We used a sequencing service and they gave us a 100GB tar file to download. After downloading, I checked the md5sum code and it matches theirs. But after I unzip the tar file and find fastq.gz files inside a folder, I tried gunzip -c filename.fastq.gz | head, I get "not in gzip format" error. I tried file filename.fastq.gz, it says "data" (not gzip compressed data as I would expect). When I just double click on a fastq.gz file, it goes into gz cpgz loop. Is it possible that they gave us corrupt files?
Finally. I wanted to leave an update here so others who run into this problem might use this post as a reference. I heard back from the sequencing providers and they had to re-do the fastq files (I'm not sure exactly what they had to re-do, but that's what they told me). The tar file size was half the original, and the fastq.gz files all behave normal (I could simply double click on one and it turned into a readable fastq file, and
file command returned
gzip compressed data, extra field on all the files). I guess their gunzip process went wrong the first time around. Thank you so much for all your help.