Divide the value of the column if not NULL.

Shell Programming and Scripting
1

File 1
---------

N_ACCT,CARD_TYPE,CARD_BAL,CUST_CODE
---------------------------------------------------
0301,38,0.00,10
0319,38,54422.92,10
0392,38,0.00,10
0418,38,35254.87,10
0442,38,0.00,10
0491,38,0.00,10
0558,38,45988.76,10
0616,38,0.00,10
0665,38,0.00,10
0699,38,0.00,10

File 2
---------------

N_ACCT,CARD_TYPE,CARD_BAL,CUST_CODE
--------------------------------------------------
0301,38,0.00,10
0319,38,56422.92,10
0392,38,0.00,10
0418,38,38254.87,10
0442,38,0.00,10
0491,38,0.00,10
0558,38,49988.76,10
0616,38,0.00,10
0665,38,0.00,10
0699,38,0.00,10

I have to apply below logic in the script.

If IsNull(File1.CARD_BAL) Then File1.CARD_BAL/2 Else (File 1.CARD_BAL+ File2.CARD_BAL)/2.

Finally output should be in below format
----------------------------------------

N_ACCT,CARD_TYPE,CARD_BAL,CUST_CODE
0301,38,0.00,10
0319,38,54422.92,10

Since I am not familiar with UNIX, appreciate your help on this.

2

Given your (hypothetical?) IsNull function returns TRUE for Zero values and not, as is done usually in e.g. DB functions, for missing, undefined, or empty variables, the File1.CARD_BAL/2 is pointless, as it will return 0.00 again.
As you seem to want the arithmetic average of file1 and file2 fields, why then is the desired output's second line value 54422.92 , and not 55422.92 ? And, where is the rest of the resulting output lines?
And, how are the files' records related, by line No.? By key (e.g. field 1) value? By what?

3 
awk '
NR < 3
FNR > 2 {line[$1]=$0; sum[$1]+=$3}
END {
   for (i in sum) {
      $0=line;
      $3=sprintf("%0.2f", sum / 2);
      print $0;
   }
}
' FS=, OFS=, file1 file2
4

While rdrtx1's proposal is based on the assumption that records are related by $1 key value, this one goes for the line No.:

awk '
NR == 1         {for (i=1; i<=NF; i++) if ($i == COL) TGF = i
                }
                {getline TMP < DFN
                 split (TMP, T)
                 if ($TGF+0 > 0) $TGF = ($TGF + T[TGF])/2
                }
1
' FS="," OFS="," COL="CARD_BAL" DFN="file2" file1
N_ACCT,CARD_TYPE,CARD_BAL,CUST_CODE
---------------------------------------------------
0301,38,0.00,10
0319,38,55422.9,10
0392,38,0.00,10
0418,38,36754.9,10
0442,38,0.00,10
0491,38,0.00,10
0558,38,47988.8,10
0616,38,0.00,10
0665,38,0.00,10
0699,38,0.00,10
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