Unit 2: Configuring Permissions and File Management

1. Configure Standard Linux Permissions

Linux uses a multi-user permission model based on file ownership and permission bits. Every file and directory is assigned an owner (User), a specific group (Group), and permissions for everyone else (Others).

The Permission Classes

• User (u): The owner of the file.
• Group (g): Users belonging to the specific group assigned to the file.
• Others (o): Any user who is not the owner and does not belong to the group.

The Permission Types

• Read (r):
  • Files: Ability to view contents (cat, less).
  • Directories: Ability to list contents (ls).
• Write (w):
  • Files: Ability to modify contents.
  • Directories: Ability to create or delete files within that directory.
• Execute (x):
  • Files: Ability to run the file as a program or script.
  • Directories: Ability to cd into the directory and access metadata of files inside.

Notation Systems

1. Symbolic Notation

Uses letters (r, w, x, -) to represent permissions.

• rwx: Read, Write, Execute.
• r--: Read only.
• rw-: Read and Write.

2. Octal (Numeric) Notation

Each permission is assigned a numeric value. To set permissions, sum the values for each class (User, Group, Others).

• Read (r) = 4
• Write (w) = 2
• Execute (x) = 1
• No permission (-) = 0

Example: chmod 755 filename

• User: 4+2+1 = 7 (rwx)
• Group: 4+0+1 = 5 (r-x)
• Others: 4+0+1 = 5 (r-x)

Management Commands

• chmod (Change Mode): Modifies permissions.
  
  BASH

      chmod u+x script.sh      # Add execute to user
      chmod 644 config.txt     # rw-r--r--
      chmod -R 755 /var/www    # Recursive change
      

• chown (Change Owner): Changes file ownership (usually requires root).
  
  BASH

      chown alice file.txt          # Change user
      chown alice:developers file.txt # Change user and group
      

• chgrp (Change Group): Changes group ownership.
  
  BASH

      chgrp developers project/
      

2. Configure Special Linux Permissions

Beyond standard permissions, Linux includes three special bits for specific security and functional use cases.

1. SUID (Set User ID)

• Function: When an executable file with SUID is run, it runs with the permissions of the file's owner, not the user who ran it.
• Representation: An s in the user execute position (e.g., -rwsr-xr-x).
• Numeric: 4000.
• Common Use: passwd command (needs root privilege to write to /etc/shadow even when run by a standard user).

2. SGID (Set Group ID)

• On Files: Runs with the permissions of the file's group.
• On Directories: New files created inside the directory automatically inherit the directory's group ownership, rather than the creator's primary group. Essential for shared collaboration folders.
• Representation: An s in the group execute position (e.g., drwxrwsr-x).
• Numeric: 2000.

3. Sticky Bit

• Function: Ensures that only the file owner, the directory owner, or root can delete or rename files within a directory.
• Representation: A t in the others execute position (e.g., drwxrwxrwt).
• Numeric: 1000.
• Common Use: /tmp directory (everyone can write, but you can't delete others' files).

3. Configure Access Control Lists (ACLs)

Standard permissions (UGO) are limited because you can only assign permissions to one specific owner and one specific group. ACLs allow for fine-grained control, enabling you to assign permissions to multiple users or groups on a single file.

Checking ACLs

Use getfacl to view current ACLs.

getfacl filename

Setting ACLs

Use setfacl to modify entries.

• Syntax: setfacl -m [u/g]:[name]:[perms] filename

Examples:

# Give user 'bob' read/write access to a file owned by 'alice'
setfacl -m u:bob:rw report.txt

# Give group 'interns' read access
setfacl -m g:interns:r report.txt

# Remove specific ACL entry
setfacl -x u:bob report.txt

# Remove all ACLs
setfacl -b report.txt

4. Understand the Linux File System

The Linux file system follows the Filesystem Hierarchy Standard (FHS). It is a single inverted tree structure starting at the root (/).

Key Directories

• /: Root - The starting point of the filesystem.
• /bin & /usr/bin: Essential user command binaries (ls, cp, cat).
• /sbin & /usr/sbin: System administration binaries (fdisk, reboot).
• /etc: System-wide configuration files (networking, users, services).
• /home: Personal directories for users.
• /root: The home directory for the root user.
• /var: Variable data (logs, spool files, mail).
• /tmp: Temporary files (cleared on reboot).
• /dev: Device nodes (representing hardware like disks sda, terminals tty).
• /proc: Virtual filesystem representing kernel and process state.

