Lab 12: Permissions

Objective Focus

Understand and use essential tools: List, set, and change standard ugo/rwx permissions

List Permissions in Linux

Permissions manage what a user can do with a file and/or directory.

	Files	Directory
Read	Viewing and copying	List items inside aka ls
Write	Modify	create, delete, rename files and subdirs
Execution	Running	Change into directory aka cd

We can use ls and stat to show us the permissions on a file or directory.

 $ ls -l

 $ stat file or directory

Set and Change Permissions

We can modify the permissions using symbolic representation or octal representation.

Symbolic

Symbolic refers to letter combinations ugo with rwx and symbols, +-=. Multiple symbolic modes can be given, separated by commas.

User Categories	Permission Types
u: user	(r) read
g: groups	(w) write
o: other or public	(x) execute
a: all

 $ chmod ug+rw testfile.txt

example, adding read and write from users and groups to text file using symbolic combinations.

 $ chmod ug-rw testfile.txt

example, removing read and write users and groups to text file using symbolic combinations.

Octal

Octal	Bit	Value
Read	r - -	4
Write	- w -	2
Execution	- - x	1

 $ chmod 660 testfile.txt

Using chmod

More Symbolic Examples

 $ chmod -v u+x testfile.txt

Adding execute to user class

 $ chmod -v +rwx testfile.txt

Adding read, write, and execute to all classes

 $ chmod -v u=rw,g-x,o-rw testfile.txt

Setting read and write to user, removing execute from group, removing read and write from other

More Octal Examples

 $ chmod -v 777 testfile.txt

Adding read, write, execute to all classes

 $ chmod -v 600 testfile.txt

Setting read and write only on user and removing all permissions from group and others

 $ chmod -v 441 testfile.txt

Setting read only to user and group then execute only on others.

Understanding Default Permissions

When a file or directory is created the default permissions applied are calculated by using the umask

umask - User Mask

The purpose of umask is to set the default permissions on newly created files and directories.

Initial permission values are 666 (rw-rw-rw-) for files

Initial permission values are 777 for directories.

 $ man 2 umask

 $ umask

Shows umask in octal form
The left most 0 does not have any meaning or weight.

 $ umask -S

Shows umask in symbolic form

Set new default permissions by using the umask command

 $ umask 020

A note about chmod

chmod never changes the permissions of symbolic links; the chmod system call cannot change their permissions. This is not a problem since the permissions of symbolic links are never used. However, for each symbolic link listed on the command line, chmod changes the permissions of the pointed-to file. In contrast, chmod ignores symbolic links encountered during recursive directory traversals. From the man page

That’s all for lab 12!