2001 12 06

                    LINUX NEWS
      Resources & Links From CramSession.com
                 December 6, 2001


1) Sean’s Notes

2) Linux News

Server On A Blade
Compaq and Red Hat Certification Deals
Snubbed by the Queen
Linux at the Movies: LOTR

3) Linux Resources

DNS In Depth
Linux Docs for Your PDA
Linux, Step By Step
Clustering by Compaq
Building Perl Projects with MakeMaker

4) App o’ the Week

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1) Sean's Notes

One of the features that separates Unix from other operating
systems is its robust file system.  In particular, symbolic
links (symlinks) allow the administrator a great deal of
flexibility when managing storage.

The basic premise behind symlinks is that they act as a sort
of transparent pointer to where a file or directory actually
resides, all the while providing the user or application
something that looks, smells, and feels like a real file.

Symlinks come in two flavors, hard and soft.  Each has its
advantages, although hard links are less popular.  A hard
link is just a pointer to the file's inode.  In the Unix
filesystem, inodes are like DOS's sector.  By duplicating
the file pointer, the link is "hard" in that both file
pointers see the file identically.

Change into /tmp, and we'll do some examples.

$ cd /tmp
$ touch orig
$ ls -l orig
-rw-rw-r--    1 sean     sean            0 Dec  5 19:37 orig

Looks normal enough.  Let's use the "ln" command to create
a symlink.  With this command, the target file (i.e. the
one that exists) goes first, and the name of the desired
link goes second.

$ ln orig new
$ ls -l orig new
-rw-rw-r--    2 sean     sean            0 Dec  5 19:37 new
-rw-rw-r--    2 sean     sean            0 Dec  5 19:37 orig

You'll notice they both look the same, except the second
column is 2 instead of 1.  This is the link count field, 2
means that two files point to the inode.  If you were coming
across the orig file for the first time and saw the 2, how
would you find the other files that point to it?  The -i
option to ls displays the actual inode, and we can also use
the find command to find a file by inode.

$ ls -i orig new
  24640 new    24640 orig
$ find /tmp -inum 24640 -print

As you can see from the ls, new and orig share the same inode.
The -inum predicate to find then searches by inode, instead
of the usual name.

Since the files share the same inode, you can delete one
and the file still stays:

$ rm orig
$ ls -li new
  24640 -rw-rw-r--    1 sean     sean      0 Dec  5 19:37 new

That's one key difference with a hard link.  Since the files
share the same inode, you have to delete all the files to
get rid of the space.  Also, inodes are a per filesystem
thing.  You can't make a hard link to a file on another
filesystem.  You also can't hard link a directory.

Soft links, by contrast, create a new file that simply points
to the other file.  Delete that other file, and you've got a
dangling pointer.  However, you get the power to symlink
across file systems, and to directories.

$ rm new
$ touch orig
$ ln -s orig new
$ ls -li orig new
  24641 lrwxrwxrwx    1 sean     sean     4 Dec  5 19:45 new -> orig
  24640 -rw-rw-r--    1 sean     sean     0 Dec  5 19:45 orig

As you can see, the link count is one and they share separate
inodes.  "new" is also a file of type 'l' (link), and has
world write permissions.  Don't worry, that's how all links
are, and doesn't mean that anyone can write to the "orig"
file.  By contrast, changing the permissions on one hard
linked file makes the changes to the others, since they share
the same inode.  Off to the right, you can see explicitly
that new points to orig.  From all other perspectives, you
can access "new" just as if it were "orig".

