Unix Tutorial 1

A very brief history of Unix

• 1970's
• early stage of AT&T Bell Lab, on PDP-11 machine
• first wrote by Ken Thompson and Dennis Ritchie
• utility programs were added by various Unix programmers
• 1974, the first OS written in the C language
• 1980's
• System V as industry standard
• BSD 4.3, AT&T Version 9
• 1990's
• FreeBSD, Linux, IBM AIX, HPUX, SunOS, many more ....

---

Relationship of Kernel and Shell

Kernel

• core of the Unix operating system
• everything below system-call interface and above the physical hardware
• provides file system, CPU scheduling, memory management and other OS functions through system calls

Systems Programs

• use the kernel-supported system calls to provide useful functions
• 3 categories of system calls :
• File manipulation
• Process control
• Information manipulation
• system calls are embedded in systems programs
• programmers & users use the already existing systems programs

Shell

• a command interpreter - users 'talk' to the shell, which interprets their commands and executes them
• called a shell because it surrounds the kernel
• a typical command is a program which may in turn request lower-level services from kernel
• the shell travels through the search path to find the command file, which is then loaded and executed
• the shell usually suspends its own execution until the command completes
• the basic form of a Unix command is: commandname [-options] [arguments]

Operating System Components

• Ref.: Figure 21.7 of 'Operating System Concepts'

fork( ), exec( ) and wait( )

• fork( ) - creates a new process
• exec( ) - is used after a fork to replace one of two processes's virtual memory space with a new program
• wait( ) - the parent may wait for a child process to terminate
• exit( ) - terminates a process

Illustration of Process Control Calls

• Ref.: Figure 21.11 of 'Operating System Concepts'

Internal and External Commands

Internal Commands

• some commands are internal, built into the shell
• the shell doesn't start a separate proces to run internal command
• for example: cd

External Commands

• some commands are external, stored in directories such as /usr/bin
• requires the shell to fork and exec a new subprocess
• for example: /usr/bin/ls

Search Path

• if the command is not built-in, the shell looks in its search path for an executable program
• search path - a list of directories that the shell should look through for a command whose name matches what is typed
• search path is not built into the shell
• search path is stored in an environment variable called PATH
• path is searched in order, if there are two commands with the same name, the one that is found first in the path will be executed

---

Commonly used Unix shells

Bourne shell (sh)

• named after its creator, Steve Bourne
• the oldest of the current Unix shells and is available on all Unix systems

C shell (csh)

• developed at Berkeley as part of their Unix implementation
• the most popular shell
• has new features such as process control & history

TC shell (tcsh)

• available in the public domain
• provides all the features of the C shell together with emacs style editing of the command line

Korn shell (ksh)

• named after its creator, David Korn
• has new features like history editing

"Bourne-again" shell (bash)

• developed by the Free Software Froundation
• fairly similar to the Korn shell

Summary of shell features

Which shell am I using?

• Bourne shell usually uses $ as a prompt.  The C shell uses %.
• printenv | grep SHELL

---

References:

• Textbook : 'Operating System Concepts', Chapter 21
• Websites :
• UNIXhelp for users
• A Basic Unix Tutorial