In Bioinformatics, you often work with very big files. Because of this, programs will often compress them (also called zipping) so they are easier to work with. You can tell that a file is compressed because it will have a .gz at the end of a filename.
zcat myreads.fastq.gz | head will allow you to inspect the first few lines of a zipped .fastq file.
zcat myreads.fastq.gz | wc -l will tell you the number of lines in a zipped .fastq.gz file.
zless myreads.fastq.gz will let you browse through a document one line at a time using the spacebar to go forward and b to go back a page.
zcat myreads.fastq.gz | head -400000 | gzip > Test100k.fastq.gz will make a new file Test100k with just the firsth 400000 lines of myreads.
If you need to unzip a file, use the command gunzip filename_tar.gz then if you receive no errors, type: tar xvf filename_tar.
The easiest way to zip a file is to use the command gzip filename