CompTIA Linux+ (XK0-005) Study Notes
Structured notes aligned to the Linux+ exam domains
Table of Contents
1.0 System Management (32%)
Filesystem Hierarchy (FHS)
- /boot: Bootloader and kernel files.
- /proc: Virtual filesystem with process/system info.
- /sys: Kernel and hardware information.
- /var: Variable data like logs and spools.
- /usr: User binaries, libraries, and docs.
- /lib: Essential shared libraries/modules.
- /dev: Device files for hardware.
- /etc: System configuration files.
- /opt: Optional/add-on software.
- /bin: Essential user binaries.
- /sbin: System binaries for admins.
- /home: User home directories.
- /media: Removable media mounts.
- /mnt: Temporary mount points.
- /root: Root user home directory.
- /tmp: Temporary files.
Disk Partitioning & Mounting
- MBR: Legacy partitioning; 2TB limit; 4 primary partitions.
- GPT: Modern partition table for large disks and many partitions.
- FUSE: Filesystems implemented in user space.
- fdisk: MBR partitioning tool.
- parted: GPT-aware partitioning tool.
- partprobe: Reload partition table without reboot.
- mount: Mount a filesystem manually.
- /etc/fstab: Persistent mount configuration file.
- systemd.mount: systemd unit for mounts.
- LUKS: Linux Unified Key Setup (disk encryption).
LVM & RAID
- pvcreate: Create a physical volume.
- vgcreate: Create a volume group.
- lvcreate: Create a logical volume.
- lvresize: Resize a logical volume.
- pvs: Display physical volume details.
- vgs: Display volume group details.
- lvs: Display logical volume details.
- RAID 0: Striping; no redundancy.
- RAID 1: Mirroring for redundancy.
- RAID 5: Single-parity redundancy with striping.
- RAID 6: Dual-parity redundancy with striping.
- mdadm: Manage RAID arrays.
- /proc/mdstat: RAID status interface.
Networking & Name Resolution
- ip: Configure routing and interfaces.
- ss: Socket statistics and connections.
- nmcli: NetworkManager CLI.
- hostname: Show or set system hostname.
- hostnamectl: systemd hostname management.
- arp: View ARP cache.
- /etc/resolv.conf: DNS resolver configuration.
- nsswitch.conf: Name lookup order.
- dig: DNS query tool.
- nslookup: Legacy DNS query tool.
- host: Simple DNS lookup tool.
- WHOIS: Domain registration lookup.
- tcpdump: CLI packet capture.
- wireshark: GUI packet analyzer.
- tshark: CLI Wireshark analyzer.
- ping: ICMP reachability test.
- traceroute: Trace network path hops.
- mtr: Combined ping/traceroute diagnostics.
System Services & Scheduling
- systemctl: Manage systemd services.
- start: Start a service immediately.
- stop: Stop a running service.
- restart: Stop and start a service.
- status: Show service status/logs.
- enable: Start service at boot.
- disable: Prevent service from starting at boot.
- mask: Block a service from being started.
- cron: Time-based job scheduler.
- crontab: Manage cron jobs.
- at: Schedule one-time jobs.
User, Group & Process Management
- useradd: Create a user account.
- userdel: Remove a user account.
- usermod: Modify a user account.
- groupadd: Create a group.
- groupdel: Remove a group.
- groupmod: Modify a group.
- /etc/passwd: User account database.
- /etc/shadow: Encrypted password database.
- /etc/group: Group definitions.
- ps: List running processes.
- top: Live process/resource view.
- htop: Enhanced interactive process viewer.
- nice: Start a process with a priority.
- renice: Change priority of a running process.
- kill: Send a signal to a PID.
- pkill: Send a signal by process name.
- pgrep: Find PIDs by name/pattern.
- Zombie: Terminated, unreaped process.
Package Management & Updates
- DNF: Modern RPM package manager (Fedora/CentOS).
- YUM: Legacy RPM package manager.
- APT: Debian-based package manager.
- RPM: Package format/manager for Red Hat systems.
- dpkg: Low-level Debian package tool.
- ZYpp: openSUSE package management system.
- Kernel updates: Updates to the Linux kernel.
- Package updates: Updates to installed software.
- snapd: Service managing Snap packages.
- Flatpak: Sandboxed cross-distro app packaging.
- AppImage: Portable self-contained application format.
2.0 Security (21%)
Permissions & Access Control
- chmod: Change file permissions.
- chown: Change file owner.
- chgrp: Change file group.
- SUID: Run as file owner when executed.
- SGID: Run as file group when executed.
- Sticky Bit: Restrict deletes in shared directories.
- ACLs: Extended permissions beyond owner/group/other.
- umask: Default permission mask for new files.
SELinux
- SELinux: Mandatory access control framework.
- Enforcing: SELinux policy is enforced.
- Permissive: Violations logged only.
- Disabled: SELinux is off.
- setenforce: Change SELinux mode.
- getenforce: Show current SELinux mode.
- semanage: Manage SELinux settings/contexts.
- audit2allow: Build allow rules from audit logs.
3.0 Scripting, Containers, and Automation (19%)
Shell Scripting Essentials
- Loops: Repeat commands until a condition is met.
- while: Loop while a condition is true.
- for: Loop over a list or range.
- until: Loop until a condition becomes true.
- Conditionals: Execute logic based on true/false tests.
- if: Run commands when a condition is true.
- case: Select a code path based on matching patterns.
- Variables: Store values for reuse in scripts.
- Globbing: Wildcard pattern matching for filenames.
- Brace expansion: Generate multiple strings from a pattern.
- Standard streams: stdin, stdout, and stderr.
- | (pipe): Pass output of one command to another.
- &&: Run next command if previous succeeds.
- ||: Run next command if previous fails.
- >: Redirect output to a file (overwrite).
- >>: Redirect output to a file (append).
- <: Read input from a file.
- <<: Here-document input block.
- &: Run process in the background.
- Exit code: Command status (0 success, non-zero failure).
- $PATH: Directories searched for commands.
- $SHELL: Current user shell path.
- $?: Exit status of last command.
- Relative path: Path based on current directory.
- Absolute path: Full path from root (/).
Containers & Automation
- Docker: Container platform for packaging apps.
- docker run: Create and start a container.
- docker start: Start a stopped container.
- docker stop: Stop a running container.
- docker ps: List containers.
- docker build: Build an image from a Dockerfile.
- docker pull: Download an image.
- docker push: Upload an image to a registry.
- Kubernetes: Container orchestration platform.
- git clone: Copy a repository.
- git add: Stage changes.
- git commit: Save a snapshot.
- git push: Upload commits.
- git pull: Fetch and merge updates.
- .gitignore: Define files to ignore in Git.
- CI/CD: Automated build/test/deploy pipelines.
4.0 Troubleshooting (28%)
Resource Troubleshooting
- df: Show filesystem free space.
- du: Show file/directory disk usage.
- OOM: Out-of-memory condition.
- iowait: CPU time waiting on disk I/O.
- journalctl: systemd log viewer.
Common Issue Areas
- Network issues: Routing, DNS, or firewall problems.
- Storage issues: RAID/LVM or filesystem failures.
- Permission issues: ACL, SELinux, or ownership conflicts.