UNIX Slides

What is this repository for?

Set of slides about UNIX operating systems
Version: 0.1

Introduction to UNIX

Why UNIX?

One of the most used operating systems nowadays
- Very popular in the academic world, government agencies, and industry
Different variants of UNIX, yet all of them share some basic features (conforming to several standards)
- Hierarchical file system
- Identical interfaces for data, devices and interprocess communication
- Background processes and daemons
- Synchronous and asynchronous processing
- Standardized tools
- High degree of portability
Linux is based on Unix
- Aims to conform to the various UNIX standard, but no Linux distribution is branded as UNIX
- The main problems are time and expenses
- Its de facto near-conformance with UNIX standards, is what have enabled Linux success in the UNIX market

A bit of UNIX history

1969: Development started by Ken Thompson at Bell Laboratories (AT&T corporation)
- Written in assembly code for a digital PDP-7 computer
- Some ideas taken from MULTICs (Multiplexed Information and Computing Service)
  - MULTICs is an earlier operating system (project collaboration between AT&T and MIT)
  - AT&T withdrew from MULTICs, frustrated at the failure to develop an economically viable and usable OS
  - Indeed Brian Kernigham (one of the fathers of the C programming language and also researcher at Bells labs at the time) called it UNICs (Uniplexed Information and Computing Service), name which derived later into UNIX
1970: Unix is rewritten in assembly language for another computer (the PDP11 back then a powerful machine)
- Vestiges of this PDP-11 heritage can be found in various names still in used in most UNIX implementations, including Linux

A bit of UNIX history

A landmark we cannot omit in the UNIX history is the design and implementation of the C programming language by Dennis Ritchie and Kernigham
1973: The C language was mature to a point where UNIX kernel could be almost entirely written in C
- UNIX became one of the earliest operating systems to be written in a high-level language
- This fact made subsequent porting of UNIX to other hardware architectures possible
- It also explains why C (also C++) have come to be used so widely as system programming languages today

Unix Versions

Between 1969 and 1979, UNIX went through a number of releases known as editions (each edition brought several milestones)
- 1st Edition (November 1971): Run on PDP-11 and contained many programs used still today: ar, cat, chmod, cp, cd, ed, find, ln, ls, mail, mkdir, mv, rm, sh, su, and who
- 2nd Edition (June 1972): Installed on 10 machines within AT&T
- 3rd Edition (February 1973): C compiler was included and provided the first implementation of pipes
- 4th Edition (November 1973): Almost totally written in the C language
- 5th Edition (June 1974): UNIX was installed on more than 50 systems
- 6th Edition (May 1975): First edition widely used outside AT&T

Inconveniences are sometimes very convenient

Over that period of time, UNIX reputation began to spread within AT&T and beyond
Problem: AT&T could not sell software at the time: it was sanctioned by the US government
The provided solution is probably one of the most important ones in computer science history: at the beginning of 1974 AT&T allowed the use of UNIX within universities

Two important variants (BSD and System V)

January 1979 saw the release of the 7th edition of UNIX, which included a) improved reliability and file systems and b) a number of new tools: awk, make, sed, tar, uucp, the Bourne shell, and a Fortran compiler
From this point UNIX derived into two important variants: BSD UNIX and System V
To understand that divergence we need to go back to 1975/1976, when Thomson (the guy who wrote the first version of UNIX) spent a year as visiting professor at the University of California Berkeley
- worked with several graduate students adding new features to UNIX such as the C shell, the vi editor, and improved file system, sendmail, a pascal compiler, and virtual memory management system
Under the name of Berkeley Software Distribution (BSD) this version of UNIX, including its source code, came to be widely distributed
In 1983, the computer research group at the University of California released the 4.2BSD version, which contained a significant amount of new code: a complete TCP/IP implementation, including sockets and other networking tools
- This version became very popular among universities around the world

Two important variants (BSD and System V)

AT&T's ban was removed during mid-70's and the breakup became effective at the beginning of the 80's
- AT&T could then sell software products and commercialize UNIX
System III was the first version sold by AT&T in 1981
- Many developers were employed to develop applications and notably improve documentation
System V developed in 1989 included many features from BSD (especially network facilities)
In addition to various BSD distributions spread through academia, by the end of the 80-s, UNIX was available in a range of commercial implementations on various hardware
Contrary to proprietary operating systems, the high availability of UNIX distributions (compliant either with System V or BSD) made UNIX systems increasingly attractive from a commercial perspective due to portability of applications

UNIX Variants

UNIX BSD
UNIX System V
Solaris
Digital Ultrix
True64 UNIX by HP
IMG's AIX
Hewlett-Packard's (HP's) HP-UX
NeXT's NeXTStep
MAC OS (Apple)
- iOS
Linux
- Android OS

