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




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?

Learn SUID with examples:

 

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?

 

1) Symbolic way(s, Stands for Set) 

2) Numerical/octal way(4)

chmod 4750 file1.txt

total 8

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

total 8

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

 

Some FAQ’s related to SUID:

 

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

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

It has the value 4

What is Sticky Bit?

Sticky Bit is mainly used on folders in order to avoid deletion of a folder and its content by other users though they having write permissions on the folder contents. If Sticky bit is enabled on a folder, the folder contents are deleted by only owner who created them and the root user. No one else can delete other users data in this folder(Where sticky bit is set). This is a security measure to avoid deletion of critical folders and their content(sub-folders and files), though other users have full permissions.

Learn Sticky Bit with examples:

 

Example: Create a project(A folder) where people will try to dump files for sharing, but they should not delete the files created by other users.
  
How can I setup Sticky Bit for a Folder?

Sticky Bit can be set in two ways

1. Symbolic way (t,represents sticky bit)
2. Numerical/octal way (1, Sticky Bit bit as value 1)

Use chmod command to set Sticky Bit on Folder: /opt/dump/

Symbolic way:

chmod o+t /opt/dump/
or
chmod +t /opt/dump/

Let me explain above command, We are setting Sticky Bit(+t) to folder /opt/dump by using chmod command.

Numerical way:

chmod 1757 /opt/dump/

Here in 1757, 1 indicates Sticky Bit set, 7 for full permissions for owner, 5 for read and execute permissions for group, and full permissions for others.

Checking if a folder is set with Sticky Bit or not?

Use ls –l to check if the x in others permissions field is replaced by t or T
For example: /opt/dump/ listing before and after Sticky Bit set

Before Sticky Bit set:
ls -l
total 8
-rwxr-xrwx 1 xyz xyzgroup 148 Dec 22 03:46 /opt/dump/

After Sticky Bit set:
ls -l
total 8
-rwxr-xrwt 1 xyz xyzgroup 148 Dec 22 03:46 /opt/dump/

Some FAQ’s related to Sticky Bit:

 

Now sticky bit is set, lets check if user “temp” can delete this folder which is created xyz user.

$ rm -rf /opt/dump
rm: cannot remove `/opt/dump’: Operation not permitted

$ ls -l /opt
total 8
drwxrwxrwt 4 xyz xyzgroup 4096 2012-01-01 17:37 dump
$


if you observe other user is unable to delete the folder /opt/dump. And now content in this folder such as files and folders can be deleted by their respective owners who created them. No one can delete other users data in this folder though they have full permissions.I am seeing “T” ie Capital s in the file permissions, what’s that?
After setting Sticky Bit to a file/folder, if you see ‘T’ in the file permission area that indicates the file/folder does not have executable permissions for all users on that particular file/folder.

Sticky bit without Executable permissions:




so if you want executable permissions, Apply executable permissions to the file.

chmod o+x /opt/dump/
ls -l command output:
-rwxr-xrwt 1 xyz xyzgroup 0 Dec 5 11:24 /opt/dump/

Sticky bit with Executable permissions:





you should see a smaller ‘t’ in the executable permission position.
How can I find all the Sticky Bit set files in Linux/Unix.

find / -perm +1000
The above find command will check all the files which is set with Sticky Bit bit(1000).

Can I set Sticky Bit for files?
Yes, but most of the time it’s not required.

How can I remove Sticky Bit bit on a file/folder?
chmod o-t /opt/dump/

This script moves on log files listed on the command line. It keeps all but the most recent one
compressed, and removes the last one once there are more than CYCLES of them. For example,
CYCLES=3 ; rotate messages Would have the following effects.

messages --> messages.1
messages.1 --> messages.2.gz
messages.2.gz --> messages.3.gz
messages.3.gz --> removed

script :

#!/bin/sh
# Rotate a log file and keep N copies
# Mostly stolen from inn
CYCLES=${CYCLES-5}
COMPRESS=/usr/local/bin/gzip
Z=.gz
for F in $* ; do
## Compress yesterday’s .1
test -f ${F}.1 \
&& ${COMPRESS} <${F}.1 >${F}.1${Z} \
&& rm -f ${F}.1 \
&& chmod 0440 ${F}.1${Z}
## Do rotation.
EXT=${CYCLES}
rm -f ${F}.${CYCLES}${Z}
while [ ${EXT} -gt 0 ] ; do
NEXT=${EXT}
EXT=‘expr ${EXT} - 1‘
test -f ${F].${EXT}${Z} \
&& rm -f ${F}.${NEXT}${Z} \
&& mv ${F}.${EXT}${Z} ${F}.${NEXT}${Z}
done
mv ${F} ${F}.1
done

RedHat Linux - Run Levels 

 

0: Halt