The setgid bit on programs

When a program with the setgid bit set is run, it runs with an 'effective' gid of the group of the file

          # cp /usr/bin/id ./setxid
          # ls -l setxid
          -rwxr-xr-x    1 root     root       13052 Apr 19  4:43 setxid

          # chgrp web setxid
          # chmod u+s setxid
          # ls -l setxid
          -rwxr-sr-x    1 root     web        13052 Apr 19  4:43 setxid

          $ /usr/bin/id
          uid=1010(bri) gid=1010(bri) groups=1010(bri)
          
          $ ./setxid
          uid=1010(bri) gid=1010(bri) egid=0(web) groups=1010(bri)