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Doc/Linux/Administration.md

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+# Linux – Administration
+
+Overview of common Linux administration tasks. Focus on day‑to‑day system management rather than distribution‑specific tools.
+
+For foundational concepts (kernel, filesystem, users, processes), see:
+
+- [Linux / Basics](./Basics.md)
+
+---
+
+## 1. User and Group Management
+
+Linux uses users and groups to control access to files, processes, and services.
+
+Typical administration tasks include:
+
+- Creating and removing users.
+- Adding users to groups (for example to grant `sudo` access).
+- Locking or disabling accounts.
+
+Key ideas:
+
+- Each user has a numeric user ID (UID) and a primary group ID (GID).
+- Additional group memberships are used to grant extra permissions (for example access to Docker or systemd‑journal).
+
+---
+
+## 2. Privileged Access and sudo
+
+Administrative operations are usually performed with elevated privileges.
+
+Approaches:
+
+- Direct `root` login (less common on modern systems for security reasons).
+- Using `sudo` to run specific commands as `root`.
+
+Principles:
+
+- Only trusted users should have `sudo` rights.
+- The set of allowed commands can be restricted via the sudoers configuration.
+
+In practice, administration is often done as an unprivileged user with `sudo` used only when needed.
+
+---
+
+## 3. Filesystem Permissions and Ownership
+
+Correct file and directory permissions are central to system security and stability.
+
+Key aspects:
+
+- Ownership (user and group) defines who primarily controls a resource.
+- Permission bits (read, write, execute) for user, group, and others control what can be done.
+- Special bits (setuid, setgid, sticky) influence execution and inheritance in specific cases.
+
+Administration tasks related to permissions include:
+
+- Adjusting ownership for application data directories.
+- Ensuring configuration files are restricted when they contain secrets.
+- Verifying that log and temporary directories have appropriate access.
+
+Additional reminders and command examples can be kept in:
+
+- [Linux / Cheat Sheet](./CheatSheet.md)
+
+---
+
+## 4. Services and systemd Units
+
+On many modern Linux distributions, **systemd** is used as the init system and service manager.
+
+Concepts:
+
+- A **unit** defines how a service, mount, timer, or other resource is started and managed.
+- Service units (for example `ssh.service`, `docker.service`) describe how to start and stop daemons.
+- Targets group units to represent boot states (for example `multi-user.target`).
+
+Typical administration actions:
+
+- Starting, stopping, and restarting services.
+- Enabling or disabling services at boot.
+- Checking service status and logs.
+
+Systemd units are usually stored under `/lib/systemd/system` or `/etc/systemd/system`. Local overrides can be created without modifying vendor files.
+
+---
+
+## 5. Logs and Troubleshooting
+
+System logs are essential for diagnosing issues.
+
+On systemd‑based systems, the journal stores logs from the kernel and many services.
+
+Common tasks:
+
+- Viewing recent logs for a specific service.
+- Filtering logs by priority or time range.
+- Inspecting boot logs to understand startup problems.
+
+In addition to the journal, many services also write to files under `/var/log`. Knowing where a service writes its logs is an important part of administration.
+
+---
+
+## 6. Software Installation and Updates
+
+Package management is used to install, update, and remove software.
+
+Administration concerns:
+
+- Keeping the system reasonably up to date for security fixes.
+- Avoiding unnecessary packages on minimal or security‑sensitive systems.
+- Understanding which repositories are enabled.
+
+The concrete commands differ per distribution (for example `apt` vs `dnf`), but the underlying tasks are similar.
+
+Distribution‑specific notes and examples can be recorded in a dedicated cheat sheet or in project‑specific documentation.
+
+---
+
+## 7. Scheduled Tasks and Automation
+
+Linux systems provide several mechanisms for scheduled or automated tasks.
+
+Common options:
+
+- `cron` jobs for periodic execution.
+- systemd timers for more structured scheduling tied to units.
+
+These mechanisms are used for activities such as log rotation, backups, or periodic health checks.
