Shell – Read columns from file, then column into an exsisting csv file

That's what awk is for:

awk '{for(i=100;i<=NF;i+=100){printf "%s ",$i;} print ""}' file > output

Or, if you can have spaces inside your fields, specify tab as the field separator:

awk -F'\t' '{for(i=100;i<=NF;i+=100){printf "%s ",$i;} print ""}' file > output

Alternatively, you could use Perl:

perl -ane 'for($i=99;$i<=$#F;$i+=100){print "$F[$i] "}' file > output

To do this for multiple files, you can use a shell loop (assuming you want to run this on all files in the current directory):

for f in *; do
  awk '{for(i=100;i<=NF;i+=100){printf "%s ",$i;} print ""}' "$f" > "$f".new;
done

Try this

$ awk -F, 'NR<2{split(gensub(/Citty/,"City","g",$0),a,FS)}NR==2{for(b=2;b<=NF;b+=2){c=c a[b]" "$b","}print gensub(/,$/,"",1,c)}NR>2{print gensub(/(^,|" *",)/,"","g",$0)}' inp
Product Name,City Location,Price Per Unit
banana,CA,5.7
apple,FL,2.3
$

Same code is more readable if split across a few lines :

$ awk -F, '
> NR<2{split(gensub(/Citty/,"City","g",$0),a,FS)}
> NR==2{for(b=2;b<=NF;b+=2){c=c a[b]" "$b","}print gensub(/,$/,"",1,c)}
> NR>2{print gensub(/(^,|" *",)/,"","g",$0)}' inp
Product Name,City Location,Price Per Unit
banana,CA,5.7
apple,FL,2.3
$

If 1st line, split the line into array elements within a. Fix the Citty->City typo.

If 2nd line, starting with the 2nd column, print the corresponding column from 1st line together with this column. Repeat for each column, going in 2 column increments. Strip the trailing ,.

After 2nd line, replace any leading , or any "<spaces>", with an empty string and then print the result.

Tested ok on GNU Awk 4.0.2

Try it online!