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Thias 07-25-2020 01:21 PM

Splitting a FastQ file into two
Hello folks,

I happen to have a small problem which seemed to be trivial at first, but keeps me busy for a while now already. Maybe you can help...


I need to split 615M paired reads currently in two FastQ files into two file pairs with 308M reads each.

Solution attempt A:

I unsuccessfully tried to use line count based tools like split or awk, but since newline characters occur in the quality scores, these tools respectively I screwed up badly.

Solution attempt B:

bbmap/  in=...  in2=... out=... out2=... reads=308000000
bbmap/  in=...  in2=... out=... out2=... skipreads=308000000

resulted in

Input is being processed as paired
Input: 615307122 reads 91326404105 bases
Output: 615307122 reads (100.00%) 91326404105 bases (100.00%)

for the first command and in

Input is being processed as paired
Input: 615307122 reads 91326404105 bases
Output: 0 reads (0.00%) 0 bases (0.00%)

for the second command. Effectively those read commands seem to be ignored (BBMap Version 38.76).

Solution attempt C:

famas --in=...  --in2=... --out=.XXXXXX.fq.gz  --out2=.XXXXXX.fq.gz  -x 308000000
flooded the output directory with thousands of subfiles files (instead of the actually needed two files each) until the file system couldn't cope with the number of open files anymore and ran out of file descriptors (famas version 0.0.12).

ERROR(famas.c|open_output_one:1056): Couldn't open =...compressed.065534.fq.gz
ERROR(famas.c|main:1163): Couldn't open output files. Exiting...

Since it took me a while to clean that mess up on the cluster again, I am somewhat reluctant to try out more now. Any ideas what I made wrong or suggestions which tools work better?

Thanks a lot for reading and help!

GenoMax 07-27-2020 05:15 AM

Cross-posted and answered at:

Thias 07-27-2020 09:15 AM

Indeed, the issue is solved by now using seqkit split2.

My apologies for not indicating this here. I had posted here first but the thread was lingering in moderation for ~24h and thus I decided to ask for help on Biostars. It had not yet shown up when I got the answer on Biostars and I subsequently forgot to check back here. Thanks a lot to everyone none the less!

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