File Access Permissions

Every file has following attributes:

• owner's user ID ( 16 bit integer )
• owner's group ID ( 16 bit integer )
• File access mode word

'r w x -r w x- r w x'
(user permission-group permission-others permission)
r-read, w-write, x-execute

To change the access mode, we use chmod (filename,mode).

r w - r - - r - - symbolic
\ / \ / \ /
110 100 100 binary
\ / \ / \ /
6 4 4 octal

Example 1:
To change mode of myfile to 'rw-rw-r--' (ie. read, write permission for user - read,write permission for group - only read permission for others) we give the args as:

chmod(myfile,0664)

Each operation is represented by discrete values
'r' is 4
'w' is 2
'x' is 1

Therefore, for 'rw' the value is 6(4+2).

Example 2:
To change mode of myfile to 'rwxr--r--' we give the args as:

chmod(myfile,0744)