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The UNIX Architecture

The UNIX Architecture

• Kernel

  • The core of the operating system.

    • The system's bootstrap program (a small piece of program code) loads the kernel into memory at startup.

  • Comprises a set of routines mostly written in C that directly communicate with the hardware.

  • User programs or the applications that need to access the hardware communicate with the kernel using a set of functions called system calls.

  • Does lots of resource management:

    • Manages the system's memory.

    • Schedules processes and decides their prioities.

    • Many more ...

  • Has work to do even if no user program is running.

  • Often called as the operating system - a program's gateway to the computer's resources.

  • Represented by the file /boot/vmlinuz.

• Shell

  • The interface between the user and the kernel. ("Outer part" of the operating system)

  • Interpreter that translates user commands into action.

  • Several shells could be run on only one kernel running on the system.

    • One for each user who is logged in.

  • Located in /bin or /usr/bin.

    xxxxxxxxxx
    2
    1
    $ echo $SHELL  
    2
    /bin/bash
    

  • Commonly used shells:

    • bash - Bourne again shell (advanced shell that is standard on Linux)

    • sh - Primitive Bourne shell

    • csh - (C shell - still popular today)

    • ksh - Korn shell

• Files

  • Containers for storing static information.

  • Everything in UNIX is considered as files even directories and devices. (Simplifying assumption in UNIX)

  • Related to another file by being part of a single hierarchical structure called the file system.

  • "Files have places and processes have life"

• Process

  • A program in execution.

  • Also form a hierarchy which is best understood when one process is considered as the child of another.

• System Calls

  • Special functions that programs invoke to access kernel services. (e.g., open, close, write, fprintf, etc.)

  • POSIX specifies the system calls that all UNIX systems must implement.

    • Once software has been developed on one UNIX system using the calls mandated by POSIX, it can be easily ported to another UNIX machine.