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Week 6 – Lesson 1:Control-Flow Statements (Continued)

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Chapter Objectives

In this chapter, you will: Learn additional shell scripting tools including:

Logic

Case Statement

Loops

Purpose

Indeterminate Loops (while , until)

Determinant Loops (for)

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Logic

case statement

The case control-flow statement is used to make a decision based upon multiple inputs that are constants.

For decision-making for ranges of values (such as letter grades based on percentage marks – eg. A for 80 – 89), then the if / elif / else control-flow statement would work better.

Refer to diagram above for flowcharting representation of a case statement….

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Logic

case statement

SYNTAX:

case $var in pattern1) command(s) ;; pattern2) command(s);; pattern3) command(s);; .. default) command(s);;esac

Marks end of case statement

Patterns can consist of letters,numbers, character class [ .. ].

;; represents break to endof case statement (esac)

Default (use * symbol) representswhat to do if no patterns arematched…

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Logic

Example #1

Based on the commands below, show the stdout:

door=2case $door in 1) echo “Door #1”;; 2) echo “Door #2”;; 3) echo “Door #3”;; *) echo “Door #?”;;esac

Question: What would happen to above case statement if value of door was set to -1?

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Logic

Example #2

Based on the commands below, show the stdout:

read –p “Pick an odd number between 11 – 19: ” number case $number in

[1][13579]) echo “The odd number is $number”;; [1][02468]) echo “That number is even – try again”;; *) echo “Must be a positive two digit number”;;esac

Question: What would happen to above case statement if the user selected 11? 19? 12? 0? 12? -2?

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Loops

Loops

A loop is a condition that causes a specific set of statements to be repeated.

Statements repeat until a condition terminates the loop.

An iteration refers to the number of complete passes the loop makes.

If a loop repeats 4 times, it has completed 4 iterations.

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Loops

Advantage – allows you to enter data without knowing the exact amount of data that needs entering.

Used in menu-based systems.

Poor programming logic design can cause infinite loops.

An infinite loop executes without end – can be terminated with the kill command.

Disadvantage of infinite loop – consumes excessive processing time.

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Loops

There are two types of loops:

Determinant

The number of repetitions (iterations) are known. For example, print “hello” six times. The for loop is a determinant loop

Indeterminate

The number of repetitions (iterations) are unknown. For example, continue prompting the user until they enter a positive number between 1 and 10. We do not know how many times or what type of data the user will enter until they get it correct! The while and until loops are indeterminant loops.

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Loops while Control-Flow Statement

The theoretical while statement tests a condition for true or false.

Syntax:

while true-condition do

Perform activity for true-condition done

If condition is true do statements execute.

If condition is false loop terminates and program flow continues with subsequent statement.

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Loops

Additional Information (while loop)

Can be used to test exit status of a command. Useful for determining if a command succeeded or

failed. Example to copy a file if it exists:

while ! cp fileX.txt fileY.txt do echo “Attempting to copy…” sleep 10echo “Unable to copy…”

done

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Loops Additional Information (while loop)

Can use true in the while statement to loop until a condition is met, then use the break command to terminate the loop.

Example:

((count=1)) while true do

if [[ $count –gt 3 ]] then

break fi count=$((count+1))

done

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Loops

Example

Based on the commands below, show the stdout:

number=23 while echo $number | grep -qv "^[1-9][0]*$" do read -p "Pick a number (1-10): " number done

echo "The number is: $number"

Question: What would happen if the user entered 1? 9? 10? 0? 11? -1? A? x? Nothing?...

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Loops until Control-Flow Statement

The until control-flow statement is similar to the while statement, but works the opposite way – it loops if the condition is false, and terminates the loop if the condition is true.

Theoretical until Logic:

until true-condition do

Perform activity for true-condition done

If condition is false do statements execute.

If condition is true loop terminates and program flow continues with subsequent statement.

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Loops Example

Based on the commands below, show the stdout:

number=5 echo "Count-Down:"

until [ $number -eq 0 ] do echo -n "$number " number=$((number - 1)) sleep 1 done echo "Lift-off!"

TASK: Describe step-by-step what how this program runs …

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Loops

for Loop

Used to repeat commands for a specified number of repetitions (iterations).

There are several methods how to use the for loop…

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Loops

for Loop (Method 1)

Word list form:

for variable in arg1 arg2 arg3 .. argN do

command(s) done

Each argument (arg1, arg2, arg3 .. argN) is used in a separate iteration…

When all arguments are used, then the for loop terminates.

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Loops

Example

Based on the commands below, show the stdout:

counter=1 for item in pens pencils paper do echo "arg #$counter: $item" counter=$((counter + 1)) done

TASK: Describe step-by-step what how this program runs …

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Loops for Loop (Method 2)

Read positional parameters:

for variable do

command(s) done

If for loop does not include reserved word “in” followed by the arugments, then any positional parameters will be used.

When all positional parameters are used, then the for loop terminates. The positional parameters stored prior to for loop still remain to be used later if necessary…

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Loops Example

Below are the contents of the file called items.bash:

counter=1 for item do echo "arg #$counter: $item" counter=$((counter + 1)) done

TASK: Assuming items.bash is in your current directory and has execute permissions. What will be the stdout if the following commands are issued:

./items.bash pigs horses hats rats books cars umbrellas

./items.bash “pigs horses hats rats” books cars umbrellas

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Loops

for Loop (Method 3)

The for loop can use arithmetic expression to increase or decrease a value for the number of iterations

Syntax:

for ((variable=start-value; variable condition; change variable value))

do command(s)

done

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Loops Example

Below are the contents of the file called count.bash:

read -p "Enter # of repetitions: " number echo -n "Repeating: "

for ((x=1; x <= number; x++)) do echo -n "$x " sleep 1 done echo "times..."

TASK: Describe step-by-step what how this program runs …

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Summary

The case control-flow statement is used to make a decision based upon multiple inputs that are constants.

A loop is a condition that causes a specific set of statements to be repeated. Statements repeat until a condition terminates the loop.

There are two types of loops:

Determinant (# of repetitions know – for ) Indeterminate (# of repetitions unknown – while, until)

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Summary

With indeterminate loops, it is important to allow the user to change the value of a variable (if prompted) or change the value of the variable by some means (eg. Mathematics).

If the value remains the same, then the loop may repeat forever (which are referred to as “infinite loops”).

Shell scripts caught in “infinite-loops” can cause reduced productivity of the computer server's operations...