+
+---
+
+## 8. Backup and Recovery Considerations
+
+From an administrative perspective, backup and recovery planning includes:
+
+- Identifying which directories and data must be backed up.
+- Choosing backup tools and storage locations.
+- Testing restore procedures.
+
+Details are often project‑specific and can be documented alongside the systems that depend on them (for example in a home‑server or application‑specific section).
+
+---
+
+## 9. Relation to Other Documents
+
+- [Linux / Cheat Sheet](./CheatSheet.md)

Doc/Linux/Basics.md

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+# Linux – Basics
+
+---
+
+## 1. What Is Linux?
+
+Linux is a family of open-source operating systems built around the Linux kernel. The kernel was created in 1991 by Linus Torvalds and is now maintained by a large community.
+
+In practice, "Linux" usually refers to a complete system that combines:
+
+- The Linux kernel (manages hardware, memory, processes).
+- User space tools and libraries (often from the GNU project).
+- Additional software such as shells, utilities, and applications.
+
+These complete systems are distributed as Linux distributions.
+
+---
+
+## 2. Linux Distributions
+
+A Linux distribution ("distro") bundles the kernel with a selection of software, configuration defaults, and a package manager.
+
+Common examples include:
+
+- **Debian / Ubuntu** – widely used on servers and desktops; focus on stability and large software repositories.
+- **Fedora / RHEL / CentOS / AlmaLinux** – often used in enterprise environments.
+- **Arch Linux** – rolling release, minimal by default, focused on simplicity and control.
+- **OpenSUSE**, **Gentoo**, and many others.
+
+Distributions differ mainly by:
+
+- Package manager (`apt`, `dnf`, `pacman`, etc.).
+- Release model (stable vs rolling release).
+- Default configuration and tooling.
+
+Despite these differences, the core concepts (filesystem layout, permissions, processes, services) are very similar across distributions.
+
+---
+
+## 3. Kernel, User Space, and Shell
+
+### 3.1 Kernel
+
+The kernel is the central part of the operating system. It is responsible for:
+
+- Managing memory and CPU time.
+- Talking to hardware devices (disks, network cards, etc.).
+- Creating and scheduling processes.
+- Enforcing basic security boundaries between processes.
+
+Users and applications normally do not talk directly to the kernel; they use system calls via libraries and tools.
+
+### 3.2 User Space
+
+User space contains all programs that run "on top" of the kernel, such as:
+
+- Command-line tools (`ls`, `cp`, `mv`, `ps`, etc.).
+- System utilities (`ip`, `systemctl`, `journalctl`, etc.).
+- Libraries (for example the C standard library `glibc`).
+
+These tools are what administrators and scripts interact with in daily work.
+
+### 3.3 Shell
+
+The shell is the command-line interpreter used to run commands and scripts. Common shells include:
+
+- **Bash** (Bourne Again SHell).
+- **Zsh**.
+- **Fish**.
+
+The shell provides features such as:
+
+- Command history and completion.
+- Variables and environment management.
+- Scripting (loops, conditions, functions).
+
+For concrete shell usage and commands, see:
+
+- [Linux / Cheat Sheet](./CheatSheet.md)
+
+---
+
+## 4. Filesystem and Paths
+
+Linux uses a single hierarchical filesystem tree that starts at the root directory `/`.
+
+Typical top-level directories include:
+
+- `/bin`, `/usr/bin` – essential user commands.
+- `/sbin`, `/usr/sbin` – system administration commands.
+- `/lib`, `/usr/lib` – shared libraries.
+- `/etc` – system-wide configuration files.
+- `/var` – variable data (logs, spool, caches).
+- `/home` – home directories for regular users.
+- `/root` – home directory for the `root` user.
+- `/tmp` – temporary files.
+
+Additional mount points (such as `/mnt`, `/srv`, or custom locations) are used for disks, network shares, or application data.
+
+Paths can be:
+
+- **Absolute** – start with `/` (for example `/etc/hosts`).
+- **Relative** – based on the current working directory (for example `../logs`).
