How to add a column at the end of multiple csv files using shell script - linux

Question

I have a couple of thousands CSV file. All of them have same structure and header. I would like to add a column at the end of the file. I found several solutions that add a column and value to that column but I didn't find anything that adds the header for that new column. For example, I have files like 1001.csv, 1002.csv, 1003.csv and so on.
Contents of 1001.csv
ID,URL
1,one.com
2,two.com
I want to modify it like this
ID,URL,FILE
1,one.com,1001
2,two.com,1001
Since I have tons of files like this, I don't want to mess up the data while adding a column. Also, I don't want to produce extra files if it's possible to do in place update.

Answer 1

I tested this on a huge number of files and it worked really fast. This code removes the header first then add a column plus value to the column and finally brings the header back.
#!/bin/bash
# How to run $ ./this-script.sh inputdir/
# here inputdir contains all csv files
# input argument is dir name
DIRNAME=`basename $1`
# go to target directory
cd $DIRNAME
# get list of all csv files
csvfiles=`ls *.csv`
for FILENAME in $csvfiles
do
echo $FILENAME
# filename without extension
CODE="${FILENAME%.*}"
echo $CODE
## remove header
tail -n +2 "$FILENAME" > "$FILENAME.tmp" && mv "$FILENAME.tmp" "$FILENAME"
## add new field at the end
sed "s/$/,$CODE/" "$FILENAME" > "$FILENAME.tmp2"
## add header with new column name
# keep filename.bak as a backup for safety
sed -i.bak 1i"id,url,file" "$FILENAME.tmp2"
# if all good then remove temp files
rm "$FILENAME"
rm "$FILENAME.tmp2.bak"
# rename output file to original name
mv "$FILENAME.tmp2" "$FILENAME"
done
# go back to parent directory
cd ..