Multilevel if-then-else

           if condition
           then
                       condition is zero (true - 0)
                       execute all commands up to elif statement
           elif condition1 
           then
                       condition1 is zero (true - 0)
                       execute all commands up to elif statement  
           elif condition2
           then
                       condition2 is zero (true - 0)
                       execute all commands up to elif statement          
           else
                       None of the above condtion,condtion1,condtion2 are true (i.e. 
                       all of the above nonzero or false)
                       execute all commands up to fi
           fi

For multilevel if-then-else statement try the following script:

Try above script as follows:
$ chmod 755 elf
$ ./elf 1
$ ./elf -2
$ ./elf 0
$ ./elf a
Here o/p for last sample run:
./elf: [: -gt: unary operator expected
./elf: [: -lt: unary operator expected
./elf: [: -eq: unary operator expected
Opps! a is not number, give number
Above program gives error for last run, here integer comparison is expected therefore error like "./elf: [: -gt: unary operator expected" occurs, but still our program notify this error to user by providing message "Opps! a is not number, give number".