what is a symlink

James Gray james at grayonline.id.au
Mon Jul 4 20:39:53 CDT 2005


On Tue, 5 Jul 2005 10:47 am, Sara Vasquez wrote:
...
> Also the only help they have is this
> http://processing.org/faq/platforms.html#linux
> where I am directed to make a symlink for the Java and the jikes folder,
> but the problem is that i din;t know what is a symlink or how to make
> one. maybe someone can tell em waht it is.
> thanks for your replies

"symlink" is a shortened version of the term "symbolic link".  As the name 
suggests it's a not a physical duplicate, but rather a "link" that points to 
something else.  In most circumstances, a symlink behaves just like the 
target it points to (there are exceptions - see below).

So if I have a file /foo/bar/myfile.txt and want to refer to that file in my 
home directory without having to use the full path, I create a symlink in my 
home directory that points to it (logically):
/home/myhome/myfile.txt -> /foo/bar/myfile.txt

Now if I edit /home/myhome/myfile.txt I am *actually* changing the 
file /foo/bar/myfile.txt.  Some goes for programs, if I create a link to a 
program, then execute the link, I'm actually executing the target.  You can 
link practically anything; files, executables, directories, even device 
nodes.

Now to actually link something, you need to use the command "ln" (short for 
"link").  There are two types of links: hard and symbolic (symbolic are 
sometimes called "soft" links too).  Don't get too concerned about hard links 
for the time being - you'd rarely need to use one and when you do, you'll 
KNOW that you do.  Start by just using symbolic links.

So, in my example above, I wanted to link /home/myhome/myfile.txt 
to /foo/bar/myfile.txt.  Here's how you would do it:
ln -s /foo/bar/myfile.txt /home/myhome/myfile.txt

...if I was in my home directory to start with, I could simply do:
ln -s /foo/bar/myfile.txt

The "-s" means "symbolic link" as ln by default tries to make hard links.

The general form is very similar to the "cp" command:
ln -s target link-name

So in my example, "/foo/bar/myfile.txt" is the target, and 
"/home/myhome/myfile.txt" is the link name.

Here's a few tips:
1. the link name can be totally different to the target name, eg;
   ln -s program-name myprogram
   This basically says "I want to refer to program-name as myprogram as well"
2. link targets can be relative to the link name, eg;
   cd ~  (ie, cd to /home/myuserid/)
   ln -s ../../etc/profile .profile
   This will link the system-wide shell config to your home directory.  BUT
   unless you can edit /etc/profile you'll be stuck with whatever your
   sysadmin has deemed "default".
3. Deleting a symbolic link only deletes the link, not the target.
4. Symbolic links inherit the permissions of the target - don't be fooled when
   you see the permissions of a link being world-read+write+exec. ;)
5. Pay careful attention to file commands (cp, mv, tar, etc) because the way
   symbolic links are handled is sometimes a little non-intuitive.  This is
   what I was referring to when I said links don't always behave like a
   regular file.  "man cp", "man mv", "man tar" etc, are your friends.

Well, that's a crash course in file system links.  For all the details:

"man ln"

Cheers,

James
-- 
Woke up this mornin' an' I had myself a beer,
Yeah, Ah woke up this mornin' an' I had myself a beer
The future's uncertain and the end is always near.
  -- Jim Morrison, "Roadhouse Blues"
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