Feaking geeks! Wish I knew what that you just said.
With the -G alone, a new file is created with the user in the new group, and overwrites the old existing file. With the -a -G. the new group and user membership are appended to the existing file, thereby not removing the user from the groups he is already a member of.
It's the same thing as doing, in a terminal;
[polack@littleboy ~]$ echo hello > ~/hello.txt [polack@littleboy ~]$ cat hello.txthello
A text file named hello.txt is created in your ~/ directory, and contains the single word hello.
[polack@littleboy ~]$ echo goodby > ~/hello.txt[polack@littleboy ~]$ cat hello.txtgoodby
Now the ~/hello.txt file contains only the word goodby.
[polack@littleboy ~]$ echo hello > ~/hello.txt[polack@littleboy ~]$ cat hello.txthello
Now the file again contains only the word hello.
[polack@littleboy ~]$ echo goodby >> ~/hello.txtBy adding the second
> the command is changed to
append the word goodby, with this result:
[polack@littleboy ~]$ cat hello.txthello
goodby
