Linux System (Review)

You are expected to be already familiar with the following concepts.

Linux Distros

Linux is only the kernel and not the full operating system. Linux distributions bundle additional utilities and software, and sometimes services to provide a full operating system. Linux distros usually include the following in their OS:

• Linux kernel (sometimes with additional modules and drivers)
• GNU coreutils utilities are standard (cd, ls, cp, rm, mv, pwd, … )
  • Official terminology: GNU/Linux Operating System:
    • Linux Distros = Linux kernel + GNU utilities + Other software + customizations
  • Some distros are switching to Rust-based core utilities instead of the GNU C-based ones
• pre-compiled software for a specific platform such as x86_64, ARM, PowerPC, Intel Itanium (PA_RISC) etc…
• Linux Distros could be fully open-source (Debian), or include additional proprietary code (Ubuntu, Red Hat)
• Linux distros may also curate and maintain software repositories for their distribution (for example, Ubuntu Universe, Multiverse and PPA repositories)

Linux Filesystem

• Everything is a file!
  • Common file types in Linux: regular files, directories, device files, links, etc.
  • No registry, no add/remove programs, no drive letters for storage devices
• Everything is located under / (root)
• File name and attributes are stored in the file system itself
  • Permissions are for User, Group, and Others (example -rwxr-xr-x = 755)
  • Every user on the system is required to have sufficient permission to access any file
• Storage is “mounted” under a directory somewhere under / (root)
• File paths:
  • Relative paths ./directory/file (starts with ./ or ../ relative to current working path)
  • Absolute path /home/user/directory/file (starts with /, pointing from root to specific file)

Users and Groups

• /etc/passwd and /etc/shadow files contain user accounts and their authentication definitions and properties
• Root user:
  • Sometimes referred to as “super user”
  • Has access to everything, WARNING! The root user can do almost anything, including irreparable damage!
• Regular users and groups
  • Each user has a username, a password, a home directory, and “usually” a login shell (if a human user)
  • Each user can be a member of at least one, or more groups
  • Groups are used to organize users and manage user permissions efficiently
• System users and groups
  • Usually have no password or login shell (log in is effectively disabled)
  • They are used to manage system processes and services
  • In Debian and Ubuntu, the system user UID and GID are set below 1000 by convention only (not technically enforced)
• Commands: adduser, addgroup, deluser, delgroup, etc.
• /etc/skel contains files that are copied to users’ home directory (like a “template”) by the adduser utility when creating a new user account

Access to System Shell

Terminal is text-only input and output. Types of terminals are:

• Local terminal
  • Serial port, or onboard management computer on some servers
  • Keyboard and Monitor
• Remote Terminal (using network protocols):
  • SSH server (sshd)
    • Allow access authenticating via password or SSH key (similar to RSA)
  • SSH Client (on the remote computer) and a terminal emulator software

The user uses the terminal to access the CLI Shell. Shell is the software program that allows users to interact with the system. There are CLI (Command Line Interface) and GUI (Graphical User Interface) shells, but GUI shells are rarely installed on servers.

• Login shell: the user’s default shell software when the user logs in
  • If the login shell process is terminated (killed), the user is logged out (kicked out)
• Example CLI shells: sh, bash, zsh, fish, etc.

Using the CLI

• Getting help: whatis and man utilities
• Switching user: su and sudo utilities
• Getting user session information: id, who, last, w
• bash shell special characters: <, >, ~, '', "", ``, |, \, ., .., etc.

Hardware Management

In Linux, device drivers are either included and compiled with the Linux kernel (preferred, easiest to use) or loaded as kernel modules (for proprietary, non-standard, or non-open-source hardware). Generally, you are not required to load or install device drivers in Linux. The system boots and loads the Linux kernel, and your hardware is either detected and functions out of the box, or is incompatible with the version of the kernel and will not work.

Hardware vendors provide proprietary drivers for their hardware in the form of special versions of the Linux kernel, or as kernel modules to load alongside your regular kernel. The kernel module version must match your kernel. Most vendors submit their open-source drivers and their firmware to the mainline Linux kernel for inclusion. Firmware can be proprietary, but the inclusion of the drivers and firmware in the mainline Linux Kernel by the hardware vendor is the most desirable and advantageous solution for users.

