INTRO

As a system administrator, you need to be able to confidently work with compressed “archives” of files. In particular two of your key responsibilities; installing new software, and managing backups, often require this.

CREATING ARCHIVES

On other operating systems, applications like WinZip, and pkzip before it, have long been used to gather a series of files and folders into one compressed file - with a .zip extension. Linux takes a slightly different approach, with the “gathering” of files and folders done in one step, and the compression in another.

So, you could create a “snapshot” of the current files in your /etc/init.d folder like this:

tar -cvf myinits.tar /etc/init.d/

This creates myinits.tar in your current directory.

Note 1: The -v switch (verbose) is included to give some feedback - traditionally many utilities provide no feedback unless they fail. Note 2: The -f switch specifies that “the output should go to the filename which follows” - so in this case the order of the switches is important.

(The cryptic “tar” name? - originally short for “tape archive”)

You could then compress this file with GnuZip like this:

gzip myinits.tar

…which will create myinits.tar.gz. A compressed tar archive like this is known as a “tarball”. You will also sometimes see tarballs with a .tgz extension - at the Linux commandline this doesn’t have any meaning to the system, but is simply helpful to humans.

In practice you can do the two steps in one with the “-z” switch, like this:

tar -cvzf myinits.tgz /etc/init.d/

This uses the -c switch to say that we’re creating an archive; -v to make the command “verbose”; -z to compress the result - and -f to specify the output file.

TASKS FOR TODAY

Check the links under “Resources” to better understand this - and to find out how to extract files from an archive!
Use tar to create an archive copy of some files and check the resulting size
Run the same command, but this time use -z to compress - and check the file size
Copy your archives to /tmp (with: cp) and extract each there to test that it works

POSTING YOUR PROGRESS

Nothing to post today - but make sure you understand this stuff, because we’ll be using it for real in the next day’s session!

EXTENSION

What is a .bz2 file - and how would you extract the files from it?
Research how absolute and relative paths are handled in tar - and why you need to be careful extracting from archives when logged in as root
You might notice that some tutorials write “tar cvf” rather than “tar -cvf” with the switch character - do you know why?

RESOURCES

Day 16 - Archiving and compressing