Managing Files

1. Linux: Everything Treated as a File

• In Linux, everything, including devices, directories, and processes, is treated as a file.

• This abstraction allows a consistent interface for interacting with diverse entities in the system.

  

2. File Creation Using touch and Timestamp Modification

• The touch command is used to create empty files or update the access and modification timestamps of existing files.

• Syntax: touch filename

  

• Multiple Files Creation: touch file1 file2 file3

  

  

• Change Timestamp: touch -c -t YYYYMMDDHHMM.SS filename

  • touch: The command itself, is used for file timestamp manipulation.

  • -c (or --no-create): This option prevents the creation of new files. If the specified file does not exist, touch will not create it.

  • -t YYYYMMDDHHMM.SS: This option allows you to specify a specific timestamp for the file in the format [[CC]YY]MMDDhhmm[.SS]. The components of the timestamp are as follows:

    • CC: First two digits of the year (the century).

    • YY: Last two digits of the year.

    • MM: Month (01-12).

    • DD: Day of the month (01-31).

    • hh: Hour (00-23).

    • mm: Minute (00-59).

    • .SS: Optional seconds (00-59).

  • filename: The name of the file you want to create or modify.

3. File Types

• File permissions are represented by a series of characters like drwxr-xr-x. In This we can identify various types of files.

  1. Regular File (-):

     • Represented by a hyphen (-).

     • It is a standard file containing data (text, binary, etc.).

     • Example: -rw-r--r--.

  2. Directory (d):

     • Represented by the letter d.

     • It is a container for other files and directories.

     • Example: drwxr-xr-x.

  3. Symbolic Link (l):

     • Represented by the letter l.

     • It is a reference to another file or directory.

     • Example: lrwxrwxrwx.

  4. Block Device (b):

     • Represented by the letter b.

     • It is a special file representing a block-oriented device (e.g., hard drive partitions).

     • Example: brw-rw----.

  5. Character Device (c):

     • Represented by the letter c.

     • It is a special file representing a character-oriented device (e.g., terminals).

     • Example: crw-rw----.

  6. Named Pipe or FIFO (p):

     • Represented by the letter p.

     • It is a special file used for inter-process communication.

     • Example: prw-r--r--.

  7. Socket (s):

     • Represented by the letter s.

     • It is a special file used for inter-process communication (IPC) between processes on the same or different hosts.

     • Example: srwxr-xr-x.

  8. Door (D):

     • Represented by the letter D.

     • It is a special file used for communication between processes on the same host.

     • Less common and not widely used.

  9. Whiteout (w):

     • Represented by the letter w.

     • It is a special file used in certain file systems to represent a deleted file.

     • Rarely seen in modern systems.

Example:

Here is an example showing different file types:

    -rw-r--r--  1 user user      0 Dec  1 10:00 regular_file
    drwxr-xr-x  2 user user   4096 Dec  1 10:00 directory
    lrwxrwxrwx  1 user user      5 Dec  1 10:00 symbolic_link -> otherfile
    brw-rw----  1 user disk    8,   1 Dec  1 10:00 block_device
    crw-rw----  1 user tty     5,   0 Dec  1 10:00 character_device
    prw-r--r--  1 user user      0 Dec  1 10:00 named_pipe
    srwxr-xr-x  1 user user      0 Dec  1 10:00 socket

4. Links

• Hard Links: Point to the same inode as the original file. Changes in one reflect in others.

  1. Creating Hard Links:

      touch originalfile
      ln originalfile hardlink1
      ln originalfile hardlink2
     

     Now, you have three files: originalfile, hardlink1, and hardlink2. They are all hard links, sharing the same content.

  2. Modifying Content:

      echo "Hello from hardlink1" > hardlink1
      cat originalfile  # Displays "Hello from hardlink1"
     

     Changes made to one hard link (hardlink1) are reflected in all other hard links (originalfile and hardlink2).

  3. Deleting a Hard Link:

      rm hardlink2
     

     Deleting one hard link (hardlink2) doesn't affect the others (originalfile and hardlink1).

• Soft Links (Symbolic Links): Point to the filename, not the inode. They are more flexible but can break if the original file is moved.

  1. Creating Symbolic Links:

      touch originalfile
      ln -s originalfile softlink
     

     Now, you have two files: originalfile and softlink. softlink is a symbolic link pointing to originalfile.

  2. Modifying Content:

      echo "Hello from originalfile" > originalfile
      cat softlink  # Displays "Hello from originalfile"
     

     Changes made to the original file (originalfile) are not automatically reflected in the symbolic link (softlink).

  3. Deleting the Original File:

      rm originalfile
     

     Deleting the original file doesn't affect the symbolic link, but now softlink is a "dangling link" since the target file is gone.

  4. Creating a Soft Link to a Directory:

      mkdir mydir
      ln -s mydir dirlink
     

     Now, you have a symbolic link (dirlink) pointing to a directory (mydir).

5. File Command

• The file command is used to determine the file type.

• Examples:

  • file image.jpeg: Displays image file format.

    

  • file /dev/sda1: Identifies a block device.

    

  • file /proc/cpuinfo: Reveals it as an empty file, dynamically updated by the kernel.

    

  • file filename.rpm: Shows information about a RPM file.

    

  • file logrotate.conf: Examines the type of configuration file.

    

File Contents

1. cat Command:

• Usage:

    cat filename
  

• Description:

  • Concatenates and displays the content of one or more files.

• Example:

    cat /etc/passwd
  

  

2. head Command:

• Usage:

    head [options] filename
  

• Description:

  • Displays the first part of a file.

• Options:

  • -n N: Display the first N lines.

• Example:

    head -n 10 filename
  

  

3. tail Command:

• Usage:

    tail [options] filename
  

• Description:

  • Displays the last part of a file.

• Options:

  • -n N: Display the last N lines.

• Example:

    tail -n 20 filename
  

4. > and >>:

• cat > newfile.txt Usage:

  • Redirects standard input to a file, creating or overwriting the file.

  • Enter text, and use Ctrl+D to stop.

• >> Usage:

  • Appends text to the end of a file.

• Custom Marker (<<) Usage:

  • Allows input until a specified marker is encountered.

• Examples:

    cat > newfile.txt
  

  

    cat >> existingfile.txt
  

  

    cat > newfile.txt << EOL
    This is a custom marker example.
    EOL
  

  

5. echo Command:

• Usage:

    echo [options] [string(s)]
  

• Description:

  • Displays a line of text or variable values.

• Example:

    echo "Hello, World!"
  

6. tac Command:

• Usage:

    tac filename
  

• Description:

  • Concatenates and displays the content of a file in reverse order (last line first).

• Example:

    tac filename
  

  

7. more Command:

• Usage:

    more filename
  

• Description:

  • Displays text one screen at a time, allowing navigation.

• Features:

  • Use the spacebar to scroll down.

  • Type / to search for a specific word.

  • Press q to exit.

    

8. less Command:

• Usage:

    less filename
  

• Description:

  • Similar to more but provides more features and allows both forward and backward navigation.

• Features:

  • Use the spacebar or f to scroll down.

  • Use the percentage to see progress.

  • Press q to exit.

    

9. Viewing Binary Files:

• Binary File with cat:

  • cat may not display binary files properly.

  • Use strings to extract readable text from binary files.

    cat /usr/bin/zsh

    

    strings /usr/bin/zsh

    

Rhel Playground:-

https://www.redhat.com/en/interactive-labs/install-software-using-package-managers