Number Conversions
Binary, Decimal, Octal and Hexadecimal
Videos
CS8 F13 Student Ahaha S. located the following list of videos to help with learning number conversions
- A good overview http://www.youtube.com/watch?v=GPnLy6YO-0M
- Just understanding Binary http://www.youtube.com/watch?v=0qjEkh3P9RE
- Binary to Octal http://www.youtube.com/watch?v=9jho2TkH6AU
- Binary to Hexadecimal http://www.youtube.com/watch?v=jFnXpMt6H_Y
- Binary to Decimal http://www.youtube.com/watch?v=tfKe8PPI2zs
- A deep look at binary, that explains WHY about binary also. https://www.youtube.com/watch?v=vT9p2QnIycI
- You can skip to 6:36 if you just want the HOW: https://www.youtube.com/watch?v=vT9p2QnIycI#t=6m36s
- Hexadecimal conversion:
Practice Problems
Practice Here: http://www.cs.ucsb.edu/~pconrad/cs8/13F/numberConversions/
Unix File Permissions in Octal
When we type ls -l at the Unix/Linux command prompt we get strings that correspond to the octal numbers used in chmod commands.
For example:
$ ls -l
total 5
-rwxr-xr-x 1 pconrad 0376 3633 Feb 7 09:01 000.SampleLectureTopics.htm
drwx------ 2 pconrad 0376 512 Feb 7 10:00 02.07
$
The second line starts with
drwx------
The d means that 02.07 is a directory. The rwx means that the owner of file, pconrad, has read, write, execute
permission.
Here are some additional examples, and how they related to octal numbers in a chmod command:
This output in ls -l |
Could be set with this chmod command |
|---|---|
-rw-r--r-- |
chmod 644 filename |
drwxr-xr-x |
chmod 755 dirname |
-rw------- |
chmod 600 filename |
As a reminder, the first d vs. - isn’t part of the octal number, but is rather an indication of whether the listing is for a file or a directory.
The meanings are:
| octal digit |
binary equivalent |
rwx format |
meaning |
|---|---|---|---|
| 0 | 000 | --- |
no permission |
| 1 | 001 | --x |
execute |
| 2 | 010 | -w- |
write |
| 4 | 100 | r-- |
read |
The three digits in an octal number have different meanings depending on their positions (text borrowed from Wikipedia). </p>
- UGO = User, Group, Other
- 777 = "-rwxrwxrwx" = rwx for all
- 754 = "-rwxr-xr--" = rwx for owner of the file, r-x for group, r-- for other
- 124 = "---x-w-r--" = x for owner of the file, w for group, r for other
For a DIRECTORY, execute has a particular peculiar meaning… It means that if you already KNOW the name of a file in that directory, you are allowed to access it… but you may not LIST the files in that directory.
If we want a directory to be available on the web, we have to open up read and execute permission to “others”. We do that with the last three letters in the permission string
Quiz yourself on octal chmod values
This web page gives you an opportunity to practice with converting between the format used in ls -l output, and the octal numbers used in chmod.
https://www.cs.ucsb.edu/~pconrad/cs8/topics/chmodQuiz1/