Shell Programming and Scripting

COL1,COL2,COL3,COL4
1,A,B,C,D
2,e,f,g,h

COL1,COL2,newcolumns 
1,A,B,CD
2,e,f,gh

awk -F, '{$4=$4 $5;NF=4}1' OFS=, file

awk -F, '{$4=$4 $5;NF=4}1' OFS=, /home/dsadm/rpt/dstempdir/SCRIPT_SEQ_LOG/test.txt

awk -F, '{$NF=$(NF-1) $NF}; {a=NF} {for(i=1;i<=NF;i++){if(i%a==0){ORS="\n"; print $i}  else {ORS=","; print $i}}}' OFS=","  filename

COL1,COL2,COL3,COL3COL4
1,A,B,C,CD
2,e,f,g,gh

awk -F, '{print $1","$2","$3","$4$5}' <filename>

Col1,Col2,Col3,Col4
1,A,B,CD
2,e,f,gh

$ cat file3
COL1,COL2,COL3,COL4,COL5
1,A,B,C,D
2,e,f,g,h

$ cat file3 | awk -F "," '{out=$1; for(i=2;i<=NF-2;i++){out=out","$i}; print out","$(NF-1)$NF}'
COL1,COL2,COL3,COL4COL5
1,A,B,CD
2,e,f,gh