How to remove setgid (linux/unix)? - linux

Question

I just changed my file permissions using $ sudo chmod g+s filename and my file permissions turned from drwxr-xr-x to drwxr-sr-x. How do I remove it?

Answer 1

Change the + for adding a permission into a - to remove it:
sudo chmod g-s filename
If you want to do this programatically, you’ll need to use some bitwise operators. Normally it’s
mode_without_suid = bitwise_and(existing_mode, bitwise_not(S_ISUID))
where S_ISUID is 0o4000, a constant that uses mode bits above the typical rwx ones of something like 0644.
For example, in python
import os
import stat
def mode_details(m):
return f"mode={oct(m)} = {stat.filemode(m)}"
mode = os.stat('foo').st_mode
print("old mode", mode_details(mode))
new_mode = mode & ~stat.S_ISUID
os.chmod('foo', new_mode)
print("new mode", mode_details(new_mode))
which prints
old mode mode=0o104654 = -rwSr-xr--
new mode mode=0o100654 = -rw-r-xr--

Answer 2

To remove setgid the numerical way the command is
sudo chmod 0664 $filename
The assumption here is the permission on file is 664 and we are not changing it. The left most bit in the above command represents setuid(4),setgid(2) and sticky(1). Now to represent these symbolically setuid is u+s, setgid is g+s and sticky is o+t
Example 1:-chmod u+s filename
This will setuid for the filename mentioned that is rwsr_xr_x
Example 2: chmod 2770 directory
This will set gid for the directory mentioned that is rwxr_sr_x

Answer 3

Regarding: "you can set (but not clear) the bits with a numeric mode"
On RHEL 7 chmod 0644 $filename did not remove the setuid(4),setgid(2) or sticky(1).
However precedeing with an extra 0 did the trick:
chmod 00644 $filename

Answer 4

Well would just like to add few points to clarify the approach of working with the numerical way for both files and directories.
Adding individual special permissions for either user/group/others.
chmod "X"755 file
Where X is the specific octal numeric mode for special permissions.
If you want to add multiple special permissions at a time, e.g. for both suid(4) and sgid(2) i.e. 4+2=6.
chmod "6"755 file
for suid(4), sgid(2) and sticky bit(1), i.e. 4+2+1=7
chmod "7"755 file
Deleting all special permissions (only applicable for a file)
chmod 00"0"755 file
Well, the trailing zeros before 4 digits doesn't add any values while changing the permission for a file but it does add values while changing permission for a directory.
The above numeric code will change the permission to 755 from 7755 only for a file but if you do the same for a directory it will be 6755 as it will only remove the sticky bit for others.
To remove all the special permissions for a directory.
chmod "000"755 file
Similarly, to remove suid permission and having sgid(2) and sticky bit(1) i.e. 2+1=3.
chmod 00"3"755 file
And solution using letters(r,w,x,X,s,,t) and operators(+/-) were already discussed and approved in the earlier answers.

Answer 5

sgid with number
#chmod 2(permission) (directory name) = for adding
#chmod 0(permission) (directory name) = for removing
sgid with word
#chmod g+s directory name = for adding
#chmod g-s directory name = for removing