Add Columns to .csv from Multiple Files

try:

 <file_list xargs -I % awk '
     { system("sed -i '\'' " NR "s/,\\?$/," $4 "/'\'' test.csv" ) }' %.txt

the xargs is reading filename from file_list file and feed to awk thought xarags's variable called % with .txt suffix as filename contains.

with sed '#s/$/something/' test.csv" command that used to append something at the end $ of a specific line number # in a file like test.csv; so at above, $4 is the string that need to appended to specific line number taking from NR; ,\?$ is saying might your line ends with comma , which I used because you have it in your test.csv. this sed command is calling by awk's system() function.

note that sed is writing in-place any changes, so you will not notice terminal ouput, do:

$ cat test.csv
0,8,4,12
1,10,6,3
2,5,9,10