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Linux Commands and Concepts

1. pwd - Print Working Directory

Description: Shows the full path of the current directory.
Examples:

  • pwd
    Example Output: /home/user/documents
  • pwd -P
    Displays the physical directory, avoiding symlinks.
    Example Output: /home/user/documents
  • pwd -L
    Displays the logical path with symlinks (default).
    Example Output: /home/user/../documents

2. ls - List Files

Description: Lists files in a directory.
Examples:

  • ls
    Example Output: file1.txt file2.txt folder
  • ls -lh
    Lists files with detailed information (size, permissions, etc.).
    Example Output: -rw-r--r-- 1 user user 1.2K Sep 1 12:00 file1.txt
  • ls -a
    Shows hidden files.
    Example Output: file1.txt .hiddenfile folder

3. cd - Change Directory

Description: Changes the current directory.
Examples:

  • cd /path/to/directory
    Example: Changes to /path/to/directory
  • cd ..
    Moves up one directory.
    Example: Moves to /path/to from /path/to/directory
  • cd ~
    Changes to the user's home directory.
    Example: Changes to /home/user
  • cd -
    Switches back to the previous directory.
    Example: Returns to /home/user from /var/log

4. mkdir - Create Directory

Description: Creates a new directory.
Examples:

  • mkdir myfolder
    Example: Creates /home/user/myfolder
  • mkdir -p /path/to/folder
    Creates nested directories.
    Example: Creates /path/to/folder, including intermediate directories.

5. cp - Copy Files

Description: Copies files or directories.
Examples:

  • cp file1.txt /backup/
    Example: Copies /home/user/file1.txt to /home/user/backup/
  • cp -p file1.txt /backup/
    Preserves file permissions.
    Example: Copies /home/user/file1.txt while preserving permissions
  • cp -r folder/ /backup/
    Recursively copies a directory and its contents.
    Example: Copies /home/user/folder/ to /home/user/backup/folder/

6. mv - Move or Rename Files

Description: Moves or renames files or directories.
Examples:

  • mv file1.txt file2.txt
    Example: Renames /home/user/file1.txt to /home/user/file2.txt
  • mv file1.txt /backup/
    Example: Moves /home/user/file1.txt to /home/user/backup/
  • mv -v file1.txt /backup/
    Verbosely shows the move operation.
    Example Output: ‘file1.txt’ -> ‘/backup/file1.txt’
  • mv -n file1.txt /backup/
    Prevents overwriting existing files.
    Example: If /backup/file1.txt exists, it will not be overwritten.
  • mv -vn file1.txt /backup/
    Verbosely and without overwriting.
    Example Output: ‘file1.txt’ -> ‘/backup/file1.txt’ (skipped: file exists)

7. rm - Remove Files

Description: Deletes files or directories.
Examples:

  • rm file1.txt
    Example: Deletes /home/user/file1.txt
  • rm -r folder/
    Recursively deletes a directory and its contents.
    Example: Deletes /home/user/folder/ and all its contents
  • rm -f file1.txt
    Force removes a file without confirmation.
    Example: Deletes /home/user/file1.txt without prompting
  • rm -rf folder/
    Forcefully and recursively deletes a folder.
    Example: Deletes /home/user/folder/ and all its contents without confirmation

8. Background and Foreground Jobs

Description: Managing process execution.
Examples:

  • command &
    Runs command in the background.
    Example: sleep 60 & runs sleep in the background.
  • jobs
    Lists all background jobs.
    Example Output: [1] 1234
  • fg %1
    Brings job number 1 to the foreground.
    Example: Moves sleep from the background to the foreground.
  • bg %1
    Resumes job number 1 in the background.
    Example: Continues sleep in the background.

9. Using screen for Management

Description: Terminal multiplexing tool.
Examples:

  • screen
    Starts a new screen session.
    Example: Creates a new session to manage multiple terminal windows.
  • screen -ls
    Lists all active screen sessions.
    Example Output: There is a screen on: 1234.pts-0.hostname (Detached)
  • screen -r 1234
    Reattaches to the screen session with ID 1234.
    Example: Reconnects to a detached session.
  • Ctrl+A followed by d
    Detaches from the current screen session.
    Example: Detaches from a session, leaving it running in the background.

10. Linux Size Conversion

Description: Convert between different file sizes.
Examples:

  • ls -lh
    Displays file sizes in human-readable format (e.g., KB, MB).
    Example Output: 1.2K
  • du -sh /path/to/dir
    Shows the size of a directory in a human-readable format.
    Example Output: 1.5G
  • stat -c %s file.txt
    Displays the file size in bytes.
    Example Output: 123456

11. How File Permissions Work in Linux

Description: Understanding file permissions.
Examples:

  • ls -l
    Displays file permissions in a detailed list.
    Example Output: -rwxr-xr-- 1 user group 12345 Sep 1 12:00 file.txt
  • chmod 755 file.txt
    Sets file permissions to rwxr-xr-x.
    Example: Owner can read/write/execute, others can read/execute.
  • chown user:group file.txt
    Changes the owner and group of a file.
    Example: Changes ownership to user and group.

12. File Permissions and Ownership

Description: Detailed permissions and ownership management.
Examples:

  • chmod
    Modify file permissions.
    Example: chmod u+x file.txt adds execute permission for the user.
  • chown
    Change file ownership.
    Example: chown user file.txt changes the owner to user.
  • chgrp
    Change the group ownership of a file.
    Example: chgrp group file.txt changes the group to group.

13. Useful Linux Commands for Data Analysis

Description: Commands for processing and analyzing data.
Examples:

  • awk '{print $1}' file.txt
    Extracts and prints the first column from a file.
    Example Output: Displays the first column from file.txt.
  • sort file.txt
    Sorts the contents of a file.
    Example Output: Sorted lines of file.txt.
  • uniq file.txt
    Removes duplicate lines from a file.
    Example Output: Unique lines from file.txt.
  • cut -d' ' -f1 file.txt
    Extracts the first field from each line using space as the delimiter.
    Example Output: First field of file.txt.

14. Combining Commands with Pipes and Redirection

Description: Using pipes and redirection to combine commands.
Examples:

  • command1 | command2
    Sends the output of command1 as input to command2.
    Example: ls | grep 'file' lists files and filters those containing 'file'.
  • command > file.txt
    Redirects the output of command to file.txt, overwriting it.
    Example: echo "Hello" > file.txt writes "Hello" to file.txt.
  • command >> file.txt
    Appends the output of command to file.txt.
    Example: echo "World" >> file.txt appends "World" to file.txt.

15. Automating Tasks with while and for Loops

Description: Automating repetitive tasks with loops.
Examples:

  • while loop 
    while [ condition ]; do
    command
done

while [ $count -lt 5 ]; do
    echo $count
    ((count++))
done
  • for loop 
    for item in list; do
  command
done

for file in *.txt; do
    cat "$file"
done

16. Understanding Data Size Conversion

Title: Data Size Conversion: Bytes, Kilobytes, Megabytes, Gigabytes

Content:

  • Basic Units of Data:

    • Bit (b): The smallest unit, 0 or 1.
    • Byte (B): 1 Byte = 8 bits.

  • Common Data Size Conversions:

    • 1 Kilobyte (KB): 1 KB = 1,024 Bytes.
    • 1 Megabyte (MB): 1 MB = 1,024 KB.
    • 1 Gigabyte (GB): 1 GB = 1,024 MB.
    • 1 Terabyte (TB): 1 TB = 1,024 GB.

  • Example:

    • A 10 MB file is equal to 10 * 1,048,576 = 10,485,760 Bytes.