Introduction to *nix Systems

Introduction to *nix Systems

HKOI Training Team

Sidney Fong

Overview of this presentation

What to expect– Brief introduction to UNIX-like systems– Brief history of Linux– Introduction to basic Linux commands and

environment– Linux as competition environment

What not to expect– Detailed instructions on how to install Linux– Setting up Linux as a server

What is *nix?

“*nix” refers to a class of UNIX-like operating systems

The term “UNIX” is sometimes used to refer to *nix systems

Examples of *nix systems:– Linux (or GNU/Linux)– BSD (FreeBSD, NetBSD, OpenBSD, etc)– Solaris– Mac OS X (Darwin)– GNU Hurd

Typical Features of *nix Systems

Geared towards being a “server” / “networking” OS– Multi-user– Multi-tasking– Tends to focus more on command line interface

The Unix Philosophy– Write programs that do one thing and do it well– Write programs to work together– Write programs to handle text streams, because that is a

universal interface

Linux - Overview

For the rest of the session, we will focus on Linux

Important Figures

Linus Torvalds– Creator of Linux Kernel

Richard M. Stallman (RMS)

– Founder of GNU and the Free Software Movement

Names

Pronunciation of “Linux”– “lee-nix” ( /linəks/ )

Pronunciation of “GNU”– “guh-noo” ( /gnu/ )

GNU– recursive acronym for “GNU’s not UNIX”

“Linux” vs “GNU/Linux” debate

The GNU Project

Started by RMS to create a complete UNIX-like system in 1983

– Kernel, system tools, compilers, documentation, editors, etc. Ideology of Free Software Movement

– Free: “Free” as in Freedom– “Copyleft” Software: GNU General Public License (GPL)– Related: Open Source Movement

Most GNU tools were ready by the early 90’s. … Except the kernel: “GNU Hurd”

– Hurd was supposed to be a microkernel– Over-design? Wrong approach?

BSD

At the same time, another open source UNIX distribution, the BSD, was being sued by UNIX System Labs. (USL v. BSDi)

The lawsuit cast doubt upon the legal status of the BSD’s. The case was finally settled in 1994, with both parties agreeing not to litigate further.

This, together with the lack of a kernel in the GNU project, created a demand for a free UNIX-like kernel, which was fulfilled by Linux

Linux

Started by Linus Torvalds as a hobby project Inspired by Minix, an OS used as an example

in teaching principles of operating systems First release of Linux in September 1991 Free of the problems of GNU Hurd and BSD Eventually joined by other developers More:

– Just for Fun: The Story of an Accidental Revolutionary

A Typical Linux System

Linux Kernel User space programs (many are GNU software)

– Shell (command line): bash– Utilities: ls, cp, mv, rm, cat, grep, ...– Compiler: gcc– … and more …

X Window System (for desktops/workstations) Various daemons (a.k.a. services)

– Eg.: syslogd, httpd, cron, inetd

User space programs

Shell

A Rough Graphical Representation of a Linux System

Kernel

Hardware

X Client

Daemons

X Server

Linux “Distributions”

Since most software on Linux can be freely modified and re-distributed, people began packaging these software into “distributions”

A “distribution” is responsible for most of the software installed on the Linux system, including the base “operating” system and any other applications the user installs

– rpm, deb/apt, tgz Each distribution has it’s own “character”, differing in

– Purpose : Server, Workstation, Firewall, etc…– Installation / upgrade method– Choice of software– Way of packaging software (tgz, rpm, deb)– Default configuration– Aims and Ideology – Commercial, Promote Free Software, etc

Linux Distributions – Redhat / Fedora

The “standard” Linux Has a relatively high level of support from third party

vendors Geared towards commercial / corporate

environments Initial set up is generally easy Rather expensive if you buy the enterprise edition

(hundreds of USD!) Maintenance is a nightmare! ;-p

www.redhat.com

Mandriva (was “Mandrake”)

