I'm trying to find a solution for the following problem. I have two sets of files:

  • Folder A has around 400 text files.
  • Folder B has around 20 000 text files total in several subfolders.

Files in folder A are either modified versions of files in folder B or they are parts of files in folder B. When I say "parts", I mean that a file in folder A might contain part of the text of a file in folder B, but not everything.

I want to match those pairs i.e. for each file in folder A I want to find the file or files in folder B that most closely resemble the file in folder A.

For example I would like to have the following kind of report:

File ./A/foo123.txt most closely matches file ./B/bar243.txt with 68% of lines identical.
File ./A/bar306.txt most closely matches file ./B/foo85.txt with 30% of lines identical.

Is there a command line tool that I could use to achieve this result? Or what would be the best way to do this?

  • That sounds like a task which takes a long time - if each pair of two is compared this is O(2), which means the number of comparisons is almost n*n where n is the number of files. – Ned64 Oct 8 at 14:41
  • Related unix.stackexchange.com/questions/1079/… – pLumo Oct 8 at 14:53
  • This could be done with a loop of course. However, I know that there are better options. For example there are plagiat testing programs that compare a small number of files to a large database of previously published work and they don't do it by comparing each pair of files separately. I would be surprised if nobody has written a command line tool for doing that kind of comparison between two folders. – ttsc Oct 8 at 15:12
  • they have a huge databases with a lot of pattern indexing .... this is very specialized and imo far beyond U&L ... – pLumo Oct 8 at 15:15

Something like this would work:

for fa in A/*; do


    for fb in B/*; do

    num_identical_lines=$(diff --unchanged-group-format='%<' --old-group-format='' --new-group-format='' --changed-group-format='' "$fa" "$fb" | wc -l)
    num_lines_file_a=$(wc -l < "$fa")

    # save permille of matching lines

    # compare with highest permille
    if [ $pm -gt $highest_pm ]; then


    # output
    [ $highest_pm -gt 0 ] \
    && printf "File %s best matches File %s with %d %% of identical lines.\n" "$fa" "$best_match" $((highest_pm/10)) \
    || printf "File %s has no match\n" "$fa"


The evaluation of num_identical_lines is based on this answer.
The rest is just a loop over the files, some comparison and some output ;-)


File A/file2 has no match
File A/filea best matches File B/fileb with 50 % of identical lines.
  • Thanks. This seems to work. It's very slow, but if there is no tools with a better algorithm available, this will do the job. There is one thing that I would like to clarify. In my example B does contain subfolders. I found that I can make this work for subfolders by adding shopt -s globstar in the beginning and using for fb in B/**; do. However, it turned out that using globstar that way also matches folders and not just files inside the folders. How could I make this work so that it would loop thrgough all files inside all subfolders of B, but not match the subfolders themselves? – ttsc Oct 19 at 12:24

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