hpr_transcripts/hpr1122.txt

Episode: 1122
Title: HPR1122: LiTS 018: ln
Source: https://hub.hackerpublicradio.org/ccdn.php?filename=/eps/hpr1122/hpr1122.mp3
Transcribed: 2025-10-17 19:23:03

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Welcome to Linux in the Shell episode 18, the link command.
My name is Dan Washco, I'm going to be your host today.
We're going to talk about the link or LN command, but before I do so I want to encourage
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Today we're going to talk about the link command, LN for short.
The link command essentially creates a link from a file or directory to in a different
location.
Some operating systems call these shortcuts, but in Linux and BSD they're called links and
they're created just by using the LN command.
LN command at its heart takes one parameter at its heart, well actually two parameters.
I recommend always two parameters to the link command, but you could actually get away
with only one parameter, get ahead of myself a little bit.
What the link command takes is LN and then it takes the target directory or file and then
the link you want created or where you want to create it.
Let's say you wanted to create a link to your file named test in your home directory and
you wanted to link that into your home directory in a subfolder called download.
All you would have to do is type LN slash home slash Dan, that's what we're going to use
as a home directory slash test and then the target and the destination will be slash home
slash Dan slash download slash test and it will create a link in download that points
to the file test in your home directory.
But that's a high level of the way that's working.
I said that it only takes really one parameter.
If I would have just said LN space slash home slash Dan slash test and I was in the download
directory and hit enter, it would create the link to the test file as if I didn't specify
the file name.
It would just create a link called test which would point back to the test in home Dan.
Now that's in a nutshell, we're going to build off of that syntax but before I continue
on the different options, you have to realize that there are two types of link files.
So a hard link which is created by default and a simlink, symbolic link which you would
have to specify the dash s and what the differences between these two really hinges around the
concept of an I node.
Now the Linux file systems, except for most Linux file systems, except for RISER FS that
uses what's called an I node and an I node is a data structure, a data structure object.
So for a file to exist in the Linux file system, you kind of have three different components.
You have the I node, you have the directory listing the file name and you have the actual
data of the file.
So what the I node is, an I node contains all the metadata for the file but it doesn't
contain the data and it doesn't contain the file name.
It contains stuff like permissions where the file is in the file system, the access time,
it contains all that metadata information but not the actual data for the file or the
file name itself.
So every file or directory in the Linux file system has an I node associated to it.
Now what a hard link is, a hard link creates a special kind of file, a link that essentially
has the same I node as a hard file.
So the hard link shares the same I node as a target file and that I node points to the
data for the file but it has a different file name because the file name is stored
in this directory.
So hard links have the same I node.
Now symbolic link does not share the same I node.
It has a separate I node but it is a special file that points to the file data and file
name in a different location.
So a symbolic link is a more of a pointer that points to the data for that file.
It has its own I node and its own file name and the directory structure but a hard link
shares an I node and has a different file name and different points to the same data structure.
So that's the two, that's the differences between a hard link and a symbolic link.
Now there are some restrictions here.
One of the restrictions is that a hard link cannot span file systems or partitions.
So a hard link has to exist in the same file system as its target and on the same partition
as its target.
A symbolic link is not restricted by that.
You can create a symbolic link that spans file systems and different partitions.
Also you generally cannot hard link to a directory.
Some special circumstances as the super user you can but most of the time if you try to hard
link to a directory even as a super user you will it will bark at you and tell you you
can't do it.
So if you want to create a link to a directory it has to be a symbolic link.
Alright, now let's cover some stuff here on links.
So just remember link two types of links, hard links symbolic links all based on I nodes.
I node contains a meta data information for a file does not contain the file name it does
not contain the data hard link creates a link that has the same I node as the target
but is in a different data directory structure and points to the same data.
Symbolic link is a different I node and points to the different directory you know file system
name of the directory structure and points to the same data the data for the file.
And you can tell you can look at I nodes if you want to with the dash I option to the
LS command.
So if you do a dash LS dash L I it will tell you the I nodes of the file in question.
So you can actually look at a link and see if it shares the same I node as its target.
Very simple.
Okay, so I had said that at its heart the LN command only needs one parameter to target.
If you are using the LN command which is the target it's going to create the link in
the current directory with the same name as a target.
So long as no other file or directory exists in that directory with the same name and
the target is not in the same directory that you're working with otherwise you would have
to specify the link name.
So I always recommend specifying the link name just to be on the safe side.
