Changing permissions with chmod

Only the file/dir owner (or root) can change permissions!

 $ mkdir music;  ls -ld music
 drwx------  1 reegen reegen  4096 Jan 12 11:17 music

 $ chmod u=rwx,g=rx,o= music;  ls -ld music
 drwxr-x---  1 reegen reegen  4096 Jan 12 11:17 music

 $ chgrp friends music;  ls -ld music
 drwxr-x---  1 reegen music   4096 Jan 12 11:17 music

Or, using octal notation

   user   group   other
    rwx    r-x     ---
=>  421    421     421
  
==   7      5       0

 $ chmod 750 music