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Symbolic links

From: Unix for Mac OS X Users

Video: Symbolic links

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.

Symbolic links

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.

Show transcript

This video is part of

Image for Unix for Mac OS X Users
Unix for Mac OS X Users

82 video lessons · 25450 viewers

Kevin Skoglund
Author

 
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  1. 3m 57s
    1. Introduction
      1m 14s
    2. Using the exercise files
      2m 43s
  2. 32m 2s
    1. What is Unix?
      7m 27s
    2. The terminal application
      4m 23s
    3. Logging in and using the command prompt
      5m 19s
    4. Command structure
      5m 22s
    5. Kernel and shells
      5m 25s
    6. Unix manual pages
      4m 6s
  3. 15m 58s
    1. The working directory
      2m 49s
    2. Listing files and directories
      3m 59s
    3. Moving around the filesystem
      4m 58s
    4. Filesystem organization
      4m 12s
  4. 1h 4m
    1. Naming files
      5m 41s
    2. Creating files
      2m 19s
    3. Unix text editors
      6m 39s
    4. Reading files
      5m 35s
    5. Reading portions of files
      3m 27s
    6. Creating directories
      2m 40s
    7. Moving and renaming files and directories
      8m 32s
    8. Copying files and directories
      3m 7s
    9. Deleting files and directories
      3m 38s
    10. Finder aliases in Unix
      4m 10s
    11. Hard links
      5m 30s
    12. Symbolic links
      6m 36s
    13. Searching for files and directories
      6m 32s
  5. 34m 58s
    1. Who am I?
      4m 3s
    2. Unix groups
      1m 52s
    3. File and directory ownership
      6m 41s
    4. File and directory permissions
      4m 27s
    5. Setting permissions using alpha notation
      6m 49s
    6. Setting permissions using octal notation
      3m 49s
    7. The root user
      1m 57s
    8. sudo and sudoers
      5m 20s
  6. 52m 34s
    1. Command basics
      4m 4s
    2. The PATH variable
      4m 13s
    3. System information commands
      3m 40s
    4. Disk information commands
      6m 8s
    5. Viewing processes
      5m 0s
    6. Monitoring processes
      3m 36s
    7. Stopping processes
      3m 19s
    8. Text file helpers
      6m 50s
    9. Utility programs
      7m 28s
    10. Using the command history
      8m 16s
  7. 20m 39s
    1. Standard input and standard output
      1m 24s
    2. Directing output to a file
      4m 13s
    3. Appending to a file
      2m 44s
    4. Directing input from a file
      5m 28s
    5. Piping output to input
      4m 40s
    6. Suppressing output
      2m 10s
  8. 41m 28s
    1. Profile, login, and resource files
      9m 11s
    2. Setting command aliases
      6m 59s
    3. Setting and exporting environment variables
      4m 54s
    4. Setting the PATH variable
      6m 10s
    5. Configuring history with variables
      6m 17s
    6. Customizing the command prompt
      6m 5s
    7. Logout file
      1m 52s
  9. 1h 25m
    1. grep: Searching for matching expressions
      5m 21s
    2. grep: Multiple files, other input
      4m 28s
    3. grep: Coloring matched text
      2m 57s
    4. Introduction to regular expressions
      3m 22s
    5. Regular expressions: Basic syntax
      3m 19s
    6. Using regular expressions with grep
      5m 20s
    7. tr: Translating characters
      8m 17s
    8. tr: Deleting and squeezing characters
      5m 30s
    9. sed: Stream editor
      7m 45s
    10. sed: Regular expressions and back-references
      7m 8s
    11. cut: Cutting select text portions
      7m 42s
    12. diff: Comparing files
      4m 35s
    13. diff: Alternative formats
      4m 30s
    14. xargs: Passing argument lists to commands
      7m 25s
    15. xargs: Usage examples
      7m 59s
  10. 42m 25s
    1. Finder integration
      4m 45s
    2. Clipboard integration
      5m 5s
    3. Screen capture
      3m 42s
    4. Shut down, reboot, and sleep
      3m 34s
    5. Text to speech
      2m 36s
    6. Spotlight integration: Searching metadata
      3m 41s
    7. Spotlight integration: Metadata attributes
      4m 24s
    8. Using AppleScript
      5m 23s
    9. System configurations: Viewing and setting
      5m 51s
    10. System configurations: Examples
      3m 24s
  11. 1m 26s
    1. Conclusion
      1m 26s

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