How to remove setgid (linux/unix)?

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

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--