Unit 4: Performing Basic Linux Tasks, Managing Users, Groups, Permissions & Storage

1. Linux Fundamentals and Design Philosophy

Identify Linux Design Philosophy

Linux follows the Unix philosophy, which emphasizes modularity and reusability. Key tenets include:

• "Everything is a file": Hardware devices, processes, and directories are represented as files in the filesystem.
• Small, single-purpose programs: Write programs that do one thing and do it well.
• Chaining: Programs are designed to work together via text streams (Standard Input/Output) using pipes (|).
• Configuration in text: System configurations are stored in human-readable text files, not binary registries.
• Case Sensitivity: Linux is natively case-sensitive (File.txt and file.txt are different).

Enter Shell Commands

The shell is the command-line interpreter (CLI). The most common shell is Bash (Bourne Again SHell).

• Syntax: command [options] [arguments]
  • ls -l /home (Command: ls, Option: -l, Argument: /home)
• Key shortcuts:
  • Tab: Autocompletes commands or filenames.
  • Ctrl+C: Terminates the running command.
  • Ctrl+R: Search command history.
  • Up/Down Arrow: Navigate history.

Get Help with Linux

Administrators rely on built-in documentation systems:

1. Man Pages (Manual): The primary reference.
   • man <command> (e.g., man ls).
   • Navigation: Space (next page), q (quit), / (search).
2. Info Pages: detailed, hyperlinked documentation.
   • info <command>
3. --help Option: Quick syntax summary.
   • <command> --help

Assume Superuser Privileges

Linux uses a permission model where the root user has unrestricted access.

• su (Switch User): Switches the current shell to another user (usually root).
  • su - (Switches to root and loads root's environment variables).
• sudo (SuperUser DO): Executes a single command with root privileges based on rules in /etc/sudoers.
  • sudo apt update
  • Benefits: granular control, logging of administrative actions.

2. Managing Users and Groups

User and group management is critical for system security and resource allocation.

Core Configuration Files

• /etc/passwd: Stores user account information (publicly readable).
• /etc/shadow: Stores encrypted password hashes and aging info (readable only by root).
• /etc/group: Stores group information.

Create, Modify, and Delete Users

• Create: useradd
  • useradd -m -s /bin/bash -c "John Doe" jdoe
  • -m: Create home directory.
  • -s: Set default shell.
  • -c: Comment (full name).
• Modify: usermod
  • usermod -aG sudo jdoe (Append user to the 'sudo' group).
  • usermod -L jdoe (Lock the account).
• Delete: userdel
  • userdel -r jdoe (Remove user AND their home directory).

Create, Modify, and Delete Groups

Groups allow permissions to be assigned to multiple users simultaneously.

• Create: groupadd developers
• Modify: groupmod -n dev_team developers (Rename group).
• Delete: groupdel dev_team

Query Users and Groups

• id <username>: Displays UID (User ID), GID (Group ID), and group memberships.
• whoami: Displays current effective user.
• w or who: Shows who is currently logged in.
• last: Shows history of user logins.

Configure Account Profiles

When a user is created, their environment is initialized using skeleton files.

• /etc/skel/: Any file placed here is automatically copied to a new user's home directory upon creation (e.g., .bashrc).
• ~/.bashrc: Script executed for non-login shells (opening a new terminal window). Used for aliases and prompt settings.
• ~/.bash_profile or ~/.profile: Executed for login shells (SSH or GUI login). Used for path variables.

3. Permissions and Ownership

Linux uses a Discretionary Access Control (DAC) model based on Read, Write, and Execute.

Modify File and Directory Permissions

Permissions apply to three entities: User (u), Group (g), and Others (o).

Permission Modes:

• Read (r): Value 4. (File: view contents; Directory: list contents).
• Write (w): Value 2. (File: modify contents; Directory: create/delete files).
• Execute (x): Value 1. (File: run as program; Directory: enter/cd into).

The chmod Command:

• Symbolic Mode: chmod g+w file.txt (Add write permission to group).
• Octal Mode: chmod 755 script.sh
  • Owner: 7 (4+2+1 = rwx)
  • Group: 5 (4+0+1 = r-x)
  • Others: 5 (4+0+1 = r-x)

Modify File and Directory Ownership

• chown (Change Owner): chown user:group filename
  • chown bob:developers project.c
  • chown -R bob /var/www/html (Recursive change).
• chgrp (Change Group): chgrp developers project.c

Configure Special Permissions and Attributes

1. SUID (Set User ID): (chmod u+s or 4xxx). Executables run with the permissions of the file owner, not the user running it (e.g., passwd command).
2. SGID (Set Group ID): (chmod g+s or 2xxx).
   • On files: Run with group privileges.
   • On directories: New files created inside inherit the group of the directory, not the user's primary group. Used for shared folders.
3. Sticky Bit: (chmod +t or 1xxx).
   • On directories: Only the file owner or root can delete a file. (e.g., /tmp).

Troubleshoot Permissions Issues

• Symptom: "Permission Denied".
• Steps:
  1. Check ls -l to view current mode.
  2. Check id to verify user group membership. Note: If a user was just added to a group, they must logout/login for it to take effect.
  3. Check directory permissions tree (user needs +x on all parent directories to reach a file).
  4. Check SELinux/AppArmor context (advanced security layers) if standard permissions look correct.

4. Storage Management

Navigate the Linux Directory Structure (FHS)

The Filesystem Hierarchy Standard defines the directory structure:

• /: Root directory.
• /boot: Kernel and bootloader files.
• /etc: System configuration files.
• /home: User home directories.
• /var: Variable data (logs, spools, web files).
• /bin & /sbin: System binaries (programs).
• /dev: Device files (hardware interfaces).

Create Partitions

• fdisk: Legacy tool for MBR partitions.
• gdisk: Tool for GPT partitions.
• parted: Versatile tool for both.
• Process:
  1. fdisk /dev/sdb
  2. n (new partition) -> Select primary/extended -> Select size.
  3. w (write changes).

Manage Logical Volumes (LVM)

LVM allows flexible resizing of storage across multiple physical disks.

1. Physical Volume (PV): Initialize the disk.
   • pvcreate /dev/sdb1
2. Volume Group (VG): Pool storage together.
   • vgcreate my_data_vg /dev/sdb1
3. Logical Volume (LV): Carve out a virtual partition.
   • lvcreate -L 10G -n my_lv my_data_vg

Manage File Systems

After partitioning (or creating an LV), a filesystem must be applied.

• Formatting:
  • mkfs.ext4 /dev/my_data_vg/my_lv (Standard Linux FS).
  • mkfs.xfs /dev/sdb1 (High performance, common in RHEL/CentOS).

Mount File Systems

To access data, the filesystem must be mounted to a directory point.

• Temporary Mount:
  • mount /dev/sdb1 /mnt/data
• Unmount:
  • umount /mnt/data
• Persistent Mount (/etc/fstab):
  • Configures mounts to occur automatically at boot.
  • Format: <Device> <Mount Point> <Type> <Options> <Dump> <Pass>
  • Example: /dev/sdb1 /var/www ext4 defaults 0 2