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

Transcription

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

2 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) 2

3 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. 3

4 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 end of case statement (esac) Default (use * symbol) represents what to do if no patterns are matched 4

5 Logic Example #1 Based on the commands below, show the stdout: door=2 case $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? 5

6 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? 6

7 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. 7

8 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. 8

9 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. 9

10 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. 10

11 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 10 echo Unable to copy done 11

12 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 12

13 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?... 13

14 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. 14

15 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 15

16 for Loop Used to repeat commands for a specified number of repetitions (iterations). There are several methods how to use the for loop 16

17 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. 17

18 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 18

19 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 19

20 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 20

21 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 done command(s) 21

22 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 22

23 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) 23

24 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... 24