Utilities for viewing hardware information:

lshw # list all recognized hardware
lspci # list all PCI devices
lsusb # list USB devices
lsblk # list block devices
lscpu # display information about the CPU architecture

lshw # list all recognized hardware
lspci # list all PCI devices
lsusb # list USB devices
lsblk # list block devices
lscpu # display information about the CPU architecture

Storage and Storage Devices

• Each block storage hardware device has a device file under /dev
  • Example /dev/sda, /dev/sdb, /dev/fda, or /dev/nvmep1s0, etc.
  • Use lsblk (list block devices) or fdisk -l to list all block storage devices and their partitions
• Each file system can be “mounted” under any directory under root (as long as the user has permissions to read and write to that directory)
  • Example /boot (partition that contains the Linux kernel images and kernel modules used at boot time)
• Use mount, umount, and eject (for removable media) utilities
• Use df command to view mounted filesystems (example: df –h # -h shows output in “human readable format”)
• Use du to see the disk usage of files
• Additional utilities such as iostat and sar can be used to monitor storage activity

System Activity Monitoring

ps, vmstat, prtstat, netstat, top (utilities that show system utilization)
many flavours or variations of the top utility available
Examples: htop, vtop, iftop, slabtop
Other utilities: memstat, mpstat, nfsiostat, cifsiostat

Here is a screenshot from htop, an equivalent to, but easier to use than, the top utility:

Package management (Ubuntu)

• Ubuntu is based on Debian: uses .deb package formats and dpkg utility for package management

• apt-get is a “front-end” utility for dpkg, to offer ease of use for end users

• apt is a “front-end” utility for the apt-get utility, to simplify the operations even more for end users

• Package managers maintain their own database

  • keeps track of all software packages available (installed, or available to install, what the versions are, where the files are located, etc.)
  • sudo apt update updates the local DB from the online repository
  • sudo apt upgrade upgrades the local software packages to the latest version available from the repository
  • sudo apt [install | remove] <package name>

• dpkg is still used, although less often, for manual and advanced tasks

  • sudo dpkg –i <packagename.deb> (-i for install)
  • sudo dpkg -L <packagename> (-L for listing all files owned by a package)
  • sudo dpkg -S <file or directory> (-S to search the database for finding which software package uses a file or directory, basically a reverse search if you want to find out who put this file here)

• For re-configuring packages (re-running the installation script):

  • sudo dpkg-reconfigure <packagename>

Example screenshots:

Linux Administration

There are different aspects of (any) system administration:

• System resource
• Users and access control
• Software, application, and services
• Networking and network services

When you are working as an IT administrator, you may have to take on one or multiple of these roles. The IT field is very vast, and there is more knowledge and skills than any one person can learn and manage. In very large deployments or large organizations, some roles are narrowly focused, for example, Cloud Specialist, Storage Specialist, Network Specialist, System or Network Security specialist, Database Specialist, etc.

The goal of this course is to provide a general baseline of skills and information required to get you started in the field. You may find yourself working as a "generalist" in one field, while you may find a need to become a "specialist" in another sub-field in your career.

Basic Linux Installation Requirements

There are multiple methods of creating a Linux instance depending on the target system and mission requirements. As examples, you may deploy an installation on bare-metal hardware, virtual machines, or create templates for cloud deployments or containers. In general, any Linux system installation needs the following information:

• Every setup at least needs:
  • Hostname
  • Network Configuration
  • First user account
• Additional configuration may include:
  • Storage
  • Software bundles and repositories
  • Localization
    • Language, keyboard layout, time zone, etc.

Interesting Files, Utilities, and Commands

• hostnamectl
• useradd, usermod, userdel
• adduser, deluser
• groupadd, groupmod, groupdel
• addgroup, delgroup
• ssh, ssh-keygen
• id, who, w, last
• mkfs, mount, umount, eject, fdisk
• lshw, lspci, lsusb, lscpu, lsblk
• iostat
• df, du, lstree
• sar
• ps, vmstat, prtstat, vmstat, top,
• memstat, mpstat, nfsiostat, free
• cifsiostat
• apt, apt-get
• dpkg, dpkg-reconfigure
• /dev
• /boot
• /etc/fstab
• /etc/default/sysstat
• /var
• /etc/hostname
• /etc/hosts
• /etc/skel
• /etc/sshd_config
• ~/.ssh