when you do an "ls -l" you will see something similar:
drwxr-xr-x 3 rom rom 4096 Sep 24 15:13 courses
-rw------- 1 root root 441 Nov 24 18:32 dead.letter
-rwxr--r-- 1 root root 15801 Dec 4 20:49 iwscanswitch.c
-rw-rw-r-- 1 rom rom 10486 Oct 15 15:10 mimeinstall.results
drwxr-xr-x 2 rom rom 4096 Dec 1 16:01 music
two users there - root, and rom. the ones starting with "d" are directories.
the others are all files with this "rwx rwx rwx".
read it as :
rwx rwx rwx
User Group Others
so, this "-rwxr--r-- 1 root root 15801 Dec 4 20:49 iwscanswitch.c" would mean, that the user has read,write,exec perms on the file. group and others have only read perms on it.
if you want everyone (user,group,others) to be able to do everything on this , you will say,
chmod ugo+rwx iwscanswitch.c = all users, all perms
chmod go+rw iwscanswitch.c = group+others, read+write
chmod go-rx iwscanswitch.c = take awat read/write perms for group+others.
and u get the idea.
the other way is to specify the numbers, like "chmod 755 iwscanswitch.c" which is similar to giving user all perms, and group+others permission to read+execute.
the numbers in 755 correspong to the binary equivalent of "rwxrwxrwx", taken 3 at a time. you know the first three from the left correspond to User, the middle 3 to Group, and the 3 on the right to Others.
User rwx: 000 = 0 - no perms
101 = 5 - read+exec
110 = 6 - read+write, but no exec
111 = 7 - all actions permitted .... and so on. hope u get the idea. so i get the 7 in 755 from the above to give the user rwx perms on the file.
ill stop now. hope that helps.