Linux Commands and Architecture Notes

Linux Commands and Architecture

Overview of Linux Architecture

  • Three Main Components:

  • Kernel: Core part of the OS, responsible for communication between hardware and applications.

  • System Libraries: Provide necessary methods for software development, ensuring applications don’t communicate directly with the kernel.

  • System Utilities: Programs performing individual tasks, e.g., command-line interfaces such as Bash.

Kernel Responsibilities

  • Device Management: Handles communication with hardware devices.

  • Memory Management: Allocates and manages computer memory.

  • Process Management: Controls execution of processes (running programs).

  • System Call Handling: Manages requests from user applications to the kernel.

Runlevels and System Boot

  • Boot Process: Initiated by UEFI/Bios reading GPT/MBR, followed by executing boot loaders like GRUB.

  • Runlevels: Different operational states (e.g., single-user, multi-user).

  • Example: Runlevel 5 for GUI mode.

Shell and File System

  • Shell: Command-line interface through which users interact with the system’s kernel.

  • File System: Organized structure of files and directories in a hierarchical tree format.

Command Structure in Linux

  • Command: A keyword representing an executable program.

  • Shell Prompt: User input interface displaying a $ for regular users or # for superusers.

Basic Command Syntax

  • Syntax: commandname [options] [arguments]

  • Commandname = the command keyword.

  • Options = modifiers that change command behavior (e.g., -l).

  • Arguments = additional input required by the command.

Examples of Basic Commands

  1. who: Display users currently logged in.

   $ who

  1. uptime: Shows how long the system has been running.

   $ uptime

  1. tty: Displays the current terminal.

   $ tty

  1. cal: Displays a calendar (default view is the current month).

   $ cal

  1. file: Returns the file type of a specified file.

   $ file /etc/passwd

  1. echo: Displays a line of text or variable values.

   $ echo "Hello World"

Advanced Command Features

  • Redirection: Redirect output to a file.

   $ command > output.txt

  • Piping: Passes the output of one command as input to another command.

   $ command1 | command2

  • Chaining Commands: Use ; to execute multiple commands sequentially.

   $ command1; command2

  • Background and Foreground Tasks: Use & to run a command in the background.

  $ command &

The grep Command

  • Search for Patterns: Searches for lines matching a pattern in a file.

  $ grep "pattern" filename

  • Common Options:

  • -i: Ignore case distinctions.

  • -c: Count matching lines only.

Searching for Files

  • locate: Quickly finds files by name.

  $ locate filename

  • find: Searches for files with broader criteria (name, type, size).

  $ find /path -name "file.txt"

Help and Documentation

  • Use man to access manual pages for detailed command usage.

  $ man command

  • --help option offers a brief help message for built-in tools.

  $ command --help

Summary of Basic Linux Commands

  • whoami: Display current username.

  • pwd: Show current directory.

  • ls: List directory contents.

  • cd: Change directory.

  • touch: Create/modify file timestamps.

  • wc: Count lines, words, characters in files.

  • echo: Print text to the terminal.

  • cat: Display file content, concatenate files.

  • more/less: View file content page by page.

  • head/tail: Display first/last n lines of files.

  • sort: Sort lines in a file.

  • mkdir: Create a directory.

Conclusion

The Linux command line and file system structure allows effective management and interaction with both the user and the operating system, paving the way for powerful system administration and software development tasks.