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Disk information commands

From: Unix for Mac OS X Users

Video: Disk information commands

In this movie we're going to be looking at some commands to give us some hard drive information, or as Unix typically refers to it a disk. The two commands we're going to be looking at are df and du. df stands for disk free space, so it's going to display the amount of disk free space available to us. If I type a line by itself and hit Return, it'll come up with the report telling you all of the volumes that it can see, the hard drives, and how much space is being used and how much is available on each of them. Notice that it has a couple of devices here that it's calling volumes that are just sort of utility devices that it's using.

Disk information commands

In this movie we're going to be looking at some commands to give us some hard drive information, or as Unix typically refers to it a disk. The two commands we're going to be looking at are df and du. df stands for disk free space, so it's going to display the amount of disk free space available to us. If I type a line by itself and hit Return, it'll come up with the report telling you all of the volumes that it can see, the hard drives, and how much space is being used and how much is available on each of them. Notice that it has a couple of devices here that it's calling volumes that are just sort of utility devices that it's using.

You can basically ignore those. The one we care about are the ones with the big numbers. Main hard drive is listed here as root. That's where it's mounted. If we had other hard drives, maybe you have your hard drive partitioned or you have external drives plugged in, you would see those under /volumes/ and then the name of the hard drive. We would see reports for those as well. Now I don't know about you but I find these numbers a bit hard to read. So one of the options we can use with df is the -h option for humanize. This will humanize those numbers so now we say, oh! 39 gigabytes used, 16 gigabytes available, 71% of it has been used, okay.

much easier to read. However, there is a caveat that comes with those numbers. There are two different ways that one can calculate gigabytes. You can calculate it using Base-2 or Base-10. It's a bit of a technical point. You may remember in the news there was some discussion about the fact that hard drive manufacturers were selling hard drives that would say, well, this is a 10 gigabyte drive and then people would get it home and it wasn't a 10 gigabyte drive once they started putting information on it. It's because of the way that the gigabytes are calculated were different. So we also have df -H which uses Base-10 to calculate it.

These give us those higher numbers, the ones that the hard drive manufacturers were using, whereas the lowercase h is giving us the actual amount of bytes that we could put on there when we're talking about the data. Now I think you want to stick with the lowercase h. I recommend that you just use that all the time. It's more widely supported, as we'll see in a moment. But that capital H, I believe, is the one that now is used in Snow Leopard. So starting in Snow Leopard, Apple started using capital H when it looks at sizes for hard drives instead of the lowercase h. It's really less of a technical point and more of an issue of customer confusion over the marketing.

The second command that we're going to use is going to be du, which stands for disk usage. And with that we'll provide it the path that we wanted to look at and tell us about the disk usage of that path. We wouldn't want to do this at the root of our hard drive because then it would tell us every single file and folder on the entire hard drive. If you accidentally do that, hit Ctrl+C to exit out of it. Instead we want to target it a little more at a direct path. disk usage for in my user folder unix_files. that's where I've got just a few files set aside. And it comes up and it tells me for each and every one of the directories that it sees, what the size is.

Now just like the other one we have a -h option that we can pass in. That's a lowercase h. unix_files, and now it gives me a human readable version. It has a -H option but it's completely unrelated to human readable output, so you wouldn't want to use it here. That's why I said that lowercase h is more widely supported as being humanized. So stick with lowercase h. Now you may notice it's only showing me the directories here. If we want it to also show us the files, we can put the -a option in front of it. Now you can see it tells us all of the files and the directories and their sizes, all right? So we get a list of all those. Fo for -a, think all.

Now this is sort of similar to what you get in the Finder if you do Get Info, right? And it calculates the sizes. But what it does is it actually summarizes the sizes. The way that we can do that is by using the depth. So for example we say du, we'll just do the directories. We'll leave out the files. Humanize the directory with the depth of 1, so that's the depth option and then the argument to the option is 1, and then we'll tell it to look at unix_files. And you'll see it just goes one directory deep. It shows me the current directory plus one.

If I did the same thing with the 0 option then it says, "Okay, just for the current directory, right?" Show me just this one directory, summarize the size of that. So a depth option can be really useful if you don't want all of this output. You really what you want to know is just this directory, to summarize and tell me how large that is. Now there's another important point that we need to talk about with disk usage. What it's reporting to us here is the amount of size that has been set aside for these files on the hard drive. That's different than the size that the files are actually using.

You notice here that all of these files report 4K as their value, right? But they're not all the same size files. In fact, we can see that. If we do ls -lah, we'll get a listing. Oops! Let's actually do that for the unix_files. -lah and unix_files and we can see those files and we see that some of them take up just 55 bytes. It's because the file system sets aside blocks of space for files. Even if they don't take up the whole space, it goes ahead and gives them a whole block on the hard drive.

So there's a minimum size for those. Disk usage is returning the space taken up by the blocks, the space allocated to the files, not the actual file size. So I want you ti understand the difference and understand that the two aren't the same. ls is reporting the actual size of the file, whereas du is reporting the allocation of space for that file, even though it may not be using it. You may have come across this before if you ever defragmented your hard drive. There are tools out there that lets you to defragment your hard drive to regain space.

One of the things that it's doing is it's taking the files and by moving them all together so that they're not fragmented, well, then the block allocation can really be best utilized. And we can really make sure that we're using only the exact number of blocks that we need, whereas if they're fragmented and part of the file is spread out at different places on the hard drive, you may end up with lots of blocks that are partially used. They've been allocated for the file but not completely used. And that's how you're able to reclaim a lot of that space when you defragment. So again, the two main commands for finding out disk information are df for disk free and du for disk usage.

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This video is part of

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

82 video lessons · 25567 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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