Shell Programming and Scripting

awk -F, -v OFS=','  '$3="xxxxx" substr($3,6)' file

cat file_1 
R.Singh test abcdefgh

awk '{split("abcdefghijklmnopqrstuvwxyz", A,"");while(j++<=26){if(j==25 || j==26){D[A[j]]=A[j];} else {D[A[j]]=A[j+2]}};while(i++<=length($3)){S=S D[substr($3,i,1)]};$3=S;print}' file_1

R.Singh test cdefghij

cat file_1.ksh 
awk '{
    split("abcdefghijklmnopqrstuvwxyz", A,"");
    while(j++<=26){        
            if(j==25 || j==26)
                { 
                    D[A[j]]=A[j]
                }
            else
                {
                    D[A[j]]=A[j+2]
                }
              };
    while(i++<=length($3)){
                S=S D[substr($3,i,1)]
                  };
    $3=S;
    print
      }' file_1

awk '{
        split("abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ", A,"");
        while(j++<=52){
                        if(j==25 || j==26 || j==51 || j==52)
                                {
                                        D[A[j]]=A[j]
                                }
                        else
                                {
                                        D[A[j]]=A[j+2]
                                }
                      };
        while(i++<=length($3)){
                                S=S D[substr($3,i,1)]
                              };
        $3=S;
        print
      }' file_1

 #####Input_file:
 cat file_1
R.Singh test abcdefghABCETRFG

 ######After running above EDIT#3 code in script as follows.
 ./file_1.ksh
R.Singh test cdefghijCDEGVTHI

lenA=split("a b c d e f g ...", A)

 cat file
abc1,abc2,abc3,abc4

 cat crypt3.sh
awk -F, -v OFS=, '
BEGIN {
  chars="abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"
  lchars=length(chars)
}
function crypt(s) {
  s1=""
  for (pos=1; pos<=length(s); pos++) {
    c=substr(s,pos,1)
    i=index(chars,c)
    c1=substr(chars,1+(1+i)%lchars,1)
    s1=s1 (i>0 ? c1 : c)
  }
  return s1
}
{
  $3=crypt($3)
  print
}
' "$@"

 sh crypt3.sh file
abc1,abc2,cde3,abc4

perl -aF, -ne '$F[2]=~tr/a-zA-Z/c-zA-Zab/; print join(",",@F)' file