halt normally invokes shutdown with the -h flag, except when already in runlevel 0, then it tells the kernel to halt the system. But first it notes in the file /var/log/wtmp that the system is being brought down. init is the mother of all processes. It reads its commands from /etc/inittab, which normally tell it which scripts to run for which runlevel, and how many gettys to spawn. killall5 sends a signal to all processes, except the processes in its own session -- so it won't kill the shell running the script that called it. last shows which users last logged in (and out), searching back through the file /var/log/wtmp. It can also show system boots and shutdowns, and runlevel changes. lastb shows the failed login attempts, as logged in /var/log/btmp. mesg controls whether other users can send messages to the current user's terminal. pidof reports the PIDs of the given programs. poweroff tells the kernel to halt the system and switch off the computer. But see halt. reboot tells the kernel to reboot the system. But see halt. runlevel reports the previous and the current runlevel, as noted in the last runlevel record in /var/run/utmp. shutdown brings the system down in a secure way, signaling all processes and notifying all logged-in users. sulogin allows the superuser to log in. It is normally invoked by init when the system goes into single user mode. telinit tells init which runlevel to enter. utmpdump displays the content of the given login file in a friendlier format. wall writes a message to all logged-in users.