Interpreting this command A few days ago I found the following command: for i in 0 1 2 3 4 5 6 S ; do ln -s /etc/rc$i.d /etc/rc.d/rc$i.d ; done As far as understand this command is going to create a symbolic link between each file using the for cycle, but **what I can't really understand is the S in that numeration, what is it supposed to do?**

Those numbers aren't randomly selected, they're the runlevels of your system. The runlevel used to determine which init scripts are run. They're mostly obsolete now. And if you're on Linux, it's highly likely that the runlevels S and 1 are the same. Your documents might be really old, or they'd probably be using `update-rc.d` instead of manually creating symlinks.

So your loop is iterating over all runlevels, 1-6 and S.