Compressing and decompressing files

UNIX systems usually support a number of utilities for backing up and compressing files. Below we will show tar,gzip and zcat utilities

tar (tape archiver)

tar backs up entire directories and files as an archive. An archive is a file that contains other files plus information about them, such as their filename, owner, timestamps, and access permissions. tar does not perform any compression by default.

To create a disk file tar archive, use

$ tar -cvf archivename filenames

where archivename will usually have a .tar extension, so tar -cvf archivename.tar filenames. The c option means create, the v option means verbose (output filenames as they are archived), and option f means file.To list the contents of a tar archive, use

$ tar -tvf archivename

To restore files from a tar archive, use

$ tar -xvf archivename

compressing files with gzip

gzip is a utility for compressing and decompressing individual files. To compress files, use:

$ gzip filename

The filename will be deleted and replaced by a compressed file called filename.Z or filename.gz. To reverse the compression process, use:

$ gzip -d filename

viewing compressed text files with zcat

zcat uncompresses the file and writes content to the standard output (which usually means the output will be on your shell screen). The following decompresses the files and writes the content on the standard output.

$ zcat geneList.gz

or in some systems:

$ gzcat geneList.gz

If the file is big and you just want to see what is in it (first few lines), you can combine zcat with head, you will see more of this in the next section, here is an example.

$ zcat geneList.gz | head

“|” allows piping commands together. head command is applied on the output of gzcat command.