Module 21: Linux and Scripting

CompTIA A+ Core 2 (220-1102)

1. Linux Basics

• Linux is an open-source, Unix-like OS; the kernel plus tools form a distribution (distro) such as Ubuntu, Debian, Fedora, Red Hat/CentOS, Mint, Arch

• Used heavily on servers, embedded devices, Android (Linux kernel), and desktops

• Desktop environments: GNOME, KDE, Xfce; the shell (usually bash) is the command interpreter

• File systems: ext4 (most common), XFS, Btrfs; can read NTFS/FAT with drivers

• Package managers install software:

2. Directory Structure (Filesystem Hierarchy)

• Everything is a file; paths use / and are case-sensitive

• ~ = home directory, . = current directory, .. = parent directory

3. Essential Commands

Navigation and Files

Pipes and Redirection

• | sends output of one command to another: ls | grep txt

• > writes output to a file (overwrites); >> appends; < reads input; 2> redirects errors

Users, Permissions, and Admin

System and Process Management

Networking

ip addr (or ifconfig), ping, traceroute, netstat / ss, nslookup / dig, curl, wget, ssh user@host, scp, iwconfig

4. Permissions

ls -l output: -rwxr-xr-- 1 user group 4096 date file

• First character: - file, d directory, l link

• Then three triplets: owner / group / others, each r (read) w (write) x (execute)

• chmod 755 file → owner rwx, group r-x, others r-x

• chmod 644 file → owner rw-, group r--, others r--

• chmod 600 → owner only; chmod 777 → everyone full (avoid)

• Symbolic: chmod u+x file, chmod go-w file

• Directories need x to be entered

• Root (UID 0) is the superuser; use sudo rather than logging in as root

5. Backup, Updates, and Software

• Back up with rsync, tar, cp -a, Timeshift (snapshots), or cloud tools; schedule with cron

• Updates: sudo apt update && sudo apt upgrade (Debian/Ubuntu); sudo dnf upgrade (Fedora/RHEL)

• Update vs. upgrade (apt): update refreshes the package list; upgrade installs newer versions

• Install: sudo apt install package; remove: sudo apt remove package

• Verify downloads with checksums and install from trusted repositories

6. Linux Security

• Use strong passwords, limit sudo, keep systems patched

• Firewalls: iptables, ufw (Ubuntu), firewalld

• SSH with keys (ssh-keygen), disable root login

• SELinux / AppArmor enforce access policy

• Encryption: LUKS for disks

• Check logs in /var/log (auth.log, syslog)

• Run services with least privilege

7. Troubleshooting Linux

• GRUB is the common boot loader; fsck repairs file systems (run on unmounted volumes)

8. Scripting Basics

What Scripting Is For

Automates repeated tasks (backups, user creation, log checks, updates, cleanups, inventory) and reduces human error.

Common Script Types and Extensions

Bash Script Essentials

#!/bin/bash
# This is a comment
name="Student"          # variable (no spaces around =)
echo "Hello, $name"      # output; $ reads a variable

if [ -f /etc/hosts ]; then
  echo "File exists"
else
  echo "Missing"
fi

for i in 1 2 3; do
  echo "Number $i"
done

• Make executable: chmod +x script.sh, run with ./script.sh

• Comments start with # (Bash/Python), REM or :: (Batch), # (PowerShell)

• Exit codes: 0 = success; non-zero = error ($? holds the last code)

Core Programming Concepts (exam level)

Batch and PowerShell Snippets

@echo off
echo Hello
pause

Get-Process | Where-Object { $_.CPU -gt 100 }
Get-Service | Sort-Object Status

• PowerShell execution policy controls whether scripts may run (Set-ExecutionPolicy)

• Windows tools to schedule scripts: Task Scheduler; Linux: cron

Script Safety Considerations

• Test in a safe environment first

• Beware of unintentional infinite loops, wrong paths, destructive commands (rm -rf, del /s)

• Don't hard-code passwords; protect credentials

• Run least privilege; review scripts from unknown sources (malware often arrives as scripts)

• Document scripts and keep them in version control

9. Linux Cheat Sheet by Task

Additional Textbook Topics: Uses for Scripting

• Automate repetitive tasks: backups, user creation, cleanup, installs

• Configuration and deployment: set up many machines the same way

• Monitoring and reports: check disk space, services, and logs

• Restart services, rename or move files in bulk, map drives at logon (login scripts)

• Scheduled jobs: Task Scheduler (Windows), cron (Linux/macOS)

• Reduce errors and save time, but test first and avoid hard-coded passwords

Quick-Reference Exam Points

• Linux is case-sensitive; / is root; ~ is home

• chmod: r=4, w=2, x=1; 755 and 644 are classics; chown changes owner

• > overwrites, >> appends, | pipes

• apt update refreshes lists; apt upgrade installs; sudo = temporary admin

• ps/top view processes, kill ends them, df = disk space, grep = search text

• Shebang #!/bin/bash starts a shell script; extensions: .bat, .ps1, .sh, .py, .js, .vbs

• Shell scripts: variables, if, loops (for/while), functions, comments

• fsck repairs file systems; GRUB is the boot loader

• Don't hard-code credentials in scripts; test before running on production