Inodes and Links

• Inode: A data structure that stores metadata about a file (permissions, owner, size, location blocks) but not the filename.
• Hard Link: A directory entry pointing to the same inode as the original file. Deleting the original does not delete the data until all hard links are removed. Cannot span partitions.
• Soft (Symbolic) Link: A file containing the path to another file (like a shortcut). If the original is deleted, the soft link becomes broken.

5. Use File Management Commands

Creating and Deleting

• mkdir: Create directory (mkdir -p dir1/subdir for nested).
• rmdir: Remove empty directory.
• touch: Create an empty file or update timestamps.
• rm: Remove files.
  • rm -r: Recursive (delete directory and contents).
  • rm -f: Force (no confirmation prompt).

Copying and Moving

• cp [source] [dest]: Copy files.
  • cp -r: Copy directories recursively.
  • cp -a: Archive mode (preserves permissions/timestamps).
• mv [source] [dest]: Move or Rename files.

Listing and Viewing

• ls: List directory contents.
  • ls -l: Long format (permissions, size, owner).
  • ls -a: Show hidden files (starting with .).
  • ls -h: Human-readable sizes (KB, MB).
• cat: Concatenate and display entire file.
• less: View file one page at a time (scrollable).
• head: View the first 10 lines (head -n 5 file).
• tail: View the last 10 lines (tail -f file follows log updates).

6. Find File Locations

The find Command

Searches the directory tree in real-time. Powerful but slower.

• Syntax: find [path] [criteria] [action]
• Examples:
  
  BASH

      find /home -name "*.txt"        # Find text files in /home
      find /var -size +100M           # Find files larger than 100MB
      find / -user bob                # Find files owned by bob
      find /data -perm 777            # Find files with 777 permissions
      find . -name "*.log" -delete    # Find and delete log files
      

The locate Command

Searches a pre-built database (mlocate.db). Very fast but may be outdated if the database hasn't updated.

• updatedb: Updates the database (requires root).
• locate config.xml: Returns path immediately.

Binary Locations

• which [command]: Shows the path of the executable that will run (e.g., /usr/bin/python).
• whereis [command]: Shows the binary, source, and manual page paths.

7. Edit Text Files

The primary command-line text editor is Vi/Vim. It operates in distinct modes.

Vi/Vim Modes

1. Normal Mode: Default mode for navigation and manipulation.
   • h, j, k, l: Move cursor (Left, Down, Up, Right).
   • dd: Delete (cut) line.
   • yy: Yank (copy) line.
   • p: Paste.
   • u: Undo.
2. Insert Mode: For typing text.
   • i: Insert before cursor.
   • a: Append after cursor.
3. Command-Line Mode: For saving and exiting.
   • :w: Save.
   • :q: Quit.
   • :wq or :x: Save and Quit.
   • :q!: Quit without saving.

Nano

A simpler, modeless editor easier for beginners.

• Commands appear at the bottom of the screen (e.g., ^O means Ctrl+O to save).

8. Manage Text Files

System administration involves heavy text processing using streams, pipes, and filters.

Input/Output Redirection

• >: Redirect standard output to a file (overwrite).
  • ls > filelist.txt
• >>: Redirect standard output to a file (append).
  • echo "New Entry" >> log.txt
• 2>: Redirect standard error.
• <: Redirect standard input.

Piping (|)

Takes the output of the command on the left and uses it as the input for the command on the right.

• ls -l | less (Scroll through a long directory list).

Text Processing Tools

• grep: Search for patterns within files.
  
  BASH

      grep "error" /var/log/syslog    # Show lines containing "error"
      grep -r "config" /etc/          # Recursive search
      

• cut: Extract sections from lines of files.
  
  BASH

      cut -d: -f1 /etc/passwd         # Extract usernames (field 1) using : as delimiter
      

• wc: Word count.
  
  BASH

      wc -l file.txt                  # Count lines
      

• sort: Sort lines of text files.
• uniq: Report or omit repeated lines (usually requires sorted input).
  
  BASH

      sort names.txt | uniq