UNIX Standards

UNIX development was not controlled by a single vendor or organization
- Many groups contributed to its evolution (AT&T, Berkeley, etc.)
- Positive side: innovations
- Negative side: implementations diverged over time, making difficult to write UNIX applications
UNIX and C language standardization
- Applications can be easily ported from one to another system when conforming to a given standard
Standards usually made by independent groups
- ISO C
- POSIX
- Single UNIX Specification
UNIX implementations are referred to the two implementations standards defined by BSD and System V

The UNIX Architecture

Kernel
- Software controlling hardware resources and providing an environment for other programs to execute
System calls
- Interface with the Kernel
Libraries
- Functions build on top of the System Calls
- Make easy to use kernel features
Shell
- Application providing an interface to other applications

The UNIX shell

Interface for every UNIX System
A shell is an interpreter
- Accepts a command
- Interprets the command
- Runs the command
- Waits for the next command
Commands have the following form: command [options] [files]

ls -l start1.txt star2.txt
cd tmp
pwd

The man command

man: shows information about how system calls, library functions, and other commands work
Basic form: man (keyword) [manual page]
Examples of use

man ls
man time 3
man -k keyword
man man

man paginates the output on the screen
- f moves one page forward
- b moves one page back
- G goes to the end of the page
- xG goes to the page x of the output
- /keyword goes to appearances of the keyword on the output
- q terminates the command
apropos (keyword): searches man pages for a given keyword

UNIX File System

Hierarchical arrangement of files and irectories
Everything stars in the root directory, denoted by /
/ is also used as separator to indicate the location of a file or directory
Every entry in the file system is a file or a directory (or a link)
Single entries can be referred either by
- absolute path (starting from the root) e.g., /usr/sbin/bzip2
- relative paths (regarding the current directory) e.g., ../../jonh
Every file has associated three types of rights
- read
- write
- execute

UNIX Files

File permissions are indicated with 9 bits
The first 3 indicate the permissions for the owner of the file for reading, writing and executing
The second group of 3 indicates the permissions for users of the same group as the owner
The third group indicates the permissions for the rest of users
These permissions are indicated with the letters r, w, and x
chmod: changes permissions on a file
chown: changes the owner of a file

UNIX Files

UNIX commands related to the File System

pwd: shows the current directory where the shell is
cd: changes the directory where the shell is

cd \usr\bin #changes the directory to \usr\bin
cd ~ #changes the directory to home folder
cd - #changes to the previous directory

mkdir: creates a new directory

mkdir lib #creates a directory called lib within the current directory

rmdir: deletes an empty existing directory
ls: lists the content of a directory

ls -l # lists the content showing detailed information about each file/folder
ls -F # lists the content indicating the type of each file (executable, regular file, folder)

file: shows information about a file

file template.txt 
# template.tex: LaTeX 2e document, ASCII text

Working with files

cat: shows the whole content of a file
less and more: shows content of a file page after page
Editors
- kate
- nano
- vi, vi(m)
- emacs
echo: prints something on the output

echo $home #prints the home directory
echo $OLDPWD #prints the previous directory if exist
echo hola #prints hola

Every shell has acess to three files
- standard input (stdin), from where the input is taken
- standard output (stdout), where the output is put
- standard error (stderr), where error messages are put
read: reads from the input and assigns it to a variable

read a # waits for user to type something

UNIX redirection

Any command can put the output and error messages to another file or take the input from a different file
- redirection
UNIX provides the following redirection options
- <, >, 2> redirect input, output, and error

cat hola.txt > adios.txt #writes all the content of hola.txt into adios.txt
read a < hola.txt #reads a line from the file hola.txt and assign it to a
ls -l out5 2> /dev/null #if ls -l out5 produces an error, it will be written into the /dev/null file

* `A|B` the output of the command `A` becomes the input of the command `B`

Copying, updating info, and removing files

cp: copy a file or a directory

cp out3 out5 #copies the file out3 into out5
cp -r /juan/home backup #copies all the content of home into backup

rm: removes files or directories

rm out5 # removes out5
rm -r Downloads # removes Download
rm -r # removes the content of the current directory

touch: updates the access time and modification time of a file

touch out4 #change the access time of out4 (assumption: out4 exist)
touch out6 #creates a new  empty file out6, with access time equal to the current time

mv: moves (rename) files or directories

Options for command execution

cmd &: executes cmd in background
cmd1 ; cmd2: executes cmd1, then cmd2, etc.
{cmd1 ; cmd; }: executes commands as group in the current shell
(cmd1 ; cmd; ): executes commands as group in the current shell
cmd $(cmd2): Uses output of cmd2 as input of cmd1

cmd1 `cmd2` #also performs command substitutions

cmd $((expression)): idem as before, but it does an arithmetic substituion
cmd1 && cmd2: executes cdm2 only if cmd1 has been sucessful
cmd1 || cmd2: executes cmd2 only if cmd1 fails

