Tell init(8) to wait sec seconds between sending processes the warning and the kill signal, before changing to another runlevel.
Don't really shutdown; only send the warning messages to everybody.
Reboot after shutdown.
Halt after shutdown.
Skip fsck on reboot.
Force fsck on reboot.
Cancel an already running shutdown. With this option it is of course not possible to give the time argument, but you can enter a explanatory message on the command line that will be sent to all users.
When to shutdown.
Message to send to all users.
The time argument can have different formats. First, it can be an absolute time in the format hh:mm, in which hh is the hour (1 or 2 digits) and mm is the minute of the hour (in two digits). Second, it can be in the format +m, in which m is the number of minutes to wait. The word now is an alias for +0.
If shutdown is called with a delay, it creates the advisory file /etc/nologin which causes programs such as login(1) to not allow new user logins. Shutdown removes this file if it is stopped before it can signal init (i.e. it is cancelled or something goes wrong). It also removes it before calling init to change the runlevel.
The -f flag means `reboot fast'. This only creates an advisory file /fastboot which can be tested by the system when it comes up again. The boot rc file can test if this file is present, and decide not to run fsck(1)? since the system has been shut down in the proper way. After that, the boot process should remove /fastboot.
The -F flag means `force fsck'. This only creates an advisory file /forcefsck which can be tested by the system when it comes up again. The boot rc file can test if this file is present, and decide to run fsck(1)? with a special `force' flag so that even properly unmounted filesystems get checked. After that, the boot process should remove /forcefsck.
shutdown can be called from init(8) when the magic keys CTRL-ALT-DEL are pressed, by creating an appropriate entry in /etc/inittab. This means that everyone who has physical access to the console keyboard can shut the system down. To prevent this, shutdown can check to see if an authorized user is logged in on one of the virtual consoles. If shutdown is called with the -a argument (add this to the invocation of shutdown in /etc/inittab), it checks to see if the file /etc/shutdown.allow is present. It then compares the login names in that file with the list of people that are logged in on a virtual console (from /var/run/utmp). Only if one of those authorized users or root is logged in, it will proceed. Otherwise it will write the message
shutdown: no authorized users logged in to the (physical) system console. The format of /etc/shutdown.allow is one user name per line. Empty lines and comment lines (prefixed by a #) are allowed. Currently there is a limit of 32 users in this file.
A lot of users forget to give the time argument and are then puzzled by the error message shutdown produces. The time argument is mandatory; in 90 percent of all cases this argument will be the word now.
Init can only capture CTRL-ALT-DEL and start shutdown in console mode. If the system is running the X window System, the X server processes all key strokes. Some X11 environments make it possible to capture CTRL-ALT-DEL, but what exactly is done with that event depends on that environment.