SGID and SUID

What is SGID?

SGID (Set Group ID up on execution) is a special type of file permissions given to a file/folder. Normally in Linux/Unix when a program runs, it inherits access permissions from the logged in user. SGID is defined as giving temporary permissions to a user to run a program/file with the permissions of the file group permissions to become member of that group to execute the file. In simple words users will get file Group’s permissions when executing a Folder/file/program/command.
SGID is similar to SUID. The difference between both is that SUID assumes owner of the file permissions and SGID assumes group’s permissions when executing a file instead of logged in user inherit permissions.

Learn SGID with examples:

Example: Linux Group quota implementation
When implementing Linux Group quota for group of people SGID plays an important role in checking the quota timer. SGID bit set on folder is used to change their inherit permissions to group’s permissions to make it as single user who is dumping data. So that group members whoever dumps the data the data will be written with group permissions and in turn quota will be reduced centrally for all the users. For clear understanding of this you have to implement group quota from the above link. Without implementation of SGID the quota will not be effective.

How can I setup SGID for a file?

SGID can be set in two ways

1) Symbolic way (s)

2) Numerical/octal way (2, SGID bit as value 2)

Use chmod command to set SGID on file: file1.txt

Symbolic way:

chmod g+s file1.txt

Let me explain above command we are setting SGID(+s) to group who owns this file.
Numerical way:

chmod 2750 file1.txt

Here in 2750, 2 indicates SGID bitset, 7 for full permissions for owner, 5 for read and execute permissions for group, and no permissions for others.
How can I check if a file is set with SGID bit or not?
Use ls –l to check if the x in group permissions field is replaced by s or S
For example: file1.txt listing before and after SGID set
Before SGID set:

ls -l

total 8

-rwxr--r-- 1 xyz xyzgroup 148 Dec 22 03:46 file1.txt

After SGID set:

ls -l

total 8

-rwxr-sr-- 1 xyz xyzgroup 148 Dec 22 03:46 file1.txt

Some FAQ’s related to SGID:
Where is SUID used?
1) When implementing Linux group disk quota.
I am seeing “S” ie Capital s in the file permissions, what’s that?
After setting SUID or SGID to a file/folder if you see ‘S’ in the file permission area that indicates that the file/folder does not have executable permissions for that user or group on that particular file/folder.

chmod g+s file1.txt

output:
-rwxrwSr-x 1 surendra surendra 0 Dec 27 11:24 file1.txt

so if you want executable permissions too, apply executable permissions to the file.

chmod g+x file1.txt

output:
-rwxrwsr-x 1 surendra surendra 0 Dec 5 11:24 file1.txt

you should see a smaller 's' in the executable permission position.
How can I find all the SGID set files in Linux/Unix.

find / -perm +2000

The above find command will check all the files which is set with SGID bit(2000).
Can I set SGID for folders?
Yes, you can if it’s required (you should remember one thing, that Linux treats everything as a file)
How can I remove SGID bit on a file/folder?

chmod g-s file1.txt

What is SUID and how to set it in Linux?

SUID (Set owner User ID
 up on execution) is a special type of file permissions given to a file.
 Normally in Linux/Unix when a program runs, it inherits access 
permissions from the logged in user. SUID is defined as giving temporary
 permissions to a user to run a program/file with the permissions of the
 file owner rather that the user who is running it. In simple words users will get file owner’s permissions as well as owner UID and GID when executing a file/program/command.




The above sentence is bit tricky and should be explained in-depth with examples.

Learn SUID with examples:

Example1: passwd command




When we try to change our password we will use passwd command which is owned by root. This passwd command
 file will try to edit some system config files such as /etc/passwd, 
/etc/shadow etc when we try to change our password. Some of these files 
cannot be opened or viewed by normal user only root user will have 
permissions. So if we try to remove SUID and give full permissions to 
this passwd command file it cannot open other files such as /etc/shadow 
file to update the changes and we will get permission denied error or 
some other error when tried to execute passwd command. So passwd command
 is set with SUID to give root user permissions to normal user so that 
it can update /etc/shadow and other files.




Example2: ping command




Similarly if we take ping command, when we have to execute this command 
internally it should open socket files and open ports in order to send 
IP packets and receive IP packets to remote server. Normal users don’t 
have permissions to open socket files and open ports. So SUID bit is set
 on this file/command so that whoever executes this will get owner (Root
 user’s) permissions to them when executing this command. So when this 
command start executing it will inherit root user permissions to this 
normal user and opens require socket files and ports.




Example3: crontab and at command.




When scheduling the jobs by using crontab or at command it is obvious to
 edit some of the crontab related configuration files located in /etc 
which are not writable for normal users. So crontab/at commands are set 
with SUID in-order to write some data.

How can I setup SUID for a file?

SUID can be set in two ways



1) Symbolic way(s, Stands for Set) 


2) Numerical/octal way(4)



Use chmod command to set SUID on file: file1.txt




Symbolic way:




chmod u+s file1.txt




Here owner permission execute bit is set to SUID with +s




Numerical way:



chmod 4750 file1.txt



Here in 4750, 4 indicates SUID bit set, 7 for full permissions for 
owner, 5 for read and execute permissions for group, and no permissions 
for others.




How can I check if a file is set with SUID bit or not?




Use ls –l to check if the x in owner permissions field is replaced by s or S




For example: file1.txt listing before and after SUID set




Before SUID set:




ls -l



total 8

-rwxr--r-- 1 xyz xyzgroup 148 Dec 22 03:46 file1.txt



After SUID set:




ls -l



total 8

-rwsr--r-- 1 xyz xyzgroup 148 Dec 22 03:46 file1.txt

Some FAQ’s related to SUID:

A) Where is SUID used?




1) Where root login is required to execute some commands/programs/scripts.




2) Where you don't want to give credentials of a particular user and but want to run some programs as the owner.




3) Where you don't want to use SUDO command but want to give execute permission for a file/script etc.




B) I am seeing “S” I.e. Capital “s” in the file permissions, what’s that?




After setting SUID to a file/folder if you see ‘S’ in the file 
permission area that indicates that the file/folder does not have 
executable permissions for that user on that particular file/folder.




For example see below example




chmod u+s file1.txt



ls -l
-rwSrwxr-x 1 surendra surendra 0 Dec 27 11:24 file1.txt



If you want to convert this S to s then add executable permissions to this file as show below



chmod u+x file1.txt
ls -l
-rwsrwxr-x 1 surendra surendra 0 Dec 5 11:24 file1.txt



you should see a smaller 's' in the executable permission position now.




SUID with execute permissions:

SUID with out execute permissions:









C) How can I find all the SUID set files in Linux/Unix.




find / -perm +4000




The above find command will check all the files which is set with SUID bit(4000).




D) Can I set SUID for folders?




Yes, you can if its required(you should remember one thing, that Linux treats everything as a file)




E) What is SUID numerical value?




It has the value 4.