+
+---
+
+## 5. Users, Groups, and Permissions
+
+Linux is a multi-user system. Access control is based on users, groups, and file permissions.
+
+### 5.1 Users and Groups
+
+- Each user has a numeric user ID (UID) and belongs to one or more groups (GID).
+- The special user `root` (UID 0) has full administrative privileges.
+- Regular users have limited permissions and may use tools like `sudo` to perform administrative tasks.
+
+### 5.2 File Permissions
+
+Every file and directory has:
+
+- An owner user.
+- An owner group.
+- Permission bits for **user**, **group**, and **others**:
+	- Read (`r`)
+	- Write (`w`)
+	- Execute (`x`)
+
+Permissions control who can read, modify, or execute a file or enter a directory.
+
+---
+
+## 6. Processes and Services
+
+### 6.1 Processes
+
+A process is an instance of a running program. Each process has:
+
+- A process ID (PID).
+- A parent process.
+- An associated user.
+
+Basic operations include listing processes, checking resource usage, and sending signals to stop or reload them.
+
+Concrete commands for process inspection are listed in:
+
+- [Linux / Cheat Sheet](./CheatSheet.md)
+
+### 6.2 Services and systemd
+
+Many modern distributions use **systemd** as the init system and service manager.
+
+systemd is responsible for:
+
+- Starting services at boot.
+- Managing service dependencies.
+- Monitoring and restarting services on failure.
+
+Service definitions are stored as **units** (for example `ssh.service`, `docker.service`).
+
+For more detailed administration topics (services, logs, units), see:
+
+- [Linux / Administration](./Administration.md)
+
+---
+
+## 7. Networking Overview
+
+Linux provides a full networking stack, including:
+
+- IPv4 and IPv6 addressing.
+- Routing between interfaces.
+- Firewalling and packet filtering.
+
+Network configuration usually involves:
+
+- Assigning IP addresses to interfaces.
+- Setting gateways and routes.
+- Configuring DNS resolvers.
+
+For networking concepts specific to this knowledge base, see:
+
+- [Network / Basics](../Network/Basics.md)
+
+---
+
+## 8. Package Management
+
+Each distribution uses a package manager to install, update, and remove software:
+
+- Debian/Ubuntu: `apt`, `apt-get`, `dpkg`.
+- RHEL/Fedora: `dnf`, `yum`, `rpm`.
+- Arch Linux: `pacman`.
+
+Package managers handle:
+
+- Downloading software from repositories.
+- Dependency resolution.
+- Keeping a local database of installed packages.
+
+Concrete command examples for package management can be added to the Cheat Sheet or distribution-specific notes.
+
+---
+
+## 9. Where to Go Next
+
+- For hands-on commands and quick reminders, see [Linux / Cheat Sheet](./CheatSheet.md).
+- For administration tasks (services, logs, users, updates), see [Linux / Administration](./Administration.md).
+- For networking details, see [Network / Basics](../Network/Basics.md).
+
+This basics document is intended as a neutral foundation that other, more specific guides can reference.

Doc/Linux/CheatSheet.md

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+# Linux Essential Commands – Cheat Sheet
+
+Each block groups related commands and includes inline comments so it can be copied and pasted as a whole.
+
+---
+
+## 1. Navigation & Help
+
+```bash
+# Show current directory
+pwd
+
+# List files (basic / long / including hidden)
+ls
+ls -l          # long listing (permissions, owner, size, date)
+ls -a          # include hidden files (starting with .)
