Linux File Ownership and Permissions

File Type in Unix

The first character in a Unix file permission string indicates the file type. It provides essential information about the nature of the file. Here are the common file types represented by the first character:

1. Ordinary File (Regular File):

   • Ordinary files contain data, text, or program instructions.

   • Examples: Text files, images, compiled programs.

   • Output:

     -rw-r--r-- 1 user group 12345 Mar 25 10:00 myfile.txt

   • Explanation:

     • -rw-r--r--: The first - indicates an ordinary file.

     • myfile.txt: The file name.

     • Permissions (rw-r--r--): Read and write permissions for the owner, read-only permissions for the group and others.

2. Directory:

   • Directories are used to organize groups of files.

   • They can contain both files and other directories.

   • Output:

     drwxr-xr-x 2 user group 4096 Mar 25 10:05 mydir

   • Explanation:

     • d: Indicates a directory.

     • mydir: The directory name.

     • Permissions (drwxr-xr-x): Read, write, and execute permissions for the owner, read and execute permissions for the group and others.

3. Character Device File:

   • Character devices communicate with hardware devices character by character (e.g., terminals, serial ports).

   • Output:

     crw-rw---- 1 root tty 4, 1 Mar 25 10:10 tty1

   • Explanation:

     • c: Indicates a character device.

     • tty1: The device name.

     • Permissions (crw-rw----): Read and write permissions for the owner and group.

4. Block Device File:

   • Block devices communicate with hardware devices in fixed-size blocks (e.g., hard drives, USB drives).

   • Output:

     brw-rw---- 1 root disk 8, 0 Mar 25 10:15 sda

   • Explanation:

     • b: Indicates a block device.

     • sda: The device name.

     • Permissions (brw-rw----): Read and write permissions for the owner and group.

5. Symbolic Links (Symlinks):

   • Symlinks are pointers to other files or directories.

   • They allow flexible organization without physically moving files.

   • Example: mylink -> myfile.

   • Output:

     lrwxrwxrwx 1 user group  4 Mar 25 10:00 my_link.txt -> my_file.txt

   • Explanation:

     • l: Indicates a symbolic link.

     • my_link.txt: The symlink name.

     • my_file.txt: The file it points to.

2. File Permissions

Explanation

File ownership in Linux is crucial for access control. Each file or directory has an owner (user) and a group associated with it. Let's explore this further.

Practical Exercise

1. Create a test file:

   touch mytestfile.txt

2. Check its ownership details:

   ls -l mytestfile.txt

   The output will show something like:

   -rw-r--r-- 1 username groupname 0 Mar 25 18:00 mytestfile.txt

   Here:

   • username is the owner.

   • groupname is the group.

   • The first - indicates it's a regular file.

3. File Permissions

Explanation

File permissions control who can read, write, or execute a file. The three basic permissions are read (r), write (w), and execute (x).

Practical Exercise

Exercise 1: Remove Write Permission

1. Create a test file:

   touch mytestfile.txt

2. Remove write permission for yourself:

   chmod -w mytestfile.txt

3. Try to modify the file:

   echo "Hello, World!" >> mytestfile.txt

   You'll receive an error because you no longer have write permission.

4. Add write permission back:

   chmod +w mytestfile.txt

5. Modify the file again:

   echo "Hello again!" >> mytestfile.txt

6. View the file content:

   cat mytestfile.txt

4. Putting It All Together

After changing permissions, let's check the updated permissions using ls -l.

5. Changing Ownership

Remember that changing ownership requires root privileges. Use sudo for this operation.

1. View ownership before:

   ls -l mytestfile.txt

2. Change ownership:

   sudo chown newuser:newgroup mytestfile.txt

3. View ownership after:

   ls -l mytestfile.txt

7. Understanding Umask

Explanation

• Umask controls default permissions for newly created files and directories.

• Default umask value: 0022 (subtract from 666 for files, 777 for directories).

Practical Example

1. Change the umask:

   umask 0027

2. Create a file and directory:

   touch myfile.txt
   mkdir mydir

3. Observe the permissions using ls -l.

9. Calculating Umask

• Umask value calculation:

  • Subtract umask from default permissions (666 for files, 777 for directories).

Example 1: 666 - 002

• Default permissions for files: 666

• Umask: 002

• Calculated permissions: 664 (rw-rw-r--)

Example 2: 777 - 002

• Default permissions for directories: 777

• Umask: 002

• Calculated permissions: 775 (rwxrwxr-x)

12. Let's explore the stat Command

The stat command provides detailed information about files. It allows you to retrieve various attributes of a file, including permissions, timestamps, and file type.

1. Show Numeric Permissions

• Display the numeric permissions (e.g., 644 for files, 755 for directories):

  stat -c %a filename

  Example Output:

  644

2. Show Access Time

• View only the access timestamp:

  stat -c %x filename

  Example Output:

  2023-03-25 18:30:00.000000000 +0000

3. Show Modification Time

• See only the modification timestamp:

  stat -c %y filename

  Example Output:

  2023-03-25 18:35:00.000000000 +0000

4. Show Change Time

• Check only the change timestamp:

  stat -c %z filename

  Example Output:

  2023-03-25 18:40:00.000000000 +0000

5. Show File Type

• Determine the file type (e.g., "regular file," "directory," etc.):

  stat -c %F filename

  Example Output:

  regular file

Advantages of Using stat:

• Provides granular information about files.

• Useful for scripting and automation.

• Helps track file changes and access patterns.

Explore these concepts, practice exercises, and master Linux permissions! 🐧🔒 Don't miss the next session whcih talk about the special permission.