Need help creatig a bash script to edit CSV File

[imkornhulio]

I need to edit 4th field in the CSV file (i need to subtract 5 from whatever number is their in the fourth field, if the number is negative then replace it with 0)
The separator is ,

Please help

[imkornhulio]

Sample for CSV i am trying to edit

Online,New World,20080819,0,1200M05,045468004016,N,,42,94,,,, ,,N
Online,New World,20080819,13,1200M055,045468004023,N,,42,94,, ,,,,N
Online,New World,20080819,0,1200M06,045468004030,N,,42,94,,,, ,,N
Online,New World,20080819,67,1200M065,045468004047,N,,42,94,, ,,,,N

Thanks in advance

[vsemaska]

Here's an example using awk. Let's assume the name of the file is cvs.txt

awk -F',' '{ nmbr=$4; nmbr=nmbr-5; if (nmbr < 0) nmbr=0; $4=nmbr; print $0 }' cvs.txt

[ghostdog74]

awk 'BEGIN{ FS=","; OFS=","}{ $4-5<=0 ? $4=0 : $4=$4-5 }$1=$1' file