8.3. More about shutdowns
It is important to follow the correct procedures when you shut
down a Linux system. If you fail do so, your filesystems probably
will become trashed and the files probably will become scrambled.
This is because Linux has a disk cache that won't write things
to disk at once, but only at intervals. This greatly improves
performance but also means that if you just turn off the power
at a whim the cache may hold a lot of data and that what is on
the disk may not be a fully working filesystem (because only
some things have been written to the disk).
Another reason against just flipping the power switch is that
in a multi-tasking system there can be lots of things going on
in the background, and shutting the power can be quite
disastrous. By using the proper shutdown sequence, you ensure
that all background processes can save their data.
The command for properly shutting down a Linux system
is shutdown. It is usually used in one of
two ways.
If you are running a system where you are the only user,
the usual way of using shutdown is to quit
all running programs, log out on all virtual consoles, log
in as root on one of them (or stay logged in as root if you
already are, but you should change to root's home directory or
the root directory, to avoid problems with unmounting), then
give the command shutdown -h now (substitute
now with a plus sign and a number in minutes
if you want a delay, though you usually don't on a single user
system).
Alternatively, if your system has many users, use the command
shutdown -h +time message, where
time
is the
time in minutes until the system is halted, and
message
is a short explanation of why the system is shutting down.
# shutdown -h +10 'We will install a new
disk. System should
> be back on-line in three hours.'
# |
This will warn everybody that the system will shut down in
ten minutes, and that they'd better get lost or lose data.
The warning is printed to every terminal on which someone is
logged in, including all
xterms:
Broadcast message from root (ttyp0) Wed Aug 2 01:03:25 1995...
We will install a new disk. System should
be back on-line in three hours.
The system is going DOWN for system halt in 10 minutes !! |
The warning is automatically repeated a few times before the boot,
with shorter and shorter intervals as the time runs out.
When the real shutting down starts after any delays, all
filesystems (except the root one) are unmounted, user processes
(if anybody is still logged in) are killed, daemons are shut down,
all filesystem are unmounted, and generally everything settles
down. When that is done, init prints out a
message that you can power down the machine. Then, and only then,
should you move your fingers towards the power switch.
Sometimes, although rarely on any good system, it is
impossible to shut down properly. For instance, if the kernel
panics and crashes and burns and generally misbehaves, it might
be completely impossible to give any new commands, hence shutting
down properly is somewhat difficult, and just about everything
you can do is hope that nothing has been too severely damaged
and turn off the power. If the troubles are a bit less severe
(say, somebody hit your keyboard with an axe), and the kernel
and the update program still run normally,
it is probably a good idea to wait a couple of minutes to give
update a chance to flush the buffer cache,
and only cut the power after that.
In the old days, some people like to shut down using the command
sync three times, waiting for the disk I/O to stop,
then turn off the power. If there are no running programs, this is
equivalent to using shutdown. However, it
does not unmount any filesystems and this can lead to problems
with the ext2fs ``clean filesystem'' flag. The triple-sync
method is not recommended.
(In case you're wondering: the reason for three syncs is
that in the early days of UNIX, when the commands were
typed separately, that usually gave sufficient time for most
disk I/O to be finished.)
