Linux:Manual Restart with sysrq-trigger
=======================================

A little over a week ago I decided it would be fun to try to write my own
multi-threaded init system in c. Without going into too much detail (I'll write
a blog post that introduces it once it's further along), I needed to find a way
to shutdown and restart my system from c (init 0 and 6, respectively).

I looked around the internet and had a surprisingly difficult time finding what
I was looking for. Every post I found indicated I should use the system
function to call "shutdown -h -P now". While not a wrong or a bad answer, it
wasn't the answer I was looking for. I wanted to know how the shutdown and
reboot commands worked, or how my current init system executed runlevels 0 and
6, respectively.

Remembering something I found a ways back about kernel triggers found in and
/sys/power/ for controlling acpi states, I started searching for something
similar. After a bit of time, I found ``/proc/sysrq-trigger``. For further
reading beyond this post, please see the `Wikipedia Article
<https://en.wikipedia.org/wiki/Magic_SysRq_key>`_.



Simple Usage
------------

The usage of sysrq-trigger is fairly simple. Before we get into exact usage for
the purposes of shutting down or rebooting though, please note that output from
``/proc/sysrq-trigger`` can only be seen from an actual tty, and not from
within a terminal emulator.

That said, execute the following command to find out all of the options
available to you from sysrq.

.. code-block:: sh

  echo h > /proc/sysrq-trigger

If you don't see output from that, either sysrq isn't enabled, or you aren't
executing from within a tty.

To check if it is enabled, ensure that the following command yields a ``1``.

.. code-block:: sh

  cat /proc/sys/kernel/sysrq



Reboot and Shutdown Sequences
-----------------------------

If you executed the ``echo h`` statement above, you likely saw the output of
supported commands. Among them you should have seen ``b`` and ``o``, for reboot
and poweroff, respectively. **Note** that it is a bad idea to execute either of
these right now.

If you read around the internet for what these two do, you'll see that both
cause a hard stop. Neither polietly asks processes to exit. They are roughly
equivelant to the hardware reset and power buttons. If rebooting or powering
off, *we want b or o to be the last command sent* to have as graceful a stop as
possible.


Here is the list of commands that should be sent to sysrq-trigger, in order,
for both a shutdown and a reboot.

* ``echo r > /proc/sysrq-trigger``: (unRaw) Takes back control of keyboard
  from X

* ``echo e > /proc/sysrq-trigger``: (tErminate) Send SIGTERM to all
  processes. If you aren't familiar with process signals, this is basically a
  friendly kill command for every process.

* ``echo i > /proc/sysrq-trigger``: (kIll) Send SIGKILL to all processes.
  If you aren't familiar with process signals, this is basically a kill -9
  command for every remaining process.

* ``echo s > /proc/sysrq-trigger``: Sync all cached disk operations to disk

* ``echo u > /proc/sysrq-trigger``: (Umount) Umounts all mounted partitions


The previous commands all get us to a ready to shutdown or reboot state. To
shutdown or reboot, execute one of the following.

``echo o > /proc/sysrq-trigger``: (pOweroff) Powers off the system

``echo b > /proc/sysrq-trigger``: (reBoot) Reboots the system


The entire list in order, using the reboot command...

.. code-block:: sh

    echo r > /proc/sysrq-trigger
    echo e > /proc/sysrq-trigger
    echo i > /proc/sysrq-trigger
    echo s > /proc/sysrq-trigger
    echo u > /proc/sysrq-trigger
    echo b > /proc/sysrq-trigger