So again in the example I used before if we wanted to create a link to that test directory
in the download directory I had said LN space slash home slash day and slash test space
slash home slash day and slash download slash test and that creates the link.
Now default is creating hard links.
If you want to create a symbolic link you have to use the dash S command option.
So if I wanted to create a, for instance, let's say I had a WWW directory and my home
directory with stuff I wanted to use this website files but I didn't necessarily want
to create a whole directory structure.
I didn't want to move that into the website and I just wanted to do a temporary.
This is a bad example but bear with me.
So my website, my Apache server hosts my website files, my document root is VAR slash
VAR slash WW slash and that's my document root and in my home directory I have a WW directory.
What I could do like if I wanted to just temporarily share that out there's a whole lot of
ways of doing it but we're just going to do a symbolic link.
I can do an LN space dash S slash home slash day and slash WW and then do slash VAR slash
WW slash day and what that does is it creates a symbolic link from the home of my WWW directory
in the VAR WW directory called day and so if you went to VAR WW day and it would take
you right into the WWW directory in my home directory.
Now some things would be aware of how that all works.
One of the issues that you need to realize with links is permissions.
When you do a hard link to a file essentially because it's the same I node it's they have
the same permissions that those files have the same permissions and they'll always have
the permissions of the file that you have created.
So if you change the permissions on the hard link itself you are actually going to change
permissions on the target file or directory well target file you can't really create links
to the director's.
Whereas on a symbolic link when a symbolic link is created the permissions on a symbolic
link are 777 the world readable writeable everything that's the way they work.
If you try to change permissions on the symbolic link you are actually going to change permissions
on the target file so just be aware of that it passes it through to the target file
or directory that's the way it works.
Let's cover some other options here.
So we talked about the dash s is for symbolic creating symbolic link and you can create
symbolic links across file systems and you can create a symbolic link to a directory.
One of the options that you have available to you is the ability when you're doing it
and option is the let's talk about backups.
By default if you try to create a link to a file and link already like your target your
link already exists in a location it's going to complain that the file or target already
exists.
So it's not going to let you create that link of that name.
You can change that with the dash f or dash dash force option which will erase any
link that's there and overwrite any link or file that's there.
To help out with that you can use a dash b or dash dash backup and that will back up
any file that exists.
Now dash dash backup also takes with it an additional options.
So it's dash dash backup equals and the type of backup that you're going to do.
There are four parameters that you can put in here.
None are off which means never make a backup so even if the dash dash backup is given so
it's kind of like equals none or off either one of those and it will make a backup.
Then there's equals numbered or t which makes a number backup and what you'll end up with
is the file is already there it's going to rename the file the new file it's going to
be the same name but it's going to attack on the end of it dot tilde a number dot tilde
dot tilde a number tilde.
So every time that you create a link and it needs to back it up it's going to increment
that number so to be dot tilde one tilde dot tilde two tilde and so on.
Then there's existing while I'll cover existing next then then simple is just a the file
with us tilde after this is simple backup now the way that it operates is the final option
was existing or no what that does is if it's a if it's a simple backup that already exists
in place it's going to use that if it's a number backup that already exists in place it'll
continue to number otherwise it's just going to go simple by default is the way to operate.
So the dash B command works just like dash dash backup but it doesn't accept an argument
it kind of goes with existing so if there's a number backup already it'll continue with
a number backup structure otherwise it's going to default to simple so that's that's
what those there is and dash I or dash dash interactive which will prompt you if the destination
or the the link already exists or file that name exists it will prompt you whether or not
you want to remove the destination so that's a dash I option right there you can change the
suffix for backups with the dash capital S or dash dash at suffix equals and in a suffix name
so instead of getting like tilde or tilde number tilde it would just create that suffix name that
you're going to use and if it was numbered it would create a number after it so that's the dash
S capital S or dash dash suffix equals and then the suffix you want to use.
