Update - verified to work also with Mac OS X
From within VI
To make this behaviour the default:
user@machine:~$ cd ~
user@machine:~$ touch .exrc
user@machiner:~$ vi .exrc
Add the following line to the file:
user@machine:~$ . .exrc
user@machine:~$ vi someTextFile.txt
Basically, if the file .exrc does not exist in your home directory create it.
Open .exrc for editing.
Add the line (w/o quotes) "set number"
Test to verify function.
Update (as I've had to look this up twice now...)
To turn off line numbers from withing VI