Shell Scripting Tutorial for Beginners 5 - If Statement ( If then , If t...

Bash Shell

Conditional Statements 

• Conditionals let us decide whether to perform an action or not, this decision is taken by evaluating an expression. The most basic form is:

      if [ expression ];

      then

            statements

      elif [ expression ];

      then

            statements

      else

            statements

      fi

• the elif (else if) and else sections are optional

• Put spaces after [ and before ], and around the operators and operands. 

Bash Shell

Expressions 

• An expression can be: String comparison, Numeric comparison, File operators and Logical operators and it is represented by [expression]:

• String Comparisons: 

=  compare if two strings are equal

!=  compare if two strings are not equal

-n  evaluate if string length is greater than zero

-z  evaluate if string length is equal to zero

• Examples:

[ s1 = s2 ]  (true if s1 same as s2, else false)

[ s1 != s2 ]  (true if s1 not same as s2, else false)

[ s1 ]   (true if s1 is not empty, else false)

[ -n s1 ]   (true if s1 has a length greater then 0, else false)

[ -z s2 ]   (true if s2 has a length of 0, otherwise false)

Bash Shell

Expressions 

• Number Comparisons:

-eq compare if two numbers are equal

-ge         compare if one number is greater than or equal to a number

-le  compare if one number is less than or equal to a number

-ne  compare if two numbers are not equal

-gt  compare if one number is greater than another number

-lt  compare if one number is less than another number

• Examples:

[ n1 -eq n2 ]  (true if n1 same as n2, else false)

[ n1 -ge n2 ]  (true if n1greater then or equal to n2, else false)

[ n1 -le n2 ]  (true if n1 less then or equal to n2, else false)

[ n1 -ne n2 ]  (true if n1 is not same as n2, else false)

[ n1 -gt n2 ]  (true if n1 greater then n2, else false)

[ n1 -lt n2 ]  (true if n1 less then n2, else false)

Bash Shell

Examples 

$ cat user.sh

 #!/bin/bash

      echo -n “Enter your login name: "

      read name

      if [ “$name” = “$USER” ];

      then

            echo “Hello, $name. How are you today ?”

      else

            echo “You are not $USER, so who are you ?”

      fi

$ cat number.sh

#!/bin/bash

      echo -n “Enter a number 1 < x < 10: "

      read num

      if [ “$num” -lt 10 ]; then

            if [ “$num” -gt 1 ]; then

                  echo “$num*$num=$(($num*$num))”

            else

                  echo “Wrong insertion !”

            fi

      else

            echo “Wrong insertion !”

      fi

#! /bin/bash

word=a

if  [[ $word == "b" ]]
then
  echo "condition b is true"
elif [[ $word == "a" ]]
then 
  echo "condition a is true" 
else
  echo "condition is false"    
fi