File Manipulation

Efficient file manipulation is essential for working with files and directories within the Linux environment. This section covers fundamental commands for managing files in the Shell.

`touch`

Description: Creates an empty file or updates the timestamp (last modified time) of an existing file.
Example Usage:
```
touch new_file.txt       # Creates a new empty file named 'new_file.txt'
touch existing_file.txt  # Updates the timestamp of 'existing_file.txt' (without changing content)
```
Sample Output: (No output is printed to the console)

Note: If existing_file.txt already exists, its modification time will be updated to the current time. If it doesn’t exist, it will be created as an empty file.

`nano`

Description: A simple, terminal-based text editor. Good for quick edits.
Example Usage:
```
nano new_file.txt  # Opens 'new_file.txt' in Nano.  Creates it if it doesn't exist.
```
Note: Within Nano:
- Type to enter text.
- Ctrl + O (then Enter): Saves the file (“Write Out”).
- Ctrl + X: Exits Nano. If you haven’t saved, it will prompt you.
Installation (if needed - usually pre-installed):
- Debian/Ubuntu: sudo apt install nano
- CentOS/Fedora: sudo yum install nano

`cat`

Description: Displays the content of a file, or concatenates (joins) multiple files.

Example Usage:

cat file1.txt          # Displays the contents of 'file1.txt'
cat file1.txt file2.txt # Displays the contents of file1.txt, then file2.txt, one after the other.

Sample Output (if file1.txt contains “Hello” and file2.txt contains “World”):

Hello
World

`echo`

Description: Prints text or the value of variables to the terminal.

Example Usage:

echo "Hello, World!"   # Prints "Hello, World!"
echo $USER             # Prints the current username
echo $HOME             # Prints the path to your home directory
echo $PATH             # Prints the list of directories where the shell looks for commands

export MY_VAR="My Value"  # Sets a custom environment variable
echo $MY_VAR            # Prints the value of MY_VAR ("My Value")

Sample Output (for echo "Hello, World!"):

  Hello, World!

Note: echo is frequently used in shell scripts for displaying messages, debugging, and generating output.

`>` (Output Redirection - Overwrite)

Description: Redirects the output of a command to a file. Overwrites the file if it already exists.

Example Usage:

echo "This is new content." > my_file.txt  # Creates 'my_file.txt' (or overwrites it) with the text.
ls -l > file_list.txt                  # Saves the output of 'ls -l' to 'file_list.txt'

Sample Output: (No output on console. Check the contents of the file with cat)

`>>` (Output Redirection - Append)

Description Redirects the output of a command to a file, adding to the end.

Example Usage:

  echo "Adding more text." >> my_file.txt

`<` (Input Redirection)

Description: Redirects the input of a command to come from a file, instead of from the keyboard.

Example Usage:

cat < my_file.txt  # Equivalent to 'cat my_file.txt'.  The '<' is often optional for simple cases.

Note: Input redirection is most useful in more complex scenarios, like shell scripting, where you might want to feed a file’s content line-by-line to a loop:

while read line; do
    echo "Line: $line"
done < my_file.txt  # Reads each line of 'my_file.txt' into the 'line' variable.

`cp`

Description: Copies files or directories.

Example Usage:

cp file1.txt file2.txt       # Creates a copy of 'file1.txt' named 'file2.txt'
cp file1.txt my_folder/      # Copies 'file1.txt' into the 'my_folder' directory
cp -r dir1/ dir2/           # Recursively copies 'dir1' (and all its contents) to 'dir2'

Sample Output: (No output is printed to the console if the command is successful) Note: The -r option is required for copying directories.

`mv`

Description: Moves or renames files or directories.

Example Usage:

mv file1.txt new_name.txt  # Renames 'file1.txt' to 'new_name.txt'
mv file1.txt my_folder/   # Moves 'file1.txt' into the 'my_folder' directory
mv dir1/ dir2/            # Moves 'dir1' *into* 'dir2' (dir1 becomes a subdirectory of dir2)

Sample Output: (No output is printed to the console if the command is successful)

`rm`

Description: Removes (deletes) files or directories. Use with caution!
Example Usage:
```
rm file1.txt         # Deletes 'file1.txt'
rm -r my_folder/     # Recursively deletes 'my_folder' and *all its contents*!
rm -f file1.txt     #forcefully deletes file1.txt
```
Sample Output: (No output is printed to the console if the command is successful)

Important Notes:
- rm is permanent. There is no “trash” or “recycle bin” by default on the command line.
- rm -r is extremely powerful and dangerous. Double-check your command before pressing Enter!
- -f option means to forcefully remove the file. It will not ask for any confirmation.

`head`

Description: Displays the first few lines of a file (10 lines by default).

Example Usage:

head my_file.txt      # Shows the first 10 lines
head -n 5 my_file.txt  # Shows the first 5 lines

`tail`

Description: Displays the last few lines of a file (10 lines by default).

Example Usage:

```bash
tail my_file.txt      # Shows the last 10 lines
tail -n 5 my_file.txt  # Shows the last 5 lines
tail -f logfile.txt # Continuously display new lines added to the file, usefull for viewing logs.
```

Note: The -f option is very useful for monitoring log files in real-time.

`less`

Description: A powerful file viewer that allows you to scroll up and down, search, and more.
Example Usage:
```
less my_file.txt  # Opens 'my_file.txt' in the 'less' pager
```
Key Commands within less:
- Spacebar: Page down.
- b: Page up.
- Arrow keys: Scroll up/down/left/right.
- /pattern: Search for “pattern” (press Enter to search, ‘n’ for next match).
- q: Quit.
- h: help
Note: less is generally preferred over more because it’s more feature-rich (e.g., it allows scrolling up, while more only allows scrolling down).