i have a file like :

File1

var1,var2,var3
var4,var5,var6
var7,var7,var8
.
.
.

$3 : var3,var6,var8, may be null in this file so when it's nul i get the information from a function serach () which return $var i want to print it in $3

i do like that :

awk -F "," '{ {first=$1; sec=$2; tro=$3 }
if (tro == "")
{


search ( parametre)
{print $1","$2","$var} }
else
{print $1","$2","$3 }}' $file1 > $file

but i doesn't print $var in $file.

any help please

where did you get $file1

If I understand correctly:

% cat data 
var1,var2,
var4,var5,var6
var7,var7
var7,var7,var8
% var="whatever"
% awk '!$3{$3=v}1' OFS="," FS="," v="$var" data
var1,var2,whatever
var4,var5,var6
var7,var7,whatever
var7,var7,var8