+
+# Change directory
+cd /path/to/dir
+cd ~           # go to home directory
+cd ..          # go up one level
+
+# Command help
+man ls         # full manual page
+ls --help      # quick help for a command
+```
+
+---
+
+## 2. Files & Directories
+
+```bash
+# Create or update an empty file
+touch file.txt
+
+# Copy files and directories
+cp source.txt dest.txt
+cp -r src_dir/ dest_dir/    # copy directory recursively
+
+# Move or rename
+mv oldname.txt newname.txt
+mv file.txt /target/dir/
+
+# Remove files and directories
+rm file.txt                 # remove file
+rm -r dir_to_remove/        # remove directory recursively
+rm -rf dir_to_force/        # force remove (use with care)
+
+# Create directories
+mkdir new_dir
+mkdir -p a/b/c              # create nested directories
+```
+
+---
+
+## 3. Viewing & Searching Content
+
+```bash
+# View file content
+cat file.txt                # print whole file
+less file.txt               # scrollable view (quit with 'q')
+
+# View start / end of a file
+head file.txt               # first 10 lines
+head -n 20 file.txt         # first 20 lines
+tail file.txt               # last 10 lines
+tail -f logfile.log         # follow log in real time
+
+# Search text
+grep "pattern" file.txt        # search in single file
+grep -r "pattern" /path/dir    # recursive in directory
+grep -ri "pattern" /path/dir   # recursive and case-insensitive
+```
+
+---
+
+## 4. Permissions & Ownership
+
+```bash
+# View permissions and ownership
+ls -l /path/to/file_or_dir
+
+# Change permissions
+chmod 644 file.txt          # owner read/write, group/others read
+chmod 600 secret.txt        # owner read/write only
+chmod +x script.sh          # add execute bit
+
+# Change owner and group
+chown user:group file.txt
+chown -R user:group /path/to/dir   # recursive
+```
+
+---
+
+## 5. Users, Groups & sudo (Debian/Ubuntu style)
+
+```bash
+# Identify current user and groups
+whoami
+id                          # show UID, GID, and groups
+
+# Run a command as root (if allowed)
+sudo command                # run single command with privileges
+
+# User management (Debian/Ubuntu)
+sudo adduser newuser        # interactive user creation
+sudo passwd newuser         # set or change password
+
+# Add user to an additional group
+sudo usermod -aG sudo newuser      # grant sudo rights (example)
+sudo usermod -aG docker newuser    # allow use of docker group
+```
+
+---
+
+## 6. Processes & System Monitoring
+
+```bash
+# List processes
+ps aux                      # all processes with details
+
+# Real-time monitoring
+top                         # built-in monitor
+htop                        # improved top (if installed)
+
+# Kill a process by PID
+kill 1234                   # ask process to terminate
+kill -9 1234                # force kill (SIGKILL)
+
+# Jobs in current shell
+jobs                        # list background jobs
+fg %1                       # bring job 1 to foreground
+bg %1                       # send job 1 to background
+```
+
+---
+
+## 7. Services & Logs (systemd)
+
+```bash
+# Service management (systemd)
+sudo systemctl status nginx.service     # check status
+sudo systemctl start nginx.service      # start service
+sudo systemctl stop nginx.service       # stop service
+sudo systemctl restart nginx.service    # restart service
+sudo systemctl enable nginx.service     # start at boot
+sudo systemctl disable nginx.service    # do not start at boot
+
+# View logs with journalctl
+sudo journalctl -u nginx.service        # logs for one unit
+sudo journalctl -u nginx.service -f     # follow logs
+sudo journalctl -b                      # logs from current boot
+```
+
+---
+
+## 8. System Information & Resources
+
+```bash
+# Basic system info
+uname -a                    # kernel and system info
+hostname                    # system hostname
+
+# Disk and filesystem usage
+df -h                       # disk usage by filesystem
+du -sh /path/to/dir         # summary size of a directory
+
+# Memory and swap usage
+free -h
+
+# Uptime and logged-in users
+uptime
+w                           # who is logged in and what they do
+```
+
+---
+
+## 9. Networking Basics
+
+```bash
+# IP addresses and links
+ip a                        # show interfaces and addresses
+ip r                        # routing table
+
+# Connectivity tests
