File Permissions

File Permissions

• Every file is owned by a user and group
• Three Access Categories are user, group, others
• Three Permission types are read, write & execute
• suid – command run with permission of owner
• sgid – command runs with group affiliation
• sticky bit – special restriction on deletion of files other than owner
• umask 002 – list permission to withhold (default value)

Permission types

Letter	Numeric	Permission
r	4	read
w	2	create, write & remove files
x	1	execute
-	0	no permission

Changing file permission

# chmod [-R] mode file|directory

where Mode is 

u	user
g	group
o	others
a	all
+	grant
-	deny

Changing file ownership

# chown [–R] user file|directory

# chown [-R] user:group file|directory

# chgrp [–R] group file|directory

SUID and SGID Executables

Set on an executable file cause it to run under the user/group security context of file owner/group

SGID Directories

File created in directory with SGID bit set, it belongs to the same group as the directory

Sticky Bit

Only owner can delete the file

# chmod o+t directory

T appears if directory execute permission for others is off

Octal Mode Number	Description
1000	Sets the sticky bit
2000	Sets the setgid bit
4000	Sets the setuid bit

Linux Read mode permissions

• Read access on a file allows you to view file 
• Read access on a directory allows you to view directory contents with ls command 

Write mode permissions

• Write access on a file allows you to write to file 
• Write access on a directory allows you to remove or add new files 

Execute mode permissions

• Execute access on a file allows to run program or script 
• Execute access on a directory allows you access file in the directory  

Default File permission

Umask used to withhold permission on file creation

Default User’s umask is 022

File will have 644

Directories will have 755