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Unix for Mac OS X Users unlocks the powerful capabilities of Unix that underlie Mac OS X, teaching how to use command-line syntax to perform common tasks such as file management, data entry, and text manipulation. The course teaches Unix from the ground up, starting with the basics of the command line and graduating to powerful, advanced tools like grep, sed, and xargs. The course shows how to enter commands in Terminal to create, move, copy, and delete files and folders; change file ownership and permissions; view and stop command and application processes; find and edit data within files; and use command-line shortcuts to speed up workflow. Exercise files accompany the course.
In this movie we are going to look at what's called symbolic links. These are also called sym links for short. So you'll often hear me just say sym link. We create them the same way as we do the hard links. We just use ln, short for link, but we use the -s option to indicate that it's a symbolic link. So ln -s and then the file that we want to link or a path to the file that we want to link, followed by the name of the link or a path with the name at the end, just like we did for hard links. We have our target followed by the name of the link.
Where symbolic links are very different from hard links is that symbolic links reference the path to the file. That's what they keep track of, is the path to get to that file. Not the file itself. So hard links keep track of the file on the hard drive and point to that file; instead symbolic links keep track of the fact, which directory is this file embedded in. As a consequence that means that they break if the file is moved. The file is no longer at that path, well, then when we try and do something with this links its going to go to that path and it's going to say, "Oh, wait I was expecting a file to be there," and it's not and you'll get an error.
And of course, if the file is deleted then, the file will definitely not be at that path any more and so will also break which is different from the way hard links works. Let's try working with symbolic links. Notice that I am inside Unix files and in here we have some files already created. You can see that I have hard link that I created in the last movie. We also have linkdir, which is just an empty directory that we'll be using again in this movie, and then I have a couple of aliases that I created in the Finder. Those aren't going to be included in the exercise file, just because they don't travel well.
They really do reference things on my file system. So you won't have those in exercise files, but they are easy for you to create. At the end of the hard link movie, we deleted our original file to see how that worked and see how it kept the data around. So what I want to do now is I want to recreate that file. So the first thing we want to do is nano, and let's create linkedfile.txt and this time just give it any kind of different text from what we had before. This is Another link test. So different than the text we had before and we'll safe that.
And the reason that I want to show you that it's different is just so that you see that this hard link still has a size of 10, while our new one has the size of 18 and if we take a look at that hard link it's still pointing at the old file. So I don't want you to think this is in any way related to this new file that we just created. So now let's see how we go back creating sym links. The way we do that is with ln and then the -s option, the path to the target file that we want to make a link to, so in our case that's going to be linkedfile.txt, and then the name of the link that we want to create.
I am going to call it symlink. Let's take a look at that and you can see the file and notice that it's very different. Notice right of the bat it says symlink and then we have this arrow and then the path to the file. We can't really see the path there because it's in the same directory. But if it were listed somewhere else on the file system we would have the full path that it would take to get to that from here. Notice also that at the beginning it's not a regular file. It's an l file. We have seen the d and we have seen the dash. This one has an l to let us know that it's a link.
The hard link doesn't do that. The hard link is just like a regular file. If we look at it in the Finder, you'll see that in the Finder it does give it this little icon to make it look like its alias just like Finder, but it's not. Looh at the difference here. Look at the size of this alias we created earlier and look at this size. Here is the difference. You see this path right here. Count up those characters. There are fourteen characters in that path. That's the fourteen characters that are being stored inside this file. So, this special type of link file and what's inside of it? Just the path to find this other thing.
So when we try and do something to the symlink, like cat symlink, what Unix does is it goes to the symlink and it says, "Oh, wait, this isn't a normal file. this is a link file. I need to read the path that's in here, go to that path, and try and find the file that's indicated and do this action to that file." The way that an alias normally works, it's more like the way the Finder does it. When we do something in the Finder, it's not pointing to the same file on the file system. It's pointing to the file itself. It's an alias to that file and if the file goes way the link breaks.
The difference is that our symlink, we can't move the file around or it breaks, because it just keeps track of this path. It's the simplest possible implementation of this. Whereas the Finder version keeps a lot more data around, so that it can keep track of this file. That's why its so much bigger is because it has this added ability that it doesn't break as you move things around. You drop something into a folder? Then no problem, your alias keeps working. But if you move something into a folder here with your symlink, let's move linkedfile into linker, now cat our sym link and it says, "Oops no such follow directory". Because it's still pointing at that same place, but when it gets there the file is not there anymore.
But in the same way, if we move it back, move linkdir linkedfile.txt, move it into this directory, now we can cat our symlink again and it works again, because it went to that path and the file was there again. It suddenly magically worked. So, just references the path. Now of course if we threw this file away, then of course it would break because the file wouldn't be at that location anymore, just the same as when we moved it. So there's really three different approaches to this and you can see why Apple came up with their alias approach.
It sort of tries to marry the best aspect of hard link and symlinks together into something new. But there's three different ways of doing it and you need to understand them all to be able to choose which one you want. Now the short version, 99% of the time, what you are going to want is symlinks. That's usually what you mean. When you are working in Unix what you are usually trying to do is give yourself a shortcut to say from this folder, boy, I really wish I had a shortcut to just shoot over to my web access logs. My access logs aren't going to move. Chances are they are going to stay in the same place.
So I'll make a symlink from here and then I can just see the end of that directory and boom, I'll be inside my web log. The other nice thing about symlinks is that they do work in the Finder. So here in the Finder, if I double- click on this, it opens it up, exactly the same as these other ones do. To the Finder they seem the same, the only difference being that symlinks are brittle and they do break if files move around.
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