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

Special Permissions
In addition to the three general-purpose permissions, 
rwx
, Linux has three 
special permissions that are slightly more complicated. These special per-
missions are set user ID (or 
SUID
), set group ID (or 
SGID
), and sticky bit. I’ll 
discuss each in turn in the next three sections.
Granting Temporary Root Permissions with SUID
As you should know by now, a user can execute a file only if they have per-
mission to execute that particular file. If the user only has read and/or 
write permissions, they cannot execute. This may seem straightforward, 
but there are exceptions to this rule.
You may have encountered a case in which a file requires the permis-
sions of the root user during execution for all users, even those who are not 
root. For example, a file that allows users to change their password would 
need access to the /etc/shadow file—the file that holds the users’ passwords 
in Linux—which requires root user privileges in order to execute. In such a 
case, you can temporarily grant the owner’s privileges to execute the file by 
setting the 
SUID
bit on the program.
Basically, the 
SUID
bit says that any user can execute the file with the per-
missions of the owner but those permissions don’t extend beyond the use of 
that file.

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Chapter 5
To set the 
SUID
bit, enter a 
4
before the regular permissions, so a file 
with a new resulting permission of 
644
is represented as 
4644
when the 
SUID
bit is set.
Setting the 
SUID
on a file is not something a typical user would do, but if 
you want to do so, you’ll use the 
chmod
command, as in 
chmod 4644 filename
.