Unix Environment What we learn this time

Unix Environment

• What we learn this time:

• Big figure of Unix environments
• basic concepts and design principles of Unix
• Shells
• File structures, naming, permissions
• Scripting

What about Unix

• Designed by programmers for programmers (i.e. hackers)

• a.k.a PWB (programmer’s work bench)

• Who wrote Unix and where?

• Ken Thompson and Dennis Ritchie
• AT&T Bell Labs
• Add-on utilities/programs designed by independent programmers

Power

• The Unix philosophy “is the idea that the power of a system comes from the relationships among programs than from the programs themselves”

• Interface is minimal, but allows programs to be strung together

• Teaches programming

• Teaches “hacking”, the ability to build a tool when one doesn’t exist

Unix Core

• The core of the Unix OS is called the kernel

• Variety of Unix flavors:

• FreeBSD
• Linux
• Darwin (Mac OS X)

• Most functionality is the same between implementation

• vi, cd, man

• Built-in commands and utilities behavior might differ

• FreeBSD make vs. GNU make

• POSIX (Portable Operating System Interface)

• IEEE standard that tries to work toward apps interoperability
• System V vs. Darwin
• are much different

Communication with Unix

• A user never “talks” directly to a Unix OS

• What do these programs use to talk to the OS?

• System calls

• Three programs that are used to communicate with the OS are:

• shell
• a command interpreter
• Interactive command
• runs inside a tty (teletype) and reads your typing
• Text editor (i.e. vi)
• GUI
• implemented with a set of running programs

Tools working together

• The Unix programs are tools

• specific in function
• multi-purpose

• To make them general-purpose:

• Must be data independent
• Output of any program should be input for another
• Info needed by a program should be contained in a data stream passed to it or passed at command prompt
• If no arguments are passed to a program, read stdin(keyboard) and print stdout(screen)

• Programs meeting the above criteria are called filters

• Pipe – take output from program on left and feed to input of program on right
• E.g. cat /etc/passwd | grep –i anyusername

Shells

• What is a shell?

• A command interpreter
• “protects you from the kernel”
• It really protects the kernel from you

Many Shells

• Bourne shell (sh)

• Creator Steve Bourne in the early 80’s
• First shell Used for shell programming

• C shell (csh)

• Created at UCB their Unix implementation in the early 80’s
• Users wanted more familiar syntax
• More features (for interactive uses) than sh (e.g. job control and history)

Many Shells

• Korn shell (ksh)

• Created by David Korn in the mid 80’s
• Compatible with sh but having most features of csh
• Features history editing (a.k.a. command-line editing)
• Was available on System V
• Public-domain version is pdksh

• T-shell (tcsh)

• has all csh features and less bugs

• Bourne-again shell (bash)

• Offered by FSF (free software foundation)
• Similar to ksh and csh
• Command-line editing

Many Shells

• Z shell (zsh)

• hybrid

• Plan 9

• Created by Bell Labs

• Tcl (tclsh), wish

What’s my shell?

• Check your prompt

• Usually bash uses $
• Usually csh uses %
• Usually tcsh uses >
• Superuser “root” usually is #

• grep yourusername /etc/passwd

• echo $SHELL

Programming the Shell

• The shell is a complete programming language

• Easy

• As simple as a single complex command saved in a file
• Look at the example used in the text mac2unix

Converting a MAC file to Unix

Internal and External Commands

• Internal commands

• built into the shell
• the shell performs the command
• E.g. chdir or cd

• External commands

• Require the shell to fork and exec and a subprocess will start
• E.g. ls

Internal & External Commands

• Shell checks what type of command the user is trying to run

• Check if built-in
• Else check if absolute path
• Else check alias (except bash)
• Check for executable in search path
• Search path is a list of dir that the shell must check
• An environment variable PATH lists these dir
• Look at an example
• Search path is specified in the shell start up files

Kernel and Daemons

• Unix is a multi-user OS

• The kernel is the main program or the heart

• Assigns memory to programs running
• Partitions time fairly
• Handles the I/O, etc.
• Think of it as a traffic cop or a manager

• Daemons

• Group of “helper” programs
• help the system with mail, network comm, tracking time, etc

Kernel and Daemons

• ps command

• prints information about active processes

• shutdown command

Unix Filenames

• Filenames can be made up of any character set except a slash ( / )

• Case sensitive
• Underscores are handled for word separation
• Periods are used by some programs to separate filenames from file extensions
• Filenames beginning with a dot (.) are treated differently by the shells

• No concept of file version

• Deleted file is gone forever.

File Extensions

• MS files are filename.extension

• Unix has no specific rules about file extensions

• Some programs such as the C compiler will look for file extensions

Wildcards

• * - matches any character

• ? – matches any single character

• filename.[character-set]

• To match a particular group of characters, say all files that end with an F or P
• E.g. filename.[FP]

• One exception

• slash (/), no matches can be made against it

Hierarchical File System

Home dir

• /home/username – your own place

• Store any file you like

Pathnames

• Locate files

• Current directory – currently working directory

• Absolute vs. relative paths

• pwd vs. cd
• . Current dir
• .. Parent dir
• ~ your home dir
• ~username username’s home dir

File Permissions

• UID – user ID

• Every user belongs to a group

• groups

• When a file is first created, its owner is the user that created and the primary group in which they belong to.

• chgrp – change group

• chown – change owner

• chmod – change permissions on file(s)

Superuser (root)

• Do we need to say more?

Unix Files

• Unix is file oriented

• Everything is treated as a file

• File is a stream of bytes with no special structure, but it is defined by the programs that use it

• Files use newline character instead of carriage return

Scripting

• Scripting languages and apps differ from compiled languages, but how?

• Interpreted as run

• Perl

• Practical Extraction and Report Language
• Created by Larry Wall

• Python

• More structured and verbose than Perl

Networking and comm

• ssh

• sftp

• scp

• telnet

• rsh

• ftp

• write

• talk

X Windows

• X Consortium and XFree86 Project

• A system that divides up the large scale screen of a workstation into multiple virtual terminal or windows

• Runs on many different kinds of hardware

• Let’s you run a program on a remote computer while the program’s windows are displayed on your local terminal