Combine two columns into new and print all columns - awk

Question

I want to combine columns 1 and 2 and add them as a new column in my data frame. Then I want to print all the old columns and the newly created column. I can combine the columns using the script below, but not sure how to print all columns, not only the combined:
awk ' { print $1 $2 "_" $NF } ' input_file
in
c1 c2 c3
12 1 12
4 4 57
out
c1 c2 c3 c4
12 1 12 12_1
4 4 57 4_4

Answer 1

If you want to print the _ between field 1 and 2, then the first output would be c1 c2 c3 c1_c2 instead of c1 c2 c3 c4
You can add a column at the end with the value of $1 and $2 and then print the whole line:
awk ' { $(NF+1) = $1"_"$2 }1' input_file
Output
c1 c2 c3 c1_c2
12 1 12 12_1
4 4 57 4_4
Or you can print the whole line followed by field $1 and $2
awk '{print $0, $1"_"$2}' input_file
Output
c1 c2 c3 c1_c2
12 1 12 12_1
4 4 57 4_4

Answer 2

Here is a Generic solution in awk. Just mention field numbers in awk variable named fields eg: 1,2,3,4,7,8(example) and it will add all fields values to last column. Written and tested in GNU awk should work in any awk.
awk -v fields="1,2" '
BEGIN{
num=split(fields,arr,",")
for(i=1;i<=num;i++){
field[arr[i]]
}
}
FNR==1{
print
next
}
{
val=""
for(i=1;i<=NF;i++){
if(i in field){
val=(val?val "_":"")$i
}
}
print $0,val
}
' Input_file

Answer 3

$ awk '{print $0, (NR>1 ? $1"_"$2 : "c4")}' file
c1 c2 c3 c4
12 1 12 12_1
4 4 57 4_4
or to get tab-separated output if your input is tab-separated:
$ awk 'BEGIN{FS=OFS="\t"} {print $0, (NR>1 ? $1"_"$2 : "c4")}' file
c1 c2 c3 c4
12 1 12 12_1
4 4 57 4_4
or if it isn't:
$ awk -v OFS='\t' '{$(NF+1)=(NR>1 ? $1"_"$2 : "c4")} 1' file
c1 c2 c3 c4
12 1 12 12_1
4 4 57 4_4

Answer 4

Another awk which at FNR==1 uses the field name in $NF to create the field name for the next field (c3 -> c4, c -> c1, etc):
$ awk '{
printf "%s%s%s\n",
$0,
OFS,
(FNR>1?$1 "_" $2:(match($3,/[0-9]+$/)?substr($3,1,RSTART-1) substr($3,RSTART)+1:$3 1))
}' file
Output:
c1 c2 c3 c4
12 1 12 12_1
4 4 57 4_4

Answer 5

golfed version
$ awk '$++NF=NR>1?$1"_"$2:"c4"' file
c1 c2 c3 c4
12 1 12 12_1
4 4 57 4_4