7-Shell_Scripting_Conditional_Statements.md

Hands-on Linux-07 : Shell Scripting/Conditional Statements

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

Learning Outcomes

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

• explain conditional statements in shell.

• use if statements in shell scripting

• use case statements in shell scripting

Outline

• Part 1 - If Statements

• Part 2 - If Else Statements

• Part 3 - If Elif Else Statements

• Part 4 - Nested If Statements

• Part 5 - Boolean Operations

• Part 6- Case Statements

Part 1 - If Statements

• Unix Shell supports conditional statements that are used to perform different actions on the basis of different conditions.

• A simple if statement essentially states, if a particular test is true, then perform a specified set of actions. If it's not true, don't take those acts.

• Create a folder and name it conditional-statements.

mkdir conditional-statements && cd conditional-statements

• Create a script file named if-statement.sh.

#!/bin/bash
read -p "Input a number: " number

if [[ $number -gt 50 ]]
then
  echo "The number is big."
fi

• Make the script executable and execute it.

chmod +x if-statement.sh
./if-statement.sh

• We can use Relational Operators, String Operators or File Test Operators inside the square brackets ( [ ] ) in the if statement above.

Relational Operators

• Bourne Shell supports the relational operators below that are specific to numeric values. These operators do not work for string values.

Operator	Description
-eq	equal
-ne	not equal
-gt	greater than
-lt	less than
-ge	greater than or equal
-le	less than or equal

String Operators

• The string operators below are supported by Bourne Shell.

Operator	Description
=	equal
!=	not equal
-z	Empty string
-n	Not empty string

• Let's see this. Create a file and name it string-operators.sh

#!/bin/bash

if [[ "a" = "a" ]]
then
  echo "They are same"
fi

if [[ "a" != "b" ]]
then
  echo "They are not same"
fi

if [[ -z "" ]]
then
  echo "It is empty"
fi

if [[ -n "text" ]]
then
  echo "It is not empty"
fi

• Notice that there are spaces between the opening bracket [ and the parameters "text" = "text", and then between the parameters and the closing bracket ]. That is precisely because the brackets here act as a command, and you are separating the command from its parameters.

• Make the script executable and execute it.

chmod +x string-operators.sh
./string-operators.sh

File Test Operators

• There are a few operators that can be used to test various properties associated with a Linux file.

Operator	Description
-d file	directory
-e file	exists
-f file	ordinary file
-r file	readable
-s file	size is > 0 bytes
-w file	writable
-x FILE	executable

• Let's try this. Create files below and configure them.

mkdir folder
touch file 
chmod 400 file

Create a file and name it file-operators.sh

#!/bin/bash

if [[ -d folder ]]
then
  echo "folder is a directory"
fi

if [[ -f file ]]
then
  echo "file is an ordinary file"
fi

if [[ -r file ]]
then
  echo "file is a readable file"
fi

if [[ -w file ]]
then
  echo "file is a writable file"
fi

if [[ -s file ]]
then
  echo "file is > 0 bytes"
fi

if [[ -x $0 ]]
then
  echo "$0 is an executable file "
fi

• Make the script executable and execute it.

chmod +x file-operators.sh
./file-operators.sh

Part 2 - If Else Statements

• Sometimes we want to execute a block of code if a statement is true, and another block of code if it is false.

• Create a script file named ifelse-statement.sh.

#!/bin/bash
read -p "Input a number: " number

if [[ $number -ge 10 ]]
then
  echo "The number is bigger than or equal to 10."
else 
  echo "The number is smaller than 10"
fi

• Make the script executable and execute it.

chmod +x ifelse-statement.sh
./ifelse-statement.sh

• Create a script. Ask user to enter a file name to create.
• If there is a file with the same name, print the message "The file already exists."
• If not, create the file and print the message "The file is created."

Part 3 - If Elif Else Statements

• The elif statement is used when it requires to specify several conditions in our program.

• Create a script file named elif-statement.sh.

#!/bin/bash
read -p "Input a number: " number

if [[ $number -eq 10 ]]
then
  echo "The number is equal to 10."
elif [[ $number -gt 10 ]]
then
  echo "The number is bigger than 10"
else 
  echo "The number is smaller than 10"
fi

• Make the script executable and execute it.

chmod +x elif-statement.sh
./elif-statement.sh

Part 4 - Nested If Statements

• If statements can be nested. Let's see the nested structure on the followig example.

• Create a script file named nested-if-statement.sh.

#!/bin/bash

read -p "Input a number: " number

if [[ $number -gt 10 ]]
then
  echo "Number is bigger than 10"

  if (( $number % 2 == 1 ))
  then
    echo "And is an odd number."
  else
    echo "And is an even number"
  fi
else 
  echo "It is not bigger than 10"
fi

• Make the script executable and execute it.

chmod +x nested-if-statement.sh
./nested-if-statement.sh

Part 5 - Boolean Operations

• The Boolean operators below are supported by the Bourne Shell.

Operator	Description
!	negation
&&	and

• ! inverts a true condition into false and vice versa.

• && is logical AND. If both the operands are true, then the condition becomes true otherwise false.

• || is logical OR. If one of the operands is true, then the condition becomes true.

• Create a script file named boolean.sh.

#!/bin/bash

read -p "Input your name: " name
read -sp "Input your password: " password

if [[ $name = $(whoami) ]] && [[ $password = Aa1234 ]]
then
  echo -e "\nWelcome $(whoami)"
else
  echo -e "\nIt is wrong account"
fi

• Make the script executable and execute it.

chmod +x boolean.sh
./boolean.sh

Part 6- Case Statements

• To execute a multiway branch, we can use several if-elif statements but that would soon become complicated. Bash case statements are similar to if-else statements but are easier and simpler. It helps to match one variable against several values.

• Create a script file named case-statement.sh.

#!/bin/bash

read -p "Input first number: " first_number
read -p "Input second number: " second_number
read -p "Select an math operation
1 - addition
2 - subtraction
3 - multiplication
4 - division
" operation

case $operation in
  "1") 
     echo "result= $(( $first_number + $second_number))"
  ;;
  "2")
     echo "result= $(( $first_number - $second_number))"
  ;;
  "3")
     echo "result= $(( $first_number * $second_number))" 
     ;;
  "4")
     echo "result= $(( $first_number / $second_number))"
  ;;
  *)
     echo "Wrong choice..." 
  ;;
esac

• Make the script executable and execute it.

chmod +x case-statement.sh
./case-statement.sh