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ShellProgramming151.ppt

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UNIX Unbounded 5th Edition
Amir Afzal

Chapter 12
Shell Programming

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Chapter 12
Exploring the Shell

This chapter explains capabilities of the shell as an interpretive high-level language
It describes shell programming constructs and particulars
Variables
Flow control
Running
Debugging

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UNDERSTANDING UNIX SHELL PROGRAMMING LANGUAGE: AN INTRODUCTION

A command language – interpreted (not compiled)
Shell program files are called shell procedures, shell scripts, or simply scripts
A file that contains a series of commands for the shell to execute

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Writing a Simple Script

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Executing a Script

Making Files Executable: The chmod Command

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Executing a Script

Making Files Executable: The chmod Command

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Executing a Script

Making Files Executable: The chmod Command

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Executing a Script

Making Files Executable: The chmod Command

chmod u=rwx,g=rx,o=r myfile

chmod 754 myfile # 754 = 1 1 1 1 0 1 1 0 0

r w x r - x r - -

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WRITING MORE SHELL SCRIPTS

Built in commands

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WRITING MORE SHELL SCRIPTS

Amir Afzal
UNIX Unbounded, 5th Edition

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WRITING MORE SHELL SCRIPTS

Amir Afzal
UNIX Unbounded, 5th Edition

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WRITING MORE SHELL SCRIPTS

Using Special Characters

When using the echo command on the prompt sign:

make sure to use –e option for the escape characters to work as specified

$echo –e “H\nE\nL\nL\nO”

When using the echo command inside script files, and you are using the bash command or chmod +x command to run this cript

make sure to use –e option for the escape characters to work as specified

echo –e “H\nE\nL\nL\nO”

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WRITING MORE SHELL SCRIPTS

Using Special Characters

When using the echo command on the prompt sign:

make sure to use –e option for the escape characters to work as specified

$echo –e “H\nE\nL\nL\nO”

When using the echo command inside script files, and you are using the bash command or chmod +x command to run this cript

make sure to use –e option for the escape characters to work as specified

echo –e “H\nE\nL\nL\nO”

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WRITING MORE SHELL SCRIPTS

Using Special Characters

When using the echo command on the prompt sign:

make sure to use –e option for the escape characters to work as specified

$echo –e “H\nE\nL\nL\nO”

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WRITING MORE SHELL SCRIPTS

Logging Off in Style

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WRITING MORE SHELL SCRIPTS

Executing Commands: The dot Command
Lets you execute a program in the current shell and prevents the shell from creating the child process

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WRITING MORE SHELL SCRIPTS

Path Modification

Command Substitution
Place the output of a command in the argument string

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WRITING MORE SHELL SCRIPTS

Reading Inputs: The read Command

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Reading Inputs: The read Command

Amir Afzal
UNIX Unbounded, 5th Edition

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EXPLORING THE SHELL: PROGRAMMING BASICS

Comments
The shell recognizes # as the comment symbol; therefore, characters after the # are ignored.

Variables
Write the name of the variable, followed by the equal sign and the value you want to store in the variable
No spaces either side of the = sign

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EXPLORING THE SHELL: PROGRAMMING BASICS

Displaying Variables
the echo command can be used to display the contents of variables

Command Substitution
You can store the output of a command in a variable

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EXPLORING THE SHELL: PROGRAMMING BASICS

Command Line Parameters

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Command Line Parameters

Amir Afzal
UNIX Unbounded, 5th Edition

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EXPLORING THE SHELL: PROGRAMMING BASICS

Assigning Values
Possible to assign values to the positional variables is to use the set command

Accents

Not single quotes

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EXPLORING THE SHELL: PROGRAMMING BASICS

Scenario – write a script file that does the following
Stores the specified file in a directory called keep in your HOME directory
Invoke the vi editor to edit the specified file

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EXPLORING THE SHELL: PROGRAMMING BASICS

Result

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EXPLORING THE SHELL: PROGRAMMING BASICS

Terminating Programs: The exit Command
Use to immediately terminate execution of your shell program

Conditions and Tests
The if-then construct

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EXPLORING THE SHELL: PROGRAMMING BASICS

The if-then-else Construct

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EXPLORING THE SHELL: PROGRAMMING BASICS

The if-then-else Construct

no #

Notice the spaces before and after

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EXPLORING THE SHELL: PROGRAMMING BASICS

The if-then-elif Construct

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EXPLORING THE SHELL: PROGRAMMING BASICS

The if-then-elif Construct

hour= `date +%H’

Replace these

two lines (8 and 9)

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EXPLORING THE SHELL: PROGRAMMING BASICS

The if-then-elif Construct

if [ “$answer” = Y ]

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EXPLORING THE SHELL: PROGRAMMING BASICS

Testing Different Categories
Numeric Values
use the test command to test (compare) two integer numbers algebraically

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EXPLORING THE SHELL: PROGRAMMING BASICS

Scenario – write a script that:
Accepts three numbers as input (command line arguments) and …
Displays the largest of the three
Output should look like this:

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EXPLORING THE SHELL: PROGRAMMING BASICS

if [ “$num1” –gt “$num2” -a “$num1” –gt “$num3” ]

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EXPLORING THE SHELL: PROGRAMMING BASICS

String Values
Compare (test) strings with the test command

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EXPLORING THE SHELL: PROGRAMMING BASICS