There's a verbose option to Ellen which is dash V or dash dash for both and that will echo
out exactly what it's going to do so if you did Ellen dash S for creating symbolic link
home dan www to var www dan it would spit out that it created a link to home dan or created a link
from home from www slash var slash dan to home dan www it's verbose it kind of makes sense right there
you can create a relative symbolic link now a relative symbolic link generally when you
when you create a symbolic link you pass the full you know the full parameters to them it'll
use the full parameters whereas if you create a relative symbolic link it will create that link
relative to the target so if you were to pass like I was saying before the full paths
creating a symbolic link from slash from slash dan slash www to slash var slash
www slash dan when you go to look at that the relative link that it would create in the
var www dan would be instead of showing the path to to dan in the home directory it would be
dot dot slash dot dot slash home slash dan slash www so that that's kind of what it does is
creates a relative path from the location of the link to the target so that that works like that
with the dash R now we are going to talk about some some kind of little more complex aspects of
the L and command and one of those is the dash P or capital P the dash capital P makes hard links
to symbolic links okay which is kind of odd you're like well why would you want to do that I have
no idea why you want to do it but you can if you were to do like if you were to have a say you had
a symbolic link and you did let's go let's go with the back to the original example where we had
a hard or a hard link to test in the home directory to test in the download directory we had a
symbolic link to test in the download directory to test in the home directory and I tried to create
a hard link against test in the download essentially what it would do is it would create a hard
link to its target so by doing the dash capital P that creates a hard link to the symbolic link
so now you have a hard link to the symbolic link as opposed directly to the target of the symbolic
link why you would want to do that I really do not know but you can let me let me correct something
there because I'm a little off on that we were afraid something if you attempt to create a hard link
to a existing symbolic link by default what it will do is create a symbolic link to the target
of the existing symbolic link so in the case if I just did Ellen
Ellen test and new in the download directory new would be a symbolic link to the target of test
which is test in my own directory whereas with the dash P creates a symbolic link
creates a hard link to the symbolic link now there's another option dash L which will tell
it to create in that case a hard link to the target of the symbolic link so the three ways
this works trying to create a hard link to a symbolic link by default will create a symbolic link
to the target of the symbolic link where if you pass the dash capital P is going to create
a hard link to the symbolic link and a dash capital L or logical maybe it is dash dash logical
dash dash logical will create a hard link to the target file instead of creating a symbolic link
so I cleared that up understand that there are two other options that I want to talk about these
are a little more complex and special in certain very limited circumstances it's the dash T
or dash dash target dash directory equals in the dash capital T dash dash no target no
dash target dash directory these are a little more complex and have very specialized usages
and generally speaking you probably will never need to use any of these unless you're actually
programming trying to talk about these these difference these more complex ones and I'm going
to throw the dash and in there also because it operates like dash T but it's a little weaker than
dash T they're really a little bit hard to talk about because they they operate on the differences
between linking to trying to link to a directory or what the target is whether a directory or a
link and they're the rather to speak of them is is getting into a bit of a wordplay and might not
be able to convey the full understanding in the audio I recommend that you go to the website
and look at the examples for these commands with the dash and it depends on what what the target is
if the target of the ln command is a symbolic link to a directory instead of treating it
as a symbolic link it treats it as like a normal file all right and if you're using it it's kind
of a weaker version of the dash dash capital T or dash dash no dash target dash directory which
essentially is kind of like the same thing it would force the ln command to treat any
director or symbolic link to a directory as a regular file and again these are used in special
conditions it's generally you want to be careful about not creating hard links to
directories but you know creating symbolic links to directories works fine but if you're trying to
if you're trying to create a link to a directory you got to be aware of how these things behave
generally speaking if your target your link name is is a directory itself you create symbolic links
to that directory but if you can you can change how the behavior is with the dash and or the
dash capital T to look at that symbolic link to a directory as if it were a regular file
the dash T and the dash dash T direct dash directory equals takes a directory after it and it
tells the ln command to use directory that you specified as the specified directory option
generally you would use this in conjunction with another command but you were actually passing
information to like x-ards would be a great example if you are listing out or using fine
defined files of a specific type in the directory you can pass that to x-ards and then pass
the to the x-ards command the output of that fine command to x-ards to pass to ln to say okay using
the dash T the specified directory name pass the directory to use that as a directory and then
create all these links in that directory go to the website check that out it has links to
some further explanations particularly the man page and the info page and there's another page
that I referenced in there for with the dash T and the dash capital T mean but chances are you will
never use the dash and dash T dash capital T in your life so the very outlying cases right there
but I've covered all the basics of the ln command if you want to see it in action head on over to
the website linuxinachial.org look up this episode and watch the video which I'll be posting up there
also I encourage you to do that anyhow to read the full write-up of the ln command get a better
understanding my name is Dan is Dan waschko and I thank you for joining me on linuxinachial
episode 18 with the link command and thank hack and public radio you have a great day
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