If given condition is true then command1 is executed otherwise command2 is executed.
Syntax:

           if condition
           then
                       condition is zero (true - 0)
                       execute all commands up to else statement

           else
                       if condition is not true then
                       execute all commands up to fi
           fi

example of nested if else

osch=0

echo "1. Unix (Sun Os)"
echo "2. Linux (Red Hat)"
echo -n "Select your os choice [1 or 2]? "
read osch

if [ $osch -eq 1 ] ; then

     echo "You Pick up Unix (Sun Os)"

else #### nested if i.e. if within if ######
            
       if [ $osch -eq 2 ] ; then
             echo "You Pick up Linux (Red Hat)"
       else
             echo "What you don't like Unix/Linux OS."
       fi
fi

if condition which is used for decision making in shell script, If given condition is true then command1 is executed.
Syntax:

 if condition 

 then 

 command1 if condition is true or if exit status of condition is 0 (zero) 

 ... 

 ... 

 fi