String Values
Compare (test) strings with the test command

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EXPLORING THE SHELL: PROGRAMMING BASICS

Files – use the test command to test file characteristics
Size, type, permissions

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EXPLORING THE SHELL: PROGRAMMING BASICS

Files – use the test command to test file characteristics
Size, type, permissions

if [ -r “$FILE” ]

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EXPLORING THE SHELL: PROGRAMMING BASICS

Files – use the test command to test file characteristics

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EXPLORING THE SHELL: PROGRAMMING BASICS

Parameter Substitution
Lets you test the value of a parameter and change its value according to a specified option

If variable has value

return this value

Else

return the string

If variable has value

return the string

Else

nothing

If variable has value

return this value

Else

variable = string

If variable has value

return this value

Else

print string

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echo ${var:-"Variable is not set"}
echo "1 - Value of var is ${var}"

echo ${var:="Variable is not set"}
echo "2 - Value of var is ${var}"

unset var
echo ${var:+"This is default value"}
echo "3 - Value of var is $var“

var="Prefix"
echo ${var:+"This is default value"}
echo "4 - Value of var is $var"

echo ${var:?"Print this message"}
echo "5 - Value of var is ${var}"

Variable is not set

1 - Value of var is

Variable is not set

2 - Value of var is Variable is not set

3 - Value of var is

This is default value

4 - Value of var is Prefix

Prefix

5 - Value of var is Prefix

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EXPLORING THE SHELL: PROGRAMMING BASICS

Parameter Substitution

Notice the exit from the script

If name is still empty

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EXPLORING THE SHELL: PROGRAMMING BASICS

Parameter Substitution

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ARITHMETIC OPERATIONS

The expr Command – Arithmetic Operators
Note: spaces between the elements of an expression are necessary, integer only

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ARITHMETIC OPERATIONS

The expr Command – Arithmetic Operators
The * (multiplication) and % (remainder) characters have special meaning to the shell, they must be preceded by a backslash [\]

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ARITHMETIC OPERATIONS

The expr Command – Arithmetic Operators
The grave accent marks [` and `] surrounding the command are necessary and cause the output of the command (expr) to be substituted.

$ x=10

$ y=20

$ z=`expr $x + $y`

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$ c=`expr $a + $b`

$ echo “the value of addition=$c”

$ d=`expr $a - $b`

$ echo “the value of subtraction=$d”

$ e=`expr $a \* $b`

$ echo “the value of multiplication=$e”

$ f=`expr $a / $b`

$ echo “the value of division=$f”

$ g=`expr $a \% $b`

$ echo “the value of modulus=$g”

Show the output of each code segment?

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ARITHMETIC OPERATIONS

Relational Operators
Relational operators that work on both numeric and nonnumeric arguments.
If both arguments are numeric, the comparison is numeric
If one or both arguments are nonnumeric, the comparison is nonnumeric and uses the ASCII values.
because > (greater than) and < (less than) characters have special meaning to the shell, they must be preceded by a backslash [\]

n1=10

n2=20

if [ “$n1” \> “$n2” ]

then

echo “$n1 is greater than $n2”

else

echo ”$n1 is not greater than $n2”

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ARITHMETIC OPERATIONS

Arithmetic Operations: The let Command
Simpler alternative to the expr command and includes all the basic arithmetic operations

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THE LOOP CONSTRUCTS

The For Loop: The for-in-done Construct
Used to execute a set of commands a specified number of times

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THE LOOP CONSTRUCTS

The For Loop: The for-in-done Construct

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for var in "$@"

echo "$var"

done

for i in {1..5}

echo "Welcome $i times"

done

for num in `seq 1 2 10`

echo "$num"

done

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for (( c=1; c<=5; c++ ))

echo "Welcome $c times"

done

exit 0

Spaces are not important here

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THE LOOP CONSTRUCTS

The For Loop: The for-in-done Construct

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THE LOOP CONSTRUCTS

The While Loop: The while-do-done Construct
The while loop continues as long as the loop condition is true.

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THE LOOP CONSTRUCTS

The While Loop: The while-do-done Construct

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THE LOOP CONSTRUCTS

The While Loop: The while-do-done Construct
Command line version of the carryon program

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THE LOOP CONSTRUCTS

The While Loop: The while-do-done Construct

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THE LOOP CONSTRUCTS

The While Loop: The while-do-done Construct

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THE LOOP CONSTRUCTS

The While Loop: The while-do-done Construct

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THE LOOP CONSTRUCTS

The Until Loop: The until-do-done Construct
Similar to the while loop, except …
It continues executing the body of the loop as long as the condition of the loop is false
Body of the until loop might never get executed if the loop condition is true

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THE LOOP CONSTRUCTS

Scenario: check whether a specified user is on the system or, if not, to be informed as soon as the user logs in

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DEBUGGING SHELL PROGRAMS

The Shell Command
Use the sh, ksh, or bash command with one of its options to make the debugging of your script files easier
Options

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DEBUGGING SHELL PROGRAMS

The Shell Command

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DEBUGGING SHELL PROGRAMS

The Shell Command

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DEBUGGING SHELL PROGRAMS

The Shell Command
Run the BOX program with the -v option, and specify some command line arguments.

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DEBUGGING SHELL PROGRAMS

The Shell Command