learning linux 3

 

User and Group Management in Linux

Managing Local Users:

• Commands:
  • adduser / useradd: Create new users.
  • usermod: Modify existing user accounts.
  • userdel: Delete users.

Managing Local Groups:

• Primary and Supplementary Groups: Each user has a primary group and can belong to multiple supplementary groups.
• Commands:
  • groupadd: Create new groups.
  • groupmod: Modify existing groups.
  • groupdel: Delete groups.

Querying User Accounts:

• Commands:
  • whoami: Displays the current logged-in user's name.
  • who: Lists users currently logged in and their login details.
  • w: Shows what users are currently doing.
  • pinky: A lightweight version of finger, provides user information.
  • last: Shows a history of user logins.
  • id username: Displays user identity information.

Managing Group Password Files:

• Password File: Located in /etc/shadow, requires root access to edit.
• Commands:
  • sudo vipw / sudo vipw -s: Safely edit the password file.
  • vigr: Edit group file.

User & Group Quotas:

• Quota Management:
  • Enable quotas during mounting by scanning the drive with sudo quotacheck.
  • Activate quotas using sudo quotaon.
  • Edit quotas for users with sudo edquota username.
• Types of Quotas:
  • Inodes: Limits the number of files a user can create.
  • Blocks: Limits the amount of disk space a user can consume.

Text Editing and Viewing

Text Editors:

• Nano: Simple and user-friendly.
• VI: Classic editor with two modes:
  • Insert Mode: Enter using i or a.
  • Command Mode: Exit using ESC, then save and quit with :wq, quit with :q!, and save only with :w.

Viewing Text:

• Commands:
  • head: Displays the first lines of a file.
  • tail: Displays the last lines of a file.
  • more / less: View files interactively; less allows searching with /search and exiting with q.

Searching Text with grep:

• Usage:
  • Regex: grep supports regular expressions.
  • Piping: Combine commands with | for complex searches.

Input/Output Redirection and Text Manipulation

Redirection:

• Stdin: < and |
• Stdout: >
• Stderr: 2>

Special Files:

• /dev/null: The "blackhole" where unwanted output is discarded.
• tee: Sends output to both a file and stdout.
• xargs: Passes stdin as arguments to another command.

Text Manipulation:

• sort: Sorts lines in a file.
• wc: Counts lines, words, and characters in a file.
• cut / paste: Cuts and pastes columns of text.

Advanced Text Processing:

• AWK: A powerful text processing language.
• SED: Stream editor for filtering and transforming text.

File Management and Links

Hard and Soft Links:

• Soft Link (Symbolic Link): Points to the file's location in the allocation table.
• Hard Link: Points directly to the data on the disk, multiple links can reference the same data block.

Finding Files:

• find: Powerful, slower, supports regex.
• locate: Fast, uses a cached database; run sudo updatedb to refresh the database.

Copying Files Over a Network

Protocols and Tools:

• SSH: Secure Shell protocol.
• SCP: Secure copy, works over SSH.
• Rsync: Copies files recursively and efficiently.

Service Management

Systemctl for Services:

• Commands:
  • enable / disable: Set services to start on boot or not.
  • start / stop: Immediately start or stop a service.

Sys-V Runlevels:

• Runlevels:
  • Debian/Ubuntu: 0, 1, 2, 6.
  • CentOS/Red Hat: 0, 1, 2, 3, 5, 6.
• Changing Runlevels: Edit /etc/inittab or use telinit.
• Managing Services:
  • Scripts: Located in /etc/init.d/.
  • Commands: chkconfig and service.

SystemD Init:

• Isolate Services: Use systemctl isolate.
• Manage Services: Use systemctl.

Network Server Roles and Virtualization

Network Server Roles:

• Key Services:
  • NTP: Network Time Protocol.
  • SSH: Secure Shell.
  • DNS: Domain Name System.
  • DHCP: Dynamic Host Configuration Protocol.
  • Docker: Container management.
• Configuration: Can be centralized or individual.

Virtualization and Server Closets:

• Impact: Virtualization reduced the need for physical servers.
• Sprawl Potential: Server sprawl was a problem, but Docker helps by containing and isolating services.

Understanding Web Servers, SSL, and More

Web Servers and SSL:

• Basics: Understanding how web servers work and the importance of SSL for secure connections.

Local Network Roles vs. Cloud:

• Shift to Cloud: Many local network roles are now handled by cloud services.

Authentication and Database Servers:

• Roles: Ensuring secure access and data management.

Centralized Logging and Monitoring:

• Syslog: Centralized logging.
• SNMP: Simple Network Management Protocol for monitoring.

VPNs (Virtual Private Networks):

• Options:
  • Site-to-Site.
  • End User to Site.
  • Protocols: IPSec, L2TP, OpenVPN, SSH.

Containers vs. Virtual Machines:

• Differences: Containers share the host OS, while VMs have separate OS instances.
• Container Systems: Tools like Docker for container management.

Clustering and Load Balancing:

• Clustering: Combining multiple servers to work as a single unit.
• Load Balancing: Distributing traffic, often using round-robin DNS.

System Maintenance and Device Management

Cron Jobs:

• System-Wide: Configure cron jobs using /etc/cron.d.
• Personal Crontab: Each user can have their own crontab.

AT Daemon:

• Use Case: Schedule one-time tasks with at.

Foreground & Background Processes:

• Management: Use &, fg, and bg commands to control process states.

Finding Local Devices:

• Commands:
  • dmesg: Kernel ring buffer.
  • lsusb: List USB devices.
  • lspci: List PCI devices.
  • lsblk: List block devices.
  • lscpu: Display CPU architecture.

Virtual File System:

• Directories:
  • /proc: Process information.
  • /sys: Kernel information.
  • /dev: Device files.

CUPS Printing:

• Commands:
  • lpr: Print a file.
  • lpq: Display print queue.
  • lprm: Remove jobs from the print queue.
• Web Interface: Manage CUPS via its web interface.

Understanding UDEV:

• UDEV: Manages device nodes in /dev.
• Sysfs: Exposes kernel device information.
• Rules: Custom UDEV rules for device management.