shell scriptingjsumey/csc400/ppts/scripting.pdfshell scripting csc400 - operating systems 4 csc400:...

Shell Scripting

CSC400 - Operating Systems 1

SHELL SCRIPTING

A Presentation for

CSC400 Operating Systems

by J. Sumey

CSC400: Shell Scripting 2 of 23

scripting concepts & overview

Bourne Shell Scripting

Scripting with other scripting languages:

VBscript, JavaScript, Perl, PHP

• oops! that's in another course…

Presentation Overview

Shell Scripting



Scripting Concept

ability to rapidly develop, test & deploy

algorithmic implementation without the

burdens of programming languages

highly popular among system

administrators & web developers

also considered a “staple1 skill” for

computer programmers in general

1. used, needed, or enjoyed constantly usually by many individuals


Scripting as Programming

although scripting is, in essence, programming; there are important differences

• compiled vs. interpreted execution

• enforcement of syntax, types, etc.

basic programming skills still needed!

• variable & stream manipulation, constructs, function design & invocation, parameters, etc.

Shell Scripting



Language Comparison

Compiled Languages

compiled: fast

execution

slower development

(edit-compile-test)

more “rules”

more portable

“application” oriented

Scripting Languages

interpreted: slower

execution

faster development

(no compilation!)

more flexible

less portable

“system” oriented


Scripting Languages

many different scripting languages exist,

some general purpose, some for

specific purposes

• Unix – sh (Bourne), csh, ksh, zsh, bash

– “shell scripting”

• MS-DOS – “batch” files (.bat)

• Windows – VBscript, MSscript

• web browsers - JavaScript

Shell Scripting



• Perl – a stable, cross platform programming language

supporting both procedural and object-oriented programming

– takes the best features from other languages, such as C, awk, sed, sh, and BASIC, among others

– database integration interface supports databases including Oracle, Sybase, Postgres, MySQL and others

– works with HTML, XML, and other mark-up languages

• PHP – PHP: Hypertext Preprocessor – a widely-used general-purpose scripting

language that is especially suited for Web development and can be embedded into HTML

The Bourne Shell

Shell Scripting



Bourne Shell Background

shell: a command programming

language that provides an interface to

the UNIX operating system

developed in late ’70s by Steve Bourne

at Bell Labs

provides variables, string substitution,

flow control constructs, parameter

passing, 2-way comm. with commands


Shell Scripting Overview

1st script: “hello world”

variables

wildcards & escape characters

external command execution

flow control constructs

functions

parameter passing

Shell Scripting



1st Shell Script

special 1st line: “#!” directive declares

path to proper interpreter

• ex.2: #!/usr/bin/perl

‘#’ begins comment to end-of-line

echo is a command

#!/bin/sh

# a comment line

echo "Hello World" # another comment


Script Execution

shell scripts may executed 1 of 2 ways:

submit the script to a (sub)shell

$ sh scriptname

make script “executable” & use as any

other command

$ chmod +x scriptname

$ scriptname

Shell Scripting



Script Variables

are typeless

do not need to be declared

are referenced with ‘$’ prefix

GREETING="Hello World"

PI=3.1416

echo $GREETING


User Input

may be read into variables…

echo "What’s your name?"

read U_NAME

echo "Welcome to scripting, $U_NAME!"

Shell Scripting



Predefined Variables

$0 : name of script

$1..$9 : 1st nine parameters to the script

$# : the number of parameters given

$@ : all the parameters

$? : return result of last command

echo "hi, my name is $0"

echo "my parameters are: $@"


Command Execution

system commands may be readily

executed from a script

command output may be captured for

use within the script…

echo "directory contents:" ls

nfiles=`ls | wc –l`

echo "$nfiles items listed"

Shell Scripting



Flow Control Constructs

for loop: known iteration

while loop: unknown iteration

if condition: 2-way statement selection

case statement: n-way selection


for Loop

iterates thru a list of given values

for i in 1 2 3 4

do

echo "$i potata"

done

Shell Scripting



while Loop

iteration as long as condition is true

"[…]" is shorthand for the test command

INPUT="?"

while [ "$INPUT" != "who's there?" ]

do

echo "knock, knock."

read INPUT

done


if Condition

1- or 2-way conditional execution

• else part is optional

if [ "`date +%p`" = "AM" ]

then

echo "good morning"

else

echo "how was lunch?"

fi

Shell Scripting



case Statement

echo "Roman converter, enter a number"

read n

case $n in

1) echo "1 = I"

;;

5) echo "5 = V"

;;

10) echo "10 = X"

;;

*) echo "I'm not that smart yet" ;;

esac


Shell Functions

the shell supports the usual function

mechanism in scripts & interactively

• also used as procedures

functions are read & "stored" until called

shell functions use $1..$9 for input

parameters

may return a result to caller or simply

output something

Shell Scripting



factorial()

{

if [ "$1" -le "1" ]; then

echo 1

else

i=`expr $1 - 1`

j=`factorial $i`

k=`expr $1 \* $j`

echo $k

fi

}

echo "Enter a number:"

read x

factorial $x

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