Thanx for quick reply, but it doesn't work for me.
I rephrase my problem:
Every row in a file should have 5 seperate columns in which values are tab seperated.
and there are some blank values, say column 4 of row 2 is blank and column 3 of row 3 is blank. I need to first fill this blank value with 'null' and then convert it into CSV
i.e the script should consider the tab value as blank and replace it with ',' but should not consider the blank value of a column as blank space.
sed will replace tabs and blank values of a column with ',' so the output becomes
row 1 has all 5 values seperated with comma
row 2 has first 3 values seperated with comma and then sed consider the tab space and blank value as blankspace and replace it with comma and then write the 5th column value so the output becomes val1,val2,val3,val5. It should be like val1,val2,val3,null,val5