+ping 8.8.8.8                # test raw connectivity
+ping example.com            # test DNS + connectivity
+
+# Open/listening ports
+sudo ss -tulpn              # TCP/UDP sockets with PIDs
+
+# HTTP requests and downloads
+curl https://example.com    # simple HTTP GET
+wget https://example.com/file.iso   # download file
+```
+
+---
+
+## 10. Storage & Mounts (Conceptual Commands)
+
+```bash
+# List block devices (disks/partitions)
+lsblk                       # tree view of devices and mount points
+
+# Show filesystems and usage
+df -h                       # mounted filesystems
+
+# Show current mounts
+mount | grep "/"
+
+# View /etc/fstab (persistent mounts)
+sudo nano /etc/fstab        # edit with care
+```
+
+---
+
+## 11. Archives & Compression
+
+```bash
+# Create and extract tar archives
+tar -cf archive.tar file1 dir2          # create tar
+tar -xf archive.tar                     # extract tar
+
+# Create and extract compressed tar.gz
+tar -czf archive.tar.gz file1 dir2      # create tar.gz
+tar -xzf archive.tar.gz                 # extract tar.gz
+
+# Zip archives
+zip archive.zip file1 file2             # create zip
+unzip archive.zip                       # extract zip
+```
+
+---
+
+## 12. Package Management (Debian / Ubuntu)
+
+```bash
+# Update package lists and upgrade
+sudo apt update                         # refresh package index
+sudo apt upgrade                        # upgrade installed packages
+
+# Install, remove, purge packages
+sudo apt install htop                   # install package
+sudo apt remove htop                    # remove package (keep config)
+sudo apt purge htop                     # remove package + config
+
+# Search and inspect packages
+apt search nginx                        # search in repositories
+apt show nginx                          # show details
+```
+
+---
+
+## 13. SSH & Remote Access
+
+```bash
+# Connect to a remote host
+ssh user@server.example.com
+
+# Use a specific key and port
+ssh -i ~/.ssh/id_ed25519 -p 2222 user@server.example.com
+
+# Copy files over SSH
+scp file.txt user@server.example.com:/remote/path/
+scp -r dir/ user@server.example.com:/remote/path/
+```
+
+---
+
+## 14. Shutdown & Reboot
+
+```bash
+# Shutdown and reboot (systemd)
+sudo shutdown now                      # immediate shutdown
+sudo shutdown -r now                   # immediate reboot
+sudo shutdown +5                       # shutdown in 5 minutes
+
+sudo reboot                            # reboot system
+sudo poweroff                          # power off system
+
+sudo systemctl reboot                  # reboot via systemd
+sudo systemctl poweroff                # power off via systemd
+```
+
+---
+
+## 15. Shell Shortcuts (Bash)
+
+- `Ctrl + C` – Cancel current command
+- `Ctrl + D` – Logout / send EOF
+- `Ctrl + R` – Search in command history
+- `↑` / `↓` – Navigate command history
+- `Tab` – Auto-completion

Doc/Linux/Storage.md

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+# Linux – Storage
+
+Overview of storage concepts on Linux systems. It Focus on how disks, partitions, file systems, and mount points fit together.
+
+General introduction to Linux and its filesystem layout:
+
+- [Linux / Basics](./Basics.md)
+
+---
+
+## 1. Disks, Partitions, and Devices
+
+On Linux, disks and partitions are exposed as device files under `/dev`.
+
+Typical examples:
+
+- `/dev/sda`, `/dev/sdb` – whole disks (SATA/SCSI).
+- `/dev/nvme0n1` – NVMe disk.
+- `/dev/sda1`, `/dev/sda2` – partitions on `/dev/sda`.
+- `/dev/nvme0n1p1` – partition on an NVMe disk.
+
+Key ideas:
+
+- A **disk** can contain one or more **partitions**.
+- A **file system** is created on a partition (or on other block devices such as LVM volumes).
+
+---
+
+## 2. File Systems
+
+Common Linux file systems include:
+
+- **ext4** – widely used default file system on many distributions.
+- **xfs** – often used on servers, good for large files and parallel workloads.
+- **btrfs**, **zfs** – advanced file systems with snapshots and additional features.
+
+Responsibilities of a file system:
+
+- Organising data into files and directories.
+- Tracking metadata (permissions, timestamps, ownership).
+- Handling allocation on the underlying block device.
+
+The choice of file system depends on requirements such as performance, robustness, and feature set (for example snapshot support).