Options for command execution

# Execute sequentially
cd; ls

# All output is redirected
(date; who; pwd) > logfile

# Sort file, page output, then print
sort file | pr -3 | lp

# Edit files found by grep
vi `grep -l ifdef *.cpp`

# Specify a list of files to search
egrep '(yes|no)' `cat list`

# POSIX version of previous
egrep '(yes|no)' $(cat list)

# Print file if it contains the pattern
grep XX file && lp file

# Otherwise, echo an error message
grep XX file || echo "XX not found"

Options for command execution

history: shows a list of the commands executed associated with a number
How to navigate the history?
- !!: executes the last command showed by history
- !-n: executes the n command in history starting from the last one
- !n: executes the command number n in history
- <Ctrl>+r: recursive search on the history
- !xt: the last command that starts with xt

Commands and wildcards

UNIX commands can be used with wildcards
- ? one character
- * 0 or more characters
- [ab] any character included in the range
- {conf,loc} conf and loc

#assume our folder contains the files out1,out2,out3, out4, progf, and prog.o
ls -l out* # detailed list of out1, out2, out3, and out4
ls -l [1-3] # detailed list of out1, out2, and out3
ls -l {1,3} # detailed list of out1, and out3

Variables

Bash allows declaring variables
A variable consists of any number of letters, digits or underscores
- case sensitive
- cannot start with a digit
A variable is assigned a value with the = operator
- there may not be any whitespace between the variable name and the value
A variable value can be accessed using the $ operator
By default, the shell treats variable values as strings, even if the value is a digit
Bash allows creating integer variables using declare -i

var1=3+4
echo  $var1 #prints 3+4
declare -i var2
var2=3+4
echo $var2 #prints 7

Built-in Variables

Automatically set by the shell and can be used by any command
$#: Number of command-line arguments
$-: options currently in effect
$?: exit value of the last executed command
$$: process number of the shell
$0: current command name
$n: Individual arguments of the command line (e.g., $1, $2, ... ${10})
$*, $@: All arguments in command line

Calling the shell

/bin/bash
- Reads several configuration files /etc/profile, ~\.bash_login, or ~\.profile
- Every nonlogin shell also reads ~\.bashrc
/bin/sh
- Reads $EVN instead of ~\.bashrc
Write commands in a file
- First line on the file is the so-called sheban #!/bin/bash
- Besides the file name, several arguments can be used
- Arguments will be executed one after the next

Evaluating conditions

test condition: Evaluate a condition and if the value is true returns 0 as exit status
- Returns nonzero as exist status if the condition is not true
[ condition ]: similar as before
[[ condition ]]: similar but does not perform word splitting and path name extensions. It also allows additional operators such as &&, ||, <, >
Some example of conditions (but there are much more)
- -a file: file exist (deprecated in favor of -e file)
- -d file: file exist and is a directory
- -f file: file exist and is a regular file
- -h file: file exist and is a symbolic link
- -p file: file exist and is a named pipe (FIFO)
- f1 -nt f2: file f1 is newer than f2
- string: string is not null
- -n s1: string s1 has nonzero length
- s1 == s2: strings s1 and s2 are identical. With [[ ]], s2 can be a wildcard pattern
- s1 = s2: same as == but only for test and [ ]

The if command

if condition1
then commands
[ elif condition 2
  then commands2 ]
.
.
.
[else commands3]
fi

Examples:

if [ $counter -lt 10 ]
then number=0$counter
else number=$counter
fi

if [ ! -d $dir ]
then
mkdir -m 7775 $dir
fi

The for command

for x [in [list]]
do
	commands
done

for ((init; cond; incr))
do
	commands
done

Examples:

for item in `cat program_list`
do
	echo "Checking chapters for"
	echo "references to program $item...$
	grep -c "$item.[co]" chap*
done

for ((x=1;$x<=20;x=x+2))
do
	cat $1 chap$x
done

The while and case commands

while condition
do
	commands
done

case value in
pattern1) cmds1;;
pattern2) cmds;;
esac

Example:

case $1 in
no|yes) response=1;;
-[tT]) rable=TURE;;
*) echo "unknown option"; exit1;;
esac

Juanjdurillo / intro_unix

UNIX Slides

What is this repository for?

Introduction to UNIX

Why UNIX?

A bit of UNIX history

A bit of UNIX history

Unix Versions

Inconveniences are sometimes very convenient

Two important variants (BSD and System V)

Two important variants (BSD and System V)

UNIX Variants

UNIX Standards

The UNIX Architecture

The UNIX shell

The man command

UNIX File System

UNIX Files

UNIX Files

UNIX commands related to the File System

Working with files

UNIX redirection

Copying, updating info, and removing files

Options for command execution

Options for command execution

Options for command execution

Commands and wildcards

Variables

Built-in Variables

Calling the shell

Evaluating conditions

The if command

The for command

The while and case commands

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