Originally based on Redhat, therefore somewhat similar to Redhat

Geared towards desktop and newbies

www.mandrivalinux.com

Slackware

Stable, simplistic One man distribution Zipslack, which works without having separat

e Linux partition(s) Relatively UNIX-like compared to other distri

butions

www.slackware.com

Gentoo

Source based installation!– The standard way of installing new software on th

e system is to compile it from source (using the “portage” system)

– This is equivalent to the BSD ports system– Takes LoOOong time to install / upgrade

www.gentoo.org

Debian

Made by a worldwide non-profit organization, with thousands of volunteers helping to maintain packages

Quality and quantity of packages are generally quite high

Slow release cycle – the last “stable” release was in July 2002!!

www.debian.org

Linux - Basics

An introduction to using Linux as a normal user

Post-Login Screen

“Message of the day”

Usually contains important notices

Username Hostname Current directoryHint: You may press

<Ctrl+Alt+F1> to <Ctrl+Alt+F6>for multiple terminals if you are

using Linux on the local machine(instead of over network)

Why is the username and hostname displayed in the command prompt?

Basic Commands

“ls” – List directory contents “cd” – Change directory “cp” – Copy file(s) “rm” – Remove file(s) “mv” – Move file(s) “echo” – Display a line (or lines) of text “mkdir” – Make (i.e. Create) Directory “cat” – Concatenate files and print to stdout “exit” / “logout” – Exit the shell

Example I

Try the following commands– ls # list directory– echo HKOI Cows # print a message on screen– ls -l # list directory (long format)– mkdir hello # create directory “hello”– cd hello # change directory to hello– cp /etc/motd . # copy file to current dir– ls -la # list directory– cat motd # display contents of file– rm /etc/motd # try to remove file (fails ;-p)– mv motd .. # move file to parent dir– cd .. # change to parent directory

Linux File System Hierarchy

In Linux, the file system is a (single) tree Everything begins from the root directory: / Non-exclusive listing of commonly used directories

– /home – User home directories– /bin, /usr/bin – Binaries (executables, programs)– /lib, /usr/lib – Libraries (similar to “DLL”s in Windows)– /etc – System configuration files– /var, /usr/var – Stores files that are often modified (eg. log files, dat

abase files, mail spool, print spool)– /dev – special device files– /tmp – Temporary files

In general, a normal user cannot modify the files outside his home directory (except /tmp)

Exploring the system

Try to navigate your way around the system with the above commands

You may view the contents of a file with the “cat” command

You can return to your home directory with the “cd” command (no arguments)

Make good use of tab completion

Text Editors

Vi (VIM)– Visual Editor– VIM: Vi Improved.

Emacs– Editor Macros– Nickname: Esc-Meta-Alt-Control-Shift

Nano– “Notepad”-like– Easy to use but less powerful than Vi / Emacs

Text Editors (How to quit)

Some programs may launch text editors that you may not know how to use -- even quitting is difficult!

VIM– <Esc> :q! (Quit without saving)

Emacs– <Ctrl-X> <Ctrl-C> (Quit)

Although not required, it is strongly recommended that you learn one of the above editors since they can increase your productivity quite significantly

Man(ual) pages

Example: man cp

Purpose of commandDifferent

usages

Detailed description of command and

its options

Hint:

Press ‘q’ to quit

*nix Quirks (compared to MSDOS)

Path separator is a slash “/” instead of backslash “\” Files are case sensitive “File extensions” are “meaningless”

– The “.” character in a file has no special meaning unless it is in the beginning of a filename

“.file” – hidden file “.” – current directory “..” – parent directory

– Executable files have an “executable bit” set, instead of a “.exe” extension

<Ctrl-D> is End of File (<Ctrl-Z> in DOS)

*nix Quirks II

There must be a space between the command and subsequent arguments

– Eg. “cd /” works, “cd/” does not Command line arguments start with “-” or “--” instead of “/”

– DOS: dir /OD– Linux: ls -t

Unix text format vs MSDOS text format– A line in *NIX systems ends with a ‘\n’ (ASCII 10)– A line in MSDOS ends with ‘\r\n’ (ASCII 13, 10)– The relevant functions of the C library in DOS/Windows converts ‘\

n’ to ‘\r\n’ automatically

*nix Quirks III

Unless the path the command is specified, the shell will only run commands located in the $PATH environment variable (why?)– Question: how to run a program on the current

directory? ./command

– To see what $PATH contains, you may issue the command: echo $PATH

– To edit the value of $PATH: PATH=(new value)

