#!/bin/ksh

# The fact that we set IFS only on new line is that in this question
# space and tabs are not considered as delimiters, only new lines
IFS='
'

FINAL_RESULT=""
RESULT=""

function concatenate
{
    VARIABLE1=$1
    VARIABLE2=$2
    
        
    CURRENT_LINE_NUMBER_IN_VARIABLE1=1
    CURRENT_LINE_NUMBER_IN_VARIABLE2=1
    
    for ITERATOR1 in $VARIABLE2
    do
        CURRENT_LINE_NUMBER_IN_VARIABLE1=1
        for ITERATOR2 in $VARIABLE1
        do
            if (( CURRENT_LINE_NUMBER_IN_VARIABLE1 < 
                 CURRENT_LINE_NUMBER_IN_VARIABLE2 ))
            then
                (( CURRENT_LINE_NUMBER_IN_VARIABLE1 = 
                      CURRENT_LINE_NUMBER_IN_VARIABLE1 + 1 ))
                
            elif (( CURRENT_LINE_NUMBER_IN_VARIABLE1 == 
                       CURRENT_LINE_NUMBER_IN_VARIABLE2 ))
            then
                FINAL_RESULT="$FINAL_RESULT\n$ITERATOR2$ITERATOR1"
                (( CURRENT_LINE_NUMBER_IN_VARIABLE1 = 
                      CURRENT_LINE_NUMBER_IN_VARIABLE1 + 1 ))
                break;
                      
            else
                break 2;
            fi
        done
    
    (( CURRENT_LINE_NUMBER_IN_VARIABLE2 = 
            CURRENT_LINE_NUMBER_IN_VARIABLE2 + 1 ))
    done
    
    
    CURRENT_LINE=1
    for ITERATOR1 in $VARIABLE1
    do
        # The following condition ignores the lines in VARIABLE1
        # that were read in the precedent loop
        if (( CURRENT_LINE < CURRENT_LINE_NUMBER_IN_VARIABLE1 ))
        then
            (( CURRENT_LINE = CURRENT_LINE + 1 ))
            
        # Here we concatenate the rest of the lines in VARIABLE1 with
        # the final result.
        else
            FINAL_RESULT="$FINAL_RESULT\n$ITERATOR1"
            (( CURRENT_LINE = CURRENT_LINE + 1 ))
            (( CURRENT_LINE_NUMBER_IN_VARIABLE1 = 
                         CURRENT_LINE_NUMBER_IN_VARIABLE1 + 1 ))
        fi
    done

(( TOTAL_LENGTH_MINUS_FIRST_NEW_LINE = ${#FINAL_RESULT} - 2 ))
FINAL_RESULT=${FINAL_RESULT:2:$TOTAL_LENGTH_MINUS_FIRST_NEW_LINE}
}

# In this example we suppose VARIABLE1 and VARIABLE2 are those lines
# that have to be concatenated, each line by its corresponding in
# another variable
VARIABLE1="var1 first line
var1 second line
var1 third line
var1 forth line
var1 fifth line
var1 sixth line
var1 seventh line"

VARIABLE2="var2 first line
var2 second line
var2 third line
var2 forth line
var2 fifth line"


# As you didn't specify in your problem whether both variables have
# the same number of lines, I considered also the case in which the
# number of lines in each variable is different


NUMBER_OF_NEW_LINES_IN_VARIABLE1=0
NUMBER_OF_NEW_LINES_IN_VARIABLE2=0

for ITERATOR in $VARIABLE1
do
    (( NUMBER_OF_NEW_LINES_IN_VARIABLE1 =
          NUMBER_OF_NEW_LINES_IN_VARIABLE1 + 1 ))
done


for ITERATOR in $VARIABLE2
do
    (( NUMBER_OF_NEW_LINES_IN_VARIABLE2 =
          NUMBER_OF_NEW_LINES_IN_VARIABLE2 + 1 ))
done




if (( $NUMBER_OF_NEW_LINES_IN_VARIABLE1 >=
               $NUMBER_OF_NEW_LINES_IN_VARIABLE2 ))
then
    concatenate "$VARIABLE1" "$VARIABLE2"
else
    concatenate "$VARIABLE2" "$VARIABLE1"
fi


print $FINAL_RESULT