tutor:File system

Permissions

Symbolic notation

Each class of permissions is represented by three characters.

• The first set of characters represents the user class.
• The second set represents the group class.
• The third and final set of three characters represents the others class.

Each of the three characters represent the read, write, and execute permissions respectively:

• r if the read bit is set, - if it is not.
• w if the write bit is set, - if it is not.
• x if the execute bit is set, - if it is not.

Three groups of three

• first - what the owner can do
• second - what the group members can do
• third - what other users can do

The triplet

• first
  • r: readable.
• second
  • w: writable.
• third
  • x: executable.
  • s: executable and setuid.
  • S: setuid but not executable.

The following are some examples of symbolic notation:

• -rwxr-xr-x for a regular file whose user class has full permissions and whose group and others classes have only the read and execute permissions.
• crw-rw-r-- for a character special file whose user and group classes have the read and write permissions and whose others class has only the read permission.
• dr-x------ for a directory whose user class has read and execute permissions and whose group and others classes have no permissions.

Octal notation

octal notation consists of a three- or four-digit base-8 value. With three-digit octal notation, each numeral represents a different component of the permission set:

• user class,
• group class,
• and "others" class respectively.

Each of these digits is the sum of its component bits, and as a result, specific bits add to the sum as it is represented by a numeral:

• The read bit adds 4 to its total (in binary 100),
• The write bit adds 2 to its total (in binary 010), and
• The execute bit adds 1 to its total (in binary 001).

These are the examples from the Symbolic notation section given in octal notation:

• -rwxr-xr-x would be represented as 755 in three-digit octal.
• -rw-rw-r-- would be represented as 664 in three-digit octal.
• -r-x------ would be represented as 500 in three-digit octal.

Here is a summary of the meanings for individual octal digit values:

0 --- no permission
1 --x execute 
2 -w- write 
3 -wx write and execute
4 r-- read
5 r-x read and execute
6 rw- read and write
7 rwx read, write and execute

Note the similarity to binary counting (starting at the right and going left):

0: 000 
1: 001 
2: 010 
3: 011 
4: 100 
5: 101 
6: 110 
7: 111 

Octal digit values can be added together to make Symbolic Notations:

(4=r)+(1=x) == (5=r-x)
 (4=r)+(2=w) == (6=rw-)

(4=r)+(2=w)+(1=x) == (7=rwx)

Here is a summary showing which octal digits affect permissions for user, group, and other:

• UGO = User, Group, Other
• 777 = "-rwxrwxrwx" = rwx for all
• 754 = "-rwxr-xr--" = rwx for owner, r-x for group, r-- for other
• 124 = "---x-w-r--" = x for owner, w for group, r for other