L in u X ba sics for h acke rs g e t t I n g s t a r t e d w I t h

5
C O N T R O L L I N G F I L E A N D 
D I R E C T O R Y P E R M I S S I O N S
Not every user of a single operating system 
should have the same level of access to files 
and directories. Like any professional or 
enterprise-level operating system, Linux has 
methods for securing file and directory access. This 
security system allows the system administrator—the 
root user—or the file owner to protect their files from 
unwanted access or tampering by granting select users permissions to read, 
write, or execute files. For each file and directory, we can specify the per-
mission status for the file’s owner, for particular groups of users, and for 
all other users. This is a necessity in a multiuser, enterprise-level operating 
system. The alternative would be quite chaotic.
In this chapter, I’ll show you how to check for and change permissions 
on files and directories for select users, how to set default file and directory 
permissions, and how to set special permissions. Finally, you will see how a 
hacker’s understanding of permissions might help them exploit a system.

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Chapter 5
Different Types of Users
As you know, in Linux, the root user is all-powerful. The root user can do 
basically anything on the system. Other users on the system have more lim-
ited capabilities and permissions and almost never have the access that the 
root user has.
These other users are usually collected into groups that generally share 
a similar function. In a commercial entity, these groups might be finance, 
engineering, sales, and so on. In an IT environment, these groups might 
include developers, network administrators, and database administrators. 
The idea is to put people with similar needs into a group that is granted 
relevant permissions; then each member of the group inherits the group 
permissions. This is primarily for the ease of administering permissions 
and, thus, security.
The root user is part of the root group by default. Each new user on the 
system must be added to a group in order to inherit the permissions of that 
group.

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