LINUX An Introduction Wan Ahmad Tajuddin Wan Abdullah Universiti Malaya KL ACGRID-II 2009

Operating Systems (OS) machine OS user System software that abstracts the interface to various resources provided by low-level system hardware and software; and that manages these resources.  distinct resources can be shared effectively  the programmer can use system resources easily [user provided with a virtual machine] * user interface * process management and scheduling * job and session management * device management * timing management * memory management * file management * error handling * reliability * security * monitoring * accounting * system management

OS: Processes independent task units which are executed e.g. a batch job, the shell of a login session, a program run The execution of processes is controlled by the kernel and is interlaced – this part of the kernel is known as the despatcher or low-level scheduler when a process ends, or uses up its scheduled time, or cannot be continued because an interrupt occurred or the process is waiting for something or the error trap suspends the execution of the process while an error is handled the despatcher executes the next process in the process queue according to priority

OS: Kernel most basic layer of operating system, which communicates with hardware provides environment where processes can exist:  handling of interrupts  changing processor usage from process to process  implements mechanisms for communication and coordination between processes (semaphores, shared memory, message passing, input/output redirection, pipes,…), including deadlock avoidance and recovery Thread – flow of control between processes; a process can spawn daughter processes and so on shares process resources

OS: File structure Files are contained in directory and subdirectory (which is also a kind of file) structure which is tree-like. Uppermost directory – main file directory or root directory (‘/’ or ‘\’). / /A/B /C /B/D/B/E /B/D/f /A/b /A/a/B/g /B/D/ e /B/D/c /C/h

Open Systems implement open specifications adequately for interfaces, services, and support formats which allow application software built appropriately to be ported onto various types of systems with minimal change, to interoperate with other applications on local and remote systems, and to interact with users in a style which facilitates user mobility portability, interoperability, non-dependency, flexibility, integrability Open specification a public specification maintained by an open public consensus process, to include current new technology and which is consistent with standards. increased consultation with respect to system and application provider due to advantage from clear description of system needs Example of open operating system: UNIX System V Example of open specifications: POSIX (Portable Operating System Interface)

LINUX UNIX-like OS initially for PCs Open source – free, standards, community, modifiable Monolithic kernel – single standard system calls Distribution suites (Red Hat, Ubuntu, Scientific Linux, …) - together with other open source (esp GNU) applications CLI: shells (c-shell, Bourne again,...) GUI: based on X-Windows Desktop environments (KDE, Gnome,...)

Legacy of UNIX programmers environment simple user interface many, simple utilities that can be combined to perform powerful functions hierarchical file system simple interface to devices consistent with file format, supports I/O redirection and piping multi-user, multi-process system architecture independent and transparent to the user. Written in C – close to C, portable

LINUX User Interface Command Line Interface – shells (processes) Line commands: command –flags arguments Arguments can be e.g. filenames stdin (standard input – keyboard), stdout (standard output – monitor), device drivers (e.g. /dev/lp) are treated like files Graphics User Interface – based on X Windows (processes)

LINUX File system Hierachical tree-like structure of (sub)directories and files Root directory: / Subdirectories usually e.g.: Filenames have extensions after a dot, usually describing type of file e.g. prog.c, files.tar, files.tar.gz binBinary (executable) programs homeHome directories of users etcMiscellaneous system files devSpecial files for I/O devices libLibraries usrUser files tmpTemporary files

File can be link to other file or (sub)directory File protection mode: defines priviledges for owner, group (of users where owner is in), (the rest of the) world, e.g. rwxr-x--x : owner can read, write, execute; group can read, execute; world can execute only (binary: ) Superuser (root) have all priviledges to all files

Networking Protocol: TCP/IP Hosts: domain name address (host.domain), IP address (number: xxx.xxx.xxx.xxx) Services:  Remote terminal – telnet, ssh  File transfer – ftp, scp  Web – http  Mail – smtp  etc

Hands on o Introduction o GUI, tools and scripts o [System: processes and threads] o [Network programming]