chmod allows any permissions changes on any file you own on a writable medium, including adding setgid. To do this, use:
chmod g+s <file>
g+s sets the setgid bit on the file.
You can also specify the full permissions of the file:
chmod 6755 <file>
This changes the permissions of the file to
In both cases, replace
<file> with the actual name of the file. You can perform this operation on multiple files by putting more than one file name on the command line in place of
For more information on
chmod and UNIX filesystem permissions, see the Wikipedia articles on
chmod and filesystem permissions.