Now that you have this power, when would you use it?  From a
programmer's perspective, you can always see how you were
invoked, so the program could perform a slightly different
action based on that.  Take for example sendmail, which when
invoked as "mailq" spits out a list of what's in the queue.
Or, some of the yp tools which change various fields
(password, shell, description) based on how they were called.
My /usr/bin directory is full of such files:

$ ls -l  /usr/bin | awk '$2 > 1 {print}'

>From the user's perspective, you can use it to place items in
your path.  If Netscape were installed as /usr/local/netscape/
bin/netscape, you could symlink that into a file that is in
your path (or the admin could do it system wide in /usr/local/
bin).  Similarly, if you had to move a file for some reason,
or install it in an unexpected place, you could provide a link
back.  Perl is usually installed in /usr/bin on Linux systems,
though people installing it from source may likely have it in
/usr/local/bin.  A symlink can easily fix the problem where a
script on one system is run on the other.

Since a soft link can be used on directories, it can be a
great help in freeing up space.  If /usr/local/someapp is
getting too big, but you've got a lot of free space on /spool,
you could move someapp to /spool/someapp, and have a pointer
in /usr/local.

As with anything in Unix, you're given enough rope to hang
yourself with.  Symlinks, when used too often, end up as a
"link farm".  Break one, and nothing works.  So, use them
judiciously, and they'll make your life a lot easier.

Long live the Penguin,


2) Linux News

Server On A Blade
HP has come out with a product much like the RLX, which
provides either an Intel or PA-RISC chip in a high-density
configuration. Initially, it supports Red Hat, Debian, and
SuSE, but will also include Windows and HP/UX at a later
date. Even though this doesn't have the power advantages
that RLX does, it's good to see the high density server on
a blade concept is picking up momentum.


Compaq and Red Hat Certification Deals
Compaq offers an Accredited Systems Engineer certification,
which focuses on the use of Compaq servers. They've added a
new track, the ASE/Linux, which is all about using Linux on
Proliants. The prerequisite is an RHCE or an advanced SAIR
cert. On the flip side, Red Hat has offered a 10% discount
on all training to ASE's who wish to get their RHCE.


Snubbed by the Queen
A couple of years ago, the British royal family moved its web
hosting from Solaris to Linux, saying that the performance
was better. That company unfortunately got out of the hosting
business, so they've moved to a new ISP that hosts everything
on IIS.


Linux at the Movies: LOTR
It would appear that the crowds in the upcoming Lord Of The
Rings movie were generated by... you guessed it, machines
running Linux! This article talks briefly about the software
used, but mostly about how Linux is taking over the movie
industry, especially those high end (and high priced) SGI


3) Linux Resources

DNS In Depth
Understanding DNS is crucial to mastering Unix. While there
are a lot of articles that tell you how to set up DNS, there
are few that tell you how it works.  This article gets into
some of the good details like TTLs and SOAs.


Linux Docs for Your PDA
This site has tons of documents you can download to your
PDA for later reading. Its computer section is full of Linux
manuals, such as the HOWTOs and LDP Guides.


Linux, Step By Step
The HOWTOs are pretty good, but some of them make even the
seasoned Linux user scratch his head. Linux, Step By Step
has many common procedures in an easily readable format.
There is a lot of stuff there, so chances are you'll find
what you need.


Clustering by Compaq
Compaq is a sponsor of the Source Forge Clustering Foundry,
a collection of projects made for various forms of clustering.
This article talks about the various forms of clustering,
focusing on the Single System Image method of providing
high availability. This is a very technical article, so
make sure you've got a full cup of coffee before starting!


Building Perl Projects with MakeMaker
"If you've used UNIX or Linux for some period of time, you've
probably written a few Perl programs to automate simple tasks.
Each of these programs does something basic and simple that
might otherwise take you 10 or 20 minutes to do by hand. In
this article, Sean will show you how to convert just such a
Perl program into a far more robust programming project, one
that will be generic enough to be widely distributed across
many disparate platforms."


4) App o' the Week

"gLabels is a lightweight program for creating labels and
business cards for the GNOME desktop environment. It is
designed to work with various laser/ink-jet peel-off label
and business card sheets that you'll find at most office
supply stores."


(C) 2001 BrainBuzz.com, Inc. All Rights Reserved.

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