Don't do this:

    ls | parallel -m -j $f "cat {} >> ../transactions_cat/transactions.csv"

It only works if all files enumerated by `ls` are smaller than `PIPE_BUF`, which is 4096 bytes on Linux (but was historically only 512 bytes). Writes larger than `PIPE_BUF` are not guaranteed to be atomic. Meaning, if you have input files larger than 512 or 4096 bytes, your output file will have chunks of file A interleaved with chunks of file B.

If I knew for sure all input files were smaller than PIPE_BUF, I'd do this instead:

    find . -type f -print0 | xargs -0 -P8 cat > ../transactions.csv

That saves you the overhead of firing up a new shell just to reopen a file for append and exec `cat`.