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Hard links

From: Unix for Mac OS X Users

Video: Hard links

In Unix we are going to want to make use of links. And the first kind of links that we are going to look at in Unix are hard links. The way that we make a hard link is just simply to have ln, short for link, space and then the file that we want to link. That could either just be a file name or could be a full path to a file, if the file is located somewhere else. Followed by a space and then followed by the name of the hard link or if we want to put that link somewhere else, the path where we want to locate it, as if we were creating a file.

Hard links

In Unix we are going to want to make use of links. And the first kind of links that we are going to look at in Unix are hard links. The way that we make a hard link is just simply to have ln, short for link, space and then the file that we want to link. That could either just be a file name or could be a full path to a file, if the file is located somewhere else. Followed by a space and then followed by the name of the hard link or if we want to put that link somewhere else, the path where we want to locate it, as if we were creating a file.

But if we are in a single directory, than we would just put the file that we want to link and then the hard link that we want to make to that file. What this will do is make a reference to a file in the file system, the same way that making a Finder alias would do. And like the Finder, it will not break if the file is moved. But there's one important difference about the way that hard links work. Remember in the Finder, if we threw away the original file, the alias still is there, but it's broken. It doesn't point anything anymore. It was just an alias to that original file.

Well, that's not the case with hard links. With hard links, they don't break if the file is deleted. Now that could be a little weird to work with it first, until you get used to it and understand what they are really doing. But when you think about it, every time we see a file listed for us, all it is is a reference to a file in the file system. So making a hard link is really no different. It's just making another reference to that same file in this file system. Both file names that we see on the screen are pointing at the same spot on our hard drive at that same file storage space.

So what we are doing with the hard link is allowing two different names to point to that same thing. That's different than the way the Mac OS X Finder works. The Mac OS X Finder there's definitely an actual file that points to that space on the hard drive and then there's this other thing called an alias which points to that original file, not to the spot on the hard drive but to that other file. Let's try that out in Unix so you can get the hang of it. So notice that here I am inside my Unix files,and in the last movie, ls -la, you can see that I have my linkedfile.txt that I'll be linking to and then I also had these other aliases here.

Now these aliases are not included in the exercise files because they don't travel well. They really are designed for making references on my hard drive, so you won't find them there. But it's easy enough to create these aliases in the Finder. What we want to focus on though is this linkedfile.txt and so we want to make a hard link to it. Well, we use ln and then a space and then the name of the file we want to link. So linkedfile.txt followed by a space and then the name of the link that we want to give it. I am going to call this one hardlink. So it's always in that format: the link, followed by the source, the thing we are targeting, and then the name of the link is the last item.

Now let's do ls -la again, and let's take a look at what's there now. So here's my hardline. Notice that the size of this is much, much more smaller than the size of the aliases I created. It is exactly the same size as linkedfile.txt and that makes sense. These are two files pointing at the same spot on the hard drive. And if we open it up, let's do cat hardlink, now you see it has the exact same text inside of it. Let's take a look over here in the Finder and just see it here. In the Finder it didn't' give it any kind of icon here because it didn't have .txt or anything after it, so it just gave it a generic file icon.

But you can see that it does not have the little arrow that the Finder uses to indicate the special alias files that it creates. So as far as the Finder is concerned, this is just another file. Let's create another directory that we can move this into. I'll make a directory. I'll just call it linkdir and then let's move the original file, linkedfile.txt, let's move that file into linkdir. So now we have ls -la, we can see we have our hardline, and then we have linkdir and if we look inside linkdir, you can see that we have our linked file.

Now, let's try opening up that hardlink again. So same thing, hardlink, yup! It's still got that same data on it. It was able to still keep track of it, even though we moved it. That's to be expected. Here's the weird part. If we delete on one of these, and it doesn't matter which one we delete, if we delete hardlink or we delete linkedfile.txt, the other one will still contain the data. It will not remove it from our hard drive. Let's try that. What I am going to do is I am going to remove the original. I'll do rm inside linkdir and we'll get rid of linkedfile.txt.

So now, the file is removed. Let's just take a look there and see, yup, it's actually gone. Now let's take a look at our hardlink again. See, it still contains that information, even though we threw away the original. The hardlink is exactly equivalent to that file. So there are times when having a hardlink can be useful. But it can be a little bit weird, because you can think that you are throwing away a file, but actually that file continues to exist. And in a shared Unix environment, that can be useful because several users could all have a hardlink to a file and if some makes a change to the file, the change happens for all users. They share this file.

But if one of them throws it away, well, it doesn't throw it away for everyone else. They still have access to it. So in that context it does make sense. But because of how we are use to working with Apple aliases or Window shortcuts, it feels a bit odd.

Show transcript

This video is part of

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

82 video lessons · 25428 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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