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Chapter 6: Shell Programming

Shell Scripts


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Using the UNIX Shell as a Programming

Objectives:

After studying this lesson, youshould be able to:

Learn about shell variables,operators

Write simple shell scripts toillustrate programming logic


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My first Shell Script

vi myfirstscript.sh

#! /bin/csh

set directory=`pwd`

echo The date today is `date`

echo The current directory is $directory

chmod u+x myfirstscript.sh

myfirstscript.sh


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Using UNIX Shell Scripts

UNIX shell scripts are text files thatcontain sequences of UNIX commands

Like high-level source files, a programmercreates shell scripts with a text editor

Shell scripts do not have to be converted

into machine language by a compiler This is because the UNIX shell acts as an

interpreterwhen reading script files


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Using UNIX Shell Scripts Continued

As an interpreter reads the statements in a

program file, it immediately translates them into

executable instructions, and causes them to run

After you create a shell script, you simply tell the

operating system that the file is a program that

can be executed

This is accomplished by using the chmodcommand to change the files mode


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Using UNIX Shell Scripts Continued

Further, the chmod command tells thecomputer who is allowed to use the file:

the owner(u), the group (g), or all otherusers (o)

Shell programs run less quickly than docompiled programs, because the shell

must interpret each UNIX command insidethe executable script file before it isexecuted


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The Programming Shell

Commonly used shell in most variant of UNIX are:

Bourne Shell (sh), first shell developed for UNIX

Bourne Again Shell (bash), written by programmersof Free Software Foundation, open source shellfrom GNU

Korn Shell (ksh), written by David Korn, superset ofBourne shell, not widely distributed.

C Shell (csh), written by Bill Joy, the author of vi,shared much of the C language structure.

Terminal Based C Shell (tcsh), enhanced version ofthe Berkeley UNIX C shell csh


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The Programming Shell

All Linux versions use the Bash shell(Bourne Again Shell) as the default

shell All UNIX system include C shell and

its predecessor Bourne shell.


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Shell Programming

programming features of the UNIXshell:

Shell variables

Operators

Logic st ruc tures


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Shell Programming

programming features of the UNIX shell:

Shell variables: Your scripts often need to keep valuesin memory for later use. Shell variables are symbolic

names that can access values stored in memory

Operators: Shell scripts support many operators,including those for performing mathematical operations

Logic struc tures: Shell scripts support sequential logic(for performing a series of commands), decision logic(for branching from one point in a script to another),looping logic (for repeating a command several times),and case logic (for choosing an action from severalpossible alternatives)


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Shell Programming


Shell variables

Operators

Logic st ruc tures


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Variables

Variables are symbolic names that representvalues stored in memory

The three types of variables discussed in thissection are configuration variables, environmentvariables, and shell variables

Use configuration variables to store informationabout the setup of the operating system, and do

not change them

You can set up environment variables with initialvalues that you can change as needed


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Variables Continued

These variables, which UNIX reads when you log in,

determine many characteristics of your session

Shell variables are those you create at thecommand line or in a shell script

Environment and configuration variables bear

standard names, such as HOME, PATH, SHELL,

USERNAME, and PWD (Configuration and environment variables are

capitalized to distinguish them from user variables)


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Variables Continued

To see a list of yourenvironment

variables:

$ printenv

or:$ printenv | more


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Variables Continued

1. Typical Environment Variables

HOME: pathname of your home directory

PATH: directories where shell is to lookfor commands USER: your user name PWD: your current working directory

MAIL: pathname of your system mailbox SHELL: pathname of your shell

2. Variable contents are accessed using $:e.g. $ echo $HOME


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Variables Continued

A shell variable take on the generalized formvariable=value (except in the C shell).

$ x=37; echo $x

$ 37$ unset x; echo $x

The C shell uses the set statement setvariables.

