8-Shell_Scripting_Tools.md

Hands-on Linux-08 : Shell Scripting/Loops

Purpose of the this hands-on training is to teach the students how to use loops in shell.

Learning Outcomes

At the end of the this hands-on training, students will be able to;

• explain loops in shell.

• use while loops in shell scripting

• use until loops in shell scripting

• use for loops in shell scripting

• use continue and break statements in shell scripting

• use select loops in shell scripting

Outline

• Part 1 - While loops

• Part 2 - Until loops

• Part 3 - For loops

• Part 4 - Continue and Break Statements

• Part 5 - Select loops

Part 1 - While loops

• When writing programs in shell, in some cases it is not enough to execute our block of code only once. The loops are used to repeat (iterate) the execution of a block of code.

• while loops have a boolean logic, similar to if statements. As long as the result of the condition returns True, the code block under while loop runs. When the condition returns to False, the loop execution is terminated and the program control moves further to the next operation.

• Create a folder and name it loops.

mkdir loops && cd loops

• Let's display numbers from 1 to ten with while loop. Create a script file named while-loop.sh.

#!/bin/bash

number=1

while [[ $number -le 10  ]]
do
  echo $number
  ((number++))
done
echo "Now, number is $number"

• Make the script executable and execute it.

chmod +x while-loop.sh
./while-loop.sh

Part 2 - Until loops

• The until loop is identical to the while loop, except that it will execute the commands within it until the test becomes true.

• Create a script file named until-loop.sh.

#!/bin/bash

number=1

until [[ $number -ge 10  ]]
do
  echo $number
  ((number++))
done
echo "Now, number is $number"

• Note that, this time we write -ge instead of -le. So in first sitiuation, condition is false. It will execute the code until the condition is true.

• Make the script executable and execute it.

chmod +x until-loop.sh
./until-loop.sh

Part 3 - For loops

• Sometimes we want to iterate a block of code for each of the items in a given list. For this we use for loop.

• Here is a simple example. Create a script file named for-loop.sh.

#!/bin/bash

echo "Numbers:"

for number in 0 1 2 3 4 5 6 7 8 9
do
   echo $number
done

echo "Names:"

for name in Joe David Matt John Timothy
do
   echo $name
done

echo "Files in current folder:"

for file in `pwd`/*
do
   echo $file
done

• Make the script executable and execute it.

chmod +x for-loop.sh
./for-loop.sh

Using arrays with the for loop

• Create a script file named for-array.sh.

#!/bin/bash

devops_tools=("docker" "kubernetes" "ansible" "terraform" "jenkins")

for tool in ${devops_tools[@]}
do
   echo $tool
done

• Make the script executable and execute it.

chmod +x for-array.sh
./for-array.sh

Part 4 - Continue and Break Statements

• A loop will continue forever unless the necessary condition is not met. A loop that runs endlessly without terminating can run for an infinite number of times. For this reason, these loops are named infinite loops.

• Create a script file named infinite-loop.sh.

#!/bin/bash

number=1

until [[ $number -lt 1  ]]
do
  echo $number
  ((number++))
done
echo "Now, number is $number"

• Make the script executable and execute it.

chmod +x infinite-loop.sh
./infinite-loop.sh

• Push the ctrl + C and terminate the process.

Break Statement

• As we see above the infinite loop run forever. The break statement is used to terminate the execution of the entire loop.

• Let's modify the infinite-loop.sh.

#!/bin/bash

number=1

until [[ $number -lt 1  ]]
do
  echo $number
  ((number++))
  if [[ $number -eq 100 ]]
  then
    break
  fi
done

• Execute the script.

./infinite-loop.sh

Continue Statement

• The Continue statement is similar to the Break command, except it causes the current iteration of the loop to exit, instead of the whole loop.

• Let's modify the infinite-loop.sh. This time we do not display 10 and its multiples (10, 20 ..)

#!/bin/bash

number=1

until [[ $number -lt 1  ]]
do
  ((number++))
  
  tens=$(($number % 10))
  
  if [[ $tens -eq 0 ]]
  then
    continue
  fi

  echo $number
    
  if [[ $number -gt 100 ]]
  then
    break
  fi
done

• Execute the script.

./infinite-loop.sh

Part 5 - Select loops

• The Select Loop generates a numbered menu from which users can select options. It's helpful when you need to ask the user to select one or more items from a list of options.

• Create a script file named select-loop.sh.

#!/bin/bash

read -p "Input first number: " first_number
read -p "Input second number: " second_number

PS3="Select the operation: "

select operation in addition subtraction multiplication division exit
do
  case $operation in
    addition) 
      echo "result= $(( $first_number + $second_number))"
    ;;
    subtraction)
       echo "result= $(( $first_number - $second_number))"
    ;;
    multiplication)
       echo "result= $(( $first_number * $second_number))" 
       ;;
    division)
       echo "result= $(( $first_number / $second_number))"
    ;;
    exit)
       break
    ;;   
    *)
       echo "Wrong choice..." 
    ;;
  esac
done

• Note that we can change the system variable PS3 to modify the prompt that is displayed.

• Make the script executable and execute it.

chmod +x select-loop.sh
./select-loop.sh