how to set field name as file name bash/awk - bash

Question

I have a file with 500 columns and I would need to split each column into a new file while printing $1 as common in all the files. Below is a sample file, and I managed to do this using the below bash/awk solution :
ID F1 F2 F4 F4
aa 1 2 3 4
bb 1 2 3 4
cc 1 2 3 4
dd 1 2 3 4
num=('1' '2' '3' '4')
for i in ${num[#]}; do awk -F "\t" -v col="$i" '{print $1,$col}' OFS="\t"
Input.txt > ${i}.txt; done
which gives the required output as:
1.txt
ID ID
aa aa
bb bb
cc cc
dd dd
2.txt
ID F1
aa 1
bb 1
cc 1
dd 1
....
However, I could not track which file corresponds to which column as the output file name is the field number but not the field name. Could it be possible to write the header of the field as prefix to the output file name?
ID.txt
ID ID
aa aa
bb bb
cc cc
dd dd
F1.txt
ID F1
aa 1
bb 1
cc 1
dd 1

Answer 1

You can do it all in one awk script. When processing the first line, put all the column headings in an array. Then when you process lines you write to the file names from that array in a loop.
awk -F'\t' 'NR == 1 { split($0, filenames) }
{for (col = 1; col <= NF; col++) {
file= filenames[col] ".txt";
print $1, $col >> file;
close(file) } }' Input.txt

Answer 2

If I understand your requirement correctly, it seems like you're very close. Try
num=('1' '2' '3' '4')
for i in ${num[#]}; do
echo "i=$i"
awk -F "\t" -v col="$i" -v OFS="\t" '
NR==1{fName=$(col+1)".out";next}
{print $1,$(col+1) > fName}' data.txt
done
1>cat F1.out
aa 1
bb 1
cc 1
dd 1
. . . .
1>cat F4.out
aa 4
bb 4
cc 4
dd 4
Edit
If you need to keep the headers as shown in your example output, just remove the ;next.
Edit 2
If you have multiple column with the same name, you can append the data to same file by using >> fName instead. One word of warning with this technique. When you use > fName, this "restarts" the file each time you rerun your script. But when using >>, you will be appending to each file each time you run the script. That can cause problems for down-stream processes ;-) ... So you'd need to add code that cleans up your previous run of the script.
Here, we're relying on the fact that awk can also write output to a file, using > fName (where fName has been defined as the value of col(Num)+1 (to skip over the first column values).
And, if you were going to do this thousands of times a day, it would be worth further optimizing per comments above to have awk read the file once and create all the outputs from internal loops. But if you only need to do this a couple of times, then your 'use the tools of unix/linux to decompose the task into manageable parts' is perfectly appropriate.
IHTH