Syntax:

           while [ condition ]
           do
                 command1
                 command2
                 command3
                 ..
                 ....
            done

Loop is executed as long as given condition is true. For e.g..the for loop program can be written using while loop as:

$cat > nt1
        #!/bin/sh
        #
        #Script to test while statement
        #
        #
        if [ $# -eq 0 ]
        then
           echo "Error - Number missing form command line
        argument"
           echo "Syntax : $0 number"
           echo " Use to print multiplication table for
        given number"
        exit 1
        fi
        n=$1
        i=1
        while [ $i -le 10 ]
        do
          echo "$n * $i = `expr $i \* $n`"
          i=`expr $i + 1`
        done

Save it and try as

$ chmod 755 nt1
$./nt1 7

Above loop can be explained as follows:

n=$1 Set the value of command line argument to variable n. (Here it's set to 7 )
i=1 Set variable i to 1
while [ $i -le 10 ]                   This is our loop condition, here if value of i is less than 10 then, shell execute all statements between do and done
do  Start loop
echo "$n  *  $i = `expr $i  \*  $n`" Print multiplication table as
7 * 1 = 7
7 * 2 = 14
....
7 * 10 = 70, Here each time value of variable n is multiply be i.
i=`expr $i + 1` Increment i by 1 and store result to i.  ( i.e. i=i+1)
Caution: If you ignore (remove) this statement  than our loop become infinite loop because value of variable i always remain less than 10 and program will only output
7 * 1 = 7
...
...
E (infinite times)
done                                   

Loop stops here if i is not less than 10 i.e. condition of loop is not true. Hence
loop is terminated.