I have this file "file.txt" which I want to split into many smaller ones. Each line of the file has an id field which looks like "id:1" for a line belonging to id 1. For each id in the file, I like to create a file named idid.txt and put all lines that belong to this id in that file. My brute force bash script solution reads as follows.

count=1
while [ $count -lt 19945 ] do
cat file.txt | grep "id:$count " >> ./sets/id$count.txt
count='expr $count + 1'
done

Now this is very inefficient as I have do read through the file about 20.000 times. Is there a way to do the same operation with only one pass through the file? - What I'm probably asking for is a way to use the value that matches for a regular expression to name the associated output file.