Single user mode is defined as a special case and although the man page for shutdown and init indicate that you can reach single user mode using those commands. However, init s definitely does not enter singloe user mode as it leaves services running and mounted filesystems. Similarly, shutdown without an option (-r or -h) can still leave filesystems mounted. So, unlike other init states, single user mode is not managed by init.
It is important to understand that single user mode is very special and to note that only 2 filesystems will be mounted: / and /stand. All sysadmins need to understand this. They also need to understand that /usr (which contains /usr/bin and /usr/lib) are not mounted and for good reason. If a problem develops with the /usr filesystem, you may need single user mode to fix the problem. All the commands you have in single user mode are located in /sbin which is part of /.
Once in single user mode, you can gain access to /usr commands such as vi by doing this:
mount /usr
mount /tmp
mount /var
and now vi (and bdf and others) will be available. Don't use mount -a since this will mount everything including network filesystems which cannot be used in single user mode (no networking). So, you don't want to change the default behavior of single user mode, but to remember the environment when it is used.
Bill Hassell, sysadmin