Write a program in Pascal that– Reads an integer n from standard input– Writes n random integers to standard output

Save it as numbers.pas Compile the program Execute the program Keep a backup copy in ~/backup/numbers.pa

s

Task I

I/O Redirection, Pipes

“A > B” redirects standard output from program A to file B. This operation will overwrite original contents of B (if any)

“A >> B” redirects standard output from program A and appends it to the end of file B.

“A < B“ redirects standard input from file B to program A “A | B” redirects standard output of program A to standard input

of program B Try the following commands:

– echo 100 | ./numbers– echo 10 > ten.txt– ./numbers < ten.txt– echo 20 | ./numbers | sort –n

Hint: Also works in MSDOS and

cmd.exe

Task II

Download http://www.hkoi.org/~sydneyfong/public/openq.zip to home directory and unzip the contents (use wget / w3m)

Change working directory to ~/prog0 Write a program to solve the HKOI “Open Question” (see HKOI

2005 website) (You may try to write a “wrong” one (i.e. “incorrect solution 4”) f

or checking answers) Save the program as “program0.pas” Run the program using inputs from the files input1.txt to input5.t

xt, redirect the output to output1.txt to output5.txt Check your answers by using `diff`

More on bash shell

alias – substitute a string as a command– Eg. alias ls=‘ls --color’ – You may list current aliases by simply issuing the command “alias”

without arguments The script ~/.bash_profile is run during login The script ~/.bashrc is run in non-login shells

– You may set aliases, environment variables and run commands in these files to customize your shell

– You may want to make the two files identical– Aliases that you may want to have by default:

alias cp=‘cp -i’ # asks for confirmation before overwriting a file alias mv=‘mv -i’ # asks for confirmation before overwriting a file alias rm=‘rm -i’ # asks for confirmation before deleting a file

More on man pages

There are different sections of man pages

– Section 1: commands– Section 2: system calls– Section 3: C library calls– Section 4: special files (in /dev)– Section 5: (config) file formats– Section 6: “games”– Section 7: Misc– Section 8: System administrati

on commands

If there are conflicts in names, the section of lower value is displayed. For a specific section:

– man <section> <page> Note that you may find secti

on 3 very useful as a reference to standard C functions (eg. printf, fopen, string functions, etc)

Special files (in /dev)

/dev/zero – returns an endless stream of ‘\0’ characters– To create an file of size 1MB:

dd if=/dev/zero of=myfile.img bs=1k count=1024

/dev/null– Always return EOF when read

cat /dev/null > abc – creates empty file of size zero

– All data written to /dev/null is discarded ls > /dev/null – mutes output of ls

Tips in competitions…

Practical advice for competition environments

Useful Commands in Competitions

bc – Arbitrary precision calculator sort – Sort lines of text files (note the “-n” option) ps – list running processes on the machine kill – Sends a signal to running programs (terminates it by default) date – Print system date/time time – reports execution time of a command diff – compares files head, tail – prints the first/last N lines of the input wc – word count (also counts characters and lines) more – pager (allows you to scroll the input for easy viewing) less – better version of more grep – print lines matching a pattern find – search for files factor – factorize numbers tsort – perform topological sort seq – print a sequence of numbers file – determine file type (sometimes displays other useful information) tar – Archive files (“glue” multiple files into one, and vice versa)

Compilers - fpc

In most cases, fpc <file> would do Useful options

– -Co (check for integer overflow)– -Cr (range check)– -O1, -O2, -O3…

In some systems the man page is outdated

Compilers – gcc/g++

use -o <output> to specify output file, otherwise the default is “a.out”

Most gcc options apply to g++ Normally you would do:

– gcc -O2 -o program1 program1.c– g++ -O2 -o program1 program1.cc

Remember to compile your program using the flags given by the judge/competition. (In particular, the optimization flags sometimes causes trouble)

If you want, you may also run `make <prog>`