Unix: File permissions

File Permissions in Unix

Every user on a Unix system has a unique username, and is a member of at least one group. This group information is held in the password file (/etc/passwd). A user can also be a member of one or more other groups. The auxiliary group information is held in the file /etc/group. Only the administrator can create new groups or add/delete group members.

Every directory and file on the system has an owner, and also an associated group. It also has a set of permission flags which specify separate read(r), write(w) and execute(x) permissions for the 'user' (owner), 'group', and 'other' (everyone else with an account on the computer) The 'ls -l' command can be used to view the permissions and group associated with files in current directory.

An example of the output produced by ls -l is shown below.

drwxr-xr-x 2 teacher staff 4096 Nov 21 15:05 Desktop

drwxr-xr-x 2 teacher staff 4096 Nov 12 10:32 Documents

drwxr-xr-x 2 teacher staff 4096 Nov 21 15:44 Downloads

drwxr-xr-x 2 teacher staff 4096 Nov 12 10:32 Music

-rw-r--r-- 1 teacher staff 406558 Nov 21 10:23 NPrecord.odt

drwxr-xr-x 2 teacher staff 4096 Nov 22 11:14 Unix Programming

Each line in the output has the following fields

Field 1: a set of ten permission flags.

Field 2: link count (don't worry about this)

Field 3: owner of the file

Field 4: associated group for the file

Field 5: size in bytes

Field 6-8: date and time of last modification

Field 9: name of file

The permission flags are interpreted as follows

Position         Meaning

1                        directory flag, 'd' if a directory, '-' if a normal file, something   else occasionally may appear here for special devices.

2,3,4                  read, write, execute permission for User (Owner) of file

5,6,7                  read, write, execute permission for Group

8,9,10                read, write, execute permission for Other

Given below is the interpretation for the symbols used in permission flag

Value             Meaning

-                        in any position means that flag is not set

r                       file is readable

w                      file is writable. On a directory, means you can add or delete files

x                       file is executable. On a directory, means you can list the files in that directory

So if we interpret the very first line of the output given above, we can understand that: Desktop is a directory owned by teacher, who belongs to the group staff, the size of the folder is 4096 bytes and was modified on Nov 21 15:05 hours. The permissions for owner is read, write and execute while the other members of the group staff has permissions only for reading and executing(i.e. no write permissions) and users who doesn't belong to the group also has permissions for reading and executing. 

Setting permissions

chmod command can be used to change the permissions of a file.
syntax: chmod {a,u,g,o} {+,-} {r,w,x} files
      Value   Meaning       
       a           all users 
      u           the owner 
      g           the owner group 
      o           others (neither u, nor g)
      +           give permission 
      -            remove permission
e.g. chmod g+rw files give the group read and write permission
 
Numbers can also be used to set the permissions
The number equivalents of r, w, x are:
                              r        w        x
OWNER(USER)    400    200    100
GROUP                40      20      10
PUBLIC                4        2        1
      e.g. (i) 700 means read write and execute permissions for owner(400 + 200 + 100) and no permissions for group(0) and others(0).
             (ii) 666 means r and w for owner, group and others   
The real stuff behind this is that the permissions for each are defined using 3 bits, with 9 bits defining the whole permission. where the last bit specifying an execute permission, the middle bit specifing write permission and the last bit read permission so a binary 100(octal 4) means r for owner and 111(octal 7). represents rwx for owner.