COMP2700 - Week 4 - Objects and Permissions

Objects

• All files in unix can be represented as a file
• Tree file system
• Each file entry in a directory is a pointer to a data structure called an inode

Information about Objects

-rwxr-x--- 1 alice alice 4648643 Aug 17 10:34 test.pdf
drwxr-xr-x 2 alice tutor    3452 Aug 17 10:33 lectures

• File type -> First character

  • - -> file
  • d -> directory
  • b -> block device file (SSD/HDD, etc)
  • c -> character device file (terminal devices; byte-by-byte query rather than block-by-block)
  • s -> socket Not covered
  • l -> symbolic link
  • p -> FIFO (queue)

• File permissions -> Next nine characters

• Link counter -> Number of links (i.e. directories pointing) to the file

• Username -> Usually user who created the file

• Group -> New file belongs to creator's group or directory's group

• File size, modification datetime and filename

• Owner and root can change permissions (chmod)

• root can change file owner and group (chown)

• Filename stored in the directory, not in inode

File and Directory Permissions

• 4 groups of 3-bits

• First group: special modes

• Next three groups define read, write, and execute access for: owner, group, and other

Special Modes

• First bit -> SUID bit
  • Allows program to change effective UID to be different from actual UID
• Second bit -> GUID bit
  • Allows program to change effective GID to be different from actual GID
• Third bit -> Sticky bit
  • Different implementations

Note: Rarely used, most files will have these set to 0

• r -> read
• w -> write
• x -> execute

rw-r--r-- -> 000 110 100 100
rwxrwxrwx -> 000 111 111 111

Special Modes Textual Representation

When special modes are present, bits in the special modes change the display of executable bits of the remaining groups.

• If SUID bit is set: Display s if the 'owner' exec bit is set; otherwise display S
• If SGID bit is set: Display s if the 'group' exec bit is set; otherwise display S
• If sticky bit is set: Display t if the 'other' exec bit is set; otherwise display T

110 111 110 100 -> rwsrwSr--
011 111 101 101 -> rwxrwsr-T
101 110 110 100 -> rwSrw-r-T

Octal Representation

Each group of three bits can be represented as an octal

000 110 100 100 -> 0644
011 111 101 101 -> 3755

Default Permissions

• Unix typically use default permissions 0666 when creating a new file, and 0777 when created a new program
• Permissions can be adjusted using the umask
  • What rights should be withheld
• Actual permissions is derived by masking the given default permissions with the umask
  • Computing the logical AND of the bits in the default permission and of the inverse of the bits in the umask

Permissions for Directories

• Read -> Required to see files in a directory (i.e. ls)
• Write -> Required to add or remove files to or from a directory
• Execute -> Required the make the directory current (i.e. cd) and opening files inside the directory