Permission Classes and Types

Permission classes

• user (u)
• group (g)
• other (o) (public)
• all (a) <- all combined

Permission types

• r,w,x
• works differently on files and directories
• hyphen (-) represents no permissions set

ls results permissions groupings

• • rwx rw- r–
    • user (owner), group, and other (public)

ls results first character meaning

• regular file d directory l symbolic link c character device file b block device file p named pipe s socket

Modifying Access Permission Bits

chmod command

• Modify permissions using symbolic or octal notation.
• Used by root or the file owner.

Flags chmod -v ::: Verbose.

Symbolic notation

• Letters (ugo/rwx) and symbols (+, -, =) used to add, revoke, or assign permission bits.

Octal Notation

Three-digit numbering system ranging from 0 to 7. 0 — 1 –x 2 -w- 3 -wx 4 r– 5 r-x 6 rw- 7 rwx

Default Permissions

• Calculated based on the umask (user mask) value subtracted from the initial permissions value.

umask

• Three-digit value (octal or symbolic) that refers to read, write, and execute permissions for owner, group, and public.
• Default umask value is 0022 for the root user and 0002 normal users.
• The left-most 0 has no significance.
• If umask is set to 000 files will get max of 666
• If the initial permissions are 666 and the umask is 002 then the default permissions are 664. (666-002)
• Any new files or directories created after changing the umask will have the new default permissions set.
• umask settings are lost when you log off. Add it to the appropriate startup file to make it permanent.

Defaults

• files 666 rw-rw-rw-
• directories 777 rwxrwxrwx

umask command

Options

• -S symbolic form

Special Permission Bits

• 3 types of special permission bits for executable files or directories for non root users
  • setuid
  • setgid
  • sticky
• setuid
  • set on exe’s to provide non-owners the ability to run them with the privileges of the owning user
  • may be set on directories and files but will have no effect.
  • example: the su command
  • shows an ’s’ in ls -l listing at the end of owners permissions
  • If the file already has the “x” bit set for the user, the long listing will show a lowercase “s”, otherwise it will list it with an uppercase “S”.
• setgid
  • set on exe’s to provide non-group members the ability to run them with the privileges of the owning group.
  • May be set on shared directories
    • allow files and subdirectories created underneath to automatically inherit the directory’s owning group.
    • saves group members who are sharing the directory contents from changing the group ID for every new file and subdirectory that they add.
  • write command has this set by default so a member of the tty group can run it. If the file already has the “x” bit set for the group, the long listing will show a lowercase “s”, otherwise it will list it with an uppercase “S”.
• Sticky bit
  • may be set on public directories for inhibiting file deletion by non-owners
  • may be set on directories and files but will have no effect.
  • Set on /tmp and /var/tmp by default
  • Letter “t” in other permission feild
  • If the directory already has the “x” bit set for public, the long listing will show a lowercase “t”, otherwise it will list it with an uppercase “T”.