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Setting permissions using octal notation

From: Unix for Mac OS X Users

Video: Setting permissions using octal notation

In the last movie we learned how to work with file permissions and to use chmod to change the permissions that we wanted for user, group, and other, and to do that we used what we called alpha notation. That's because we're using the alphabet, u, g, and o for user, group, and other and r, w, and x for read, write, and execute. That's everything you need to know to change file permissions. That's it. You could get by do everything with that. However, I want you to also learn about octal notation, because it's extremely popular. It's the way that I change file permissions and I think a lot of other Unix users do and you're bound to run into it.

Setting permissions using octal notation

In the last movie we learned how to work with file permissions and to use chmod to change the permissions that we wanted for user, group, and other, and to do that we used what we called alpha notation. That's because we're using the alphabet, u, g, and o for user, group, and other and r, w, and x for read, write, and execute. That's everything you need to know to change file permissions. That's it. You could get by do everything with that. However, I want you to also learn about octal notation, because it's extremely popular. It's the way that I change file permissions and I think a lot of other Unix users do and you're bound to run into it.

So I want you to at least have some exposure to it. So remember here we have u, g, and o, r, w, and x and we use that to come up with those nine digits rwx, rw-, r--, right? Now imagine that for r, w, and x that we establish a conversion. Each one of those letters is instead going to be represented by a number, 4, 2, and 1. r=4, w=2, x=1. So in place of the yes's and no's for each one of those, let's drop in those numbers.

So for the user, the user can read, write, and execute, so they get 4, 2, and 1. The group can read and write, so they get 4, 2 and 0, and for other, they can only read, so they get 4 and 0 and 0. Take all those numbers and add them up. You get 7, 6 and 4. And those three digits are the octal notation representation of rwxrw-r--. So we're not adding permissions or taking away permissions; we're defining all nine of those just using these three numbers.

We're saying each one of these numbers will represent the state of read, write, and execute for each one of those. Let me show you some examples. So for example, let's say we want to say that a file ought to have all the permissions. Well, read, write, and execute, all added together, 4, 2 and 1 those add up to 7. That's the highest number we can have. So 777 means give all the permissions wide open, so everybody can do everything. When we're just looking at, we had 764. That means that the user can read, write, and execute. For the group they have a 6.

That's made up of 4 and 2. That's the only way we can come up with that number is with 4 and 2, therefore they have read and write privileges, not execute. And then for other, they have only read privileges. That's the 4. A very common one that you will see is 755. That means that I as the user have read, write, and execute. Everyone else just has read and execute privileges. They don't have write privileges. And then of course, if they didn't have any of them, it will be 000. That would essentially take away all privileges from everybody.

More commonly what you would see is something like 700, which would mean I get all privileges, but everyone else gets none. 7 represents read, write, and execute. So that's it. That is all that is there to it. It's just a really compact concise way to spell out exactly what you want to have happen with those permissions. Like I said, you can still do it the other way, but this is a really powerful way once you get the hang of it. What I like you to do now is just pause the movie with this alpha to octal conversion up, r, w, and x, just to remind you, and try a few of these.

Just try setting the permissions on a test file. Go ahead and just try 777, see what it does. Try 755, see what it does. Remember that the numbers will always have to be combinations between 0 and 7. They can't be any higher than that. So, you can just play around and see what it does, see if you can get the hang of it. Maybe even if you start out using alpha notation, just every now and then just try the octal one and see, and if you're going to do all of them wide open anyway, just use 777. It's certainly a lot faster, instead of typing out user, group, and other equals read, write, and execute, right? It's a lot more characters to type.

Once you feel like you have to hang of it, we'll move on to talking about how we can switch to a different user identity.

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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 · 26967 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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