$ set x = 37 You can set a pathname or a command to a

variable or substitute to set the variable.$ setmydir=`pwd`; echo $mydir


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Variables Continued

To create lists:$ set Y = (UNL 123 CS251)

To set a list element:$ set Y[2] = HUSKER

To view a list element:$ echo $Y[2]


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Variables Continued

vi myinputs.sh#! /bin/csh

echo Total number of inputs: $#argvecho First input: $argv[1]echo Second input: $argv[2]

chmod u+x myinputs.sh myinputs.sh HUSKER UNL CSE


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Shell Programming


Shell variables

Operators

Logic st ruc tures


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Shell Operators

The Bash shell operators are divided into

three groups: defining and evaluating

operators, arithmetic operators, and

redirecting and piping operators


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Arithmetic Operators

exprsupports the following operators:

arithmetic operators: +,-,*,/,%

comparison operators:

boolean/logical operators: &, |

parentheses: (, )

precedence is the same as C, Java


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Arithmetic Operators (example)

vi math.sh

#!/bin/csh

set count=5

set count=`expr $count + 1`

echo $count

chmod u+x math.sh

math.sh


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Shell Programming


Shell variables

Operators

Logic st ruc tures


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Shell Logic Structures

The four basic logic structures needed for

program development are:

Sequential logic

Decision logic

Looping logicCase logic


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Sequential Logic

Sequential logic states that commands

will be executed in the order in which they

appear in the program

The only break in this sequence comes

when a branch instruction changes theflow of execution


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Decision Logic

Decision logic enables your programto execute a statement or series of

statements only if a certain conditionexists

The if statement is the primary

decision-making control structure inthis type of logic


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Decision Logic (continued)

if-then

if ( expr ) simple-command

if-then-elseif ( expr ) then

command-set-1

[elsecommand-set-2]

endif


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A simple example

#!/bin/cshif ($#argv != 2) then

echo $0 needs two parameters!echo You are inputting $#argv parameters.

else

set par1 = $argv[1]set par2 = $argv[2]

endif


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Another example:#! /bin/csh# number is positive, zero or negative

echo "enter a number:"set number = $ /dev/null )} thenecho UNIX occurs in $argv[1]

elseecho No!

echo UNIX does not occur in $argv[1]endif

Redirect intermediate results into >/dev/null, instead of

showing on the screen.


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Looping Logic

In looping logic, a control structure (or loop)repeats until some condition exists or someaction occurs

You will learn two looping mechanisms in thissection: the forloop and the while loop

You use the forcommand for looping through arange of values.

It causes a variable to take on each value in aspecified set, one at a time, and perform someaction while the variable contains each individualvalue


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The while Loop

A different pattern for looping is created using

the while statement

The while statement best illustrates how to set upa loop to test repeatedly for a matching condition

The while loop tests an expression in a manner

similar to the if statement

As long as the statement inside the brackets istrue, the statements inside the do and done

statements repeat


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Looping Logic

while ( expr )

command_set

end

foreach var ( worddlist )

command_set

end


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Looping Logic

Program:

#!/bin/csh

foreach person (Bob Susan Joe Gerry)echo Hello $person

end

Output:Hello BobHello SusanHello JoeHello Gerry


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Looping Logic

Adding integers from 1 to 10

#!/bin/csh

set i=1set sum=0while ($i


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Switch Logic

The switch logic structure simplifies the

selection of a match when you have a list

of choices

It allows your program to perform one of

many actions, depending upon the value

of a variable


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Switch Logic

switch ( var )

case string1:

command_set_1

breaksw

case string2:

command_set_2

breaksw

default

command_set_3

endsw


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Switch Logic

#!/bin/cshif ($#argv == 0 ) then

echo "No arguments supplied...exiting"

else

switch ($argv[1])

case [yY]:

echo Argument one is yes.

breaksw

case [nN]:

echo Argument one is no.

breaksw

default:

echo Argument one is neither yes nor no.

breaksw

endsw

endif


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Chapter Summary

A high-level language must be converted into a low-level (machine) language before the computer canexecute it

The shell interprets UNIX shell scripts UNIX shell scripts, created with the vi or other

editor, contain instructions that do not need to bewritten from scratch, but rather can be selectivelychosen from the operating systems inventory of

executable commands


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Chapter Summary Section AContinued

Linux shells are derived from the UNIX Bourne,Korn, and C shells

UNIX keeps three types of variables:

Configuration

Environment

Shell

The shell supports numerous operators, includingmany for performing arithmetic operations

The logic structures supported by the shell aresequential, decision, looping, and case


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Homework 1

Please refer to handout.

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