LINUX NEWS
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December 6, 2001TABLE OF CONTENTS
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: LOTR3) Linux Resources
DNS In Depth
Linux Docs for Your PDA
Linux, Step By Step
Clustering by Compaq
Building Perl Projects with MakeMaker4) 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 /tmp/orig /tmp/new
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,
Sean mailto:swalberg@cramsession.com
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.
http://www.hpservernews.com/blades/release1.html
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.
http://www.compaq.com/certification/na/ase_proliant_linux.html http://www.redhat.com/training/ase.html
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.
http://www.zdnet.com/zdnn/stories/news/0,4586,2830122,00.html
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 workstations.
http://www.stuff.co.nz/inl/index/0,1008,1025220a28,FF.html
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.
http://www.8wire.com/articles/?AID$84
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.
http://www.memoware.com/Category=Computers_ResultSet=0.htm
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.
http://linux.nf/stepbystep.html
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!
http://foundries.sourceforge.net/clusters/index.pl?node_id8692
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.”
http://www-106.ibm.com/developerworks/linux/library/l-make.html
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.”
http://snaught.com/glabels/
(C) 2001 BrainBuzz.com, Inc. All Rights Reserved.
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