+
+---
+
+## 3. Mount Points and the Unified Tree
+
+Linux presents storage through a single unified directory tree.
+
+To make a file system available, it is **mounted** at a directory path (the mount point).
+
+Examples:
+
+- A root file system on `/`.
+- Additional storage mounted under `/srv`, `/mnt`, or `/data`.
+- Removable media mounted under `/media` or `/run/media`.
+
+Important aspects:
+
+- The same directory path can hide existing contents when a new file system is mounted on top of it.
+- Consistent mount points simplify backups and documentation.
+
+---
+
+## 4. /etc/fstab and Persistent Mounts
+
+The file `/etc/fstab` defines which file systems are mounted at boot.
+
+Each entry typically specifies:
+
+- The device (for example a UUID or `/dev/sda1`).
+- The mount point (for example `/srv/data`).
+- The file system type (for example `ext4`).
+- Mount options.
+
+Conceptual points:
+
+- Using UUIDs or labels makes mounts more stable than relying on `/dev/sdX` names.
+- Mount options can influence performance and safety (for example `noatime`, `defaults`, `ro`).
+- Misconfigured entries can prevent the system from booting cleanly, so changes should be tested carefully.
+
+Project‑specific mount layouts (for example for application data) can be documented alongside the systems that use them.
+
+---
+
+## 5. LVM and RAID (Overview)
+
+In addition to simple disk‑partition‑filesystem setups, Linux supports more advanced storage layers.
+
+### 5.1 LVM (Logical Volume Manager)
+
+LVM introduces an extra abstraction layer:
+
+- Physical volumes (PVs) are typically partitions or whole disks.
+- Volume groups (VGs) combine one or more PVs.
+- Logical volumes (LVs) are created inside VGs and used like regular block devices.
+
+Advantages:
+
+- Flexible resizing of logical volumes.
+- Ability to span volumes across multiple physical disks.
+
+### 5.2 RAID (mdadm and hardware RAID)
+
+RAID (Redundant Array of Independent Disks) is used for redundancy, performance, or both.
+
+Common levels:
+
+- RAID 1 – mirroring for redundancy.
+- RAID 5/6 – striping with parity.
+- RAID 10 – combination of striping and mirroring.
+
+Implementation options:
+
+- Software RAID with `md` devices (managed by tools such as `mdadm`).
+- Hardware RAID controllers that present a single virtual disk.
+
+LVM and RAID are often combined for flexibility and resilience.
+
+---
+
+## 6. Swap Space
+
+Swap provides additional virtual memory by using disk space when physical RAM is exhausted.
+
+Swap can be configured as:
+
+- A dedicated swap partition.
+- A swap file on an existing file system.
+
+Considerations:
+
+- Swap is much slower than RAM, but can prevent out‑of‑memory conditions.
+- On some systems, hibernation relies on swap configuration.
+
+---
+
+## 7. Monitoring Space Usage (Conceptual)
+
+Administrators need to keep track of available disk space and inode usage.
+
+Key concepts:
+
+- Total vs used vs available space for each mounted file system.
+- Inode counts (maximum number of files/directories a file system can track).
+- Distinguishing usage on the root file system from usage on separate data file systems.
+
+Concrete commands for checking disk and inode usage can be listed in:
+
+- [Linux / Cheat Sheet](./CheatSheet.md)
+
+---
+
+## 8. Backup‑Friendly Layouts
+
+Storage layout has a strong impact on backup and restore strategies.
+
+General considerations:
+
+- Separating system files from application and user data simplifies targeted backups.
+- Grouping related data under well‑defined mount points (for example `/srv`) makes it easier to snapshot or back up.
+- Avoiding unnecessary writes on system partitions can reduce wear on SSDs.
+
+Backup strategies themselves are usually documented alongside the systems or applications that depend on them.
+
+---
+
+## 9. Relation to Other Documents
+
+- High‑level Linux concepts: [Linux / Basics](./Basics.md)
+- Administration topics (permissions, services, backups): [Linux / Administration](./Administration.md)