 1 through 3

example:

$ sed -n '1,3p' lines.txt
line 1
line 2
line 3

To Show only bottom three fruits from the file:-

Using Tail

The tail command is used to display the last few lines of a file. To display the last 3 lines

tail -n 3 filename.txt

The -n option specifies the number of lines to display from the end of the file.

example:

$ tail -n 3 lines.txt
second last line  
third last line
last line

Using Head and Tail Together

You can pipe the output of head into tail to display the last 3 lines:

head -n 1000 filename.txt | tail -n 3

This works because:

• head -n 1000 displays the first 1000 lines

• Piping that output to tail -n 3 will then display the last 3 lines from that output.

  To find the difference between fruits.txt and Colors.txt file:-

  We can use the diff command to compare two text files and see the differences.

  example:

          diff file1.txt file2.txt
  

  It will output all lines that are different between the two files.

---

Happy Learning!

Thanks for Reading :)

Afroz Shaik ✨