Linux:Manual Restart with sysrq-trigger
=======================================
:author: Aaron Ball
:email: nullspoon@oper.io
:revdate: February 15, 2016


== {doctitle}

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
https://en.wikipedia.org/wiki/Magic_SysRq_key[Wikipedia Article].



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.

    # 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*.

    # 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:: (un*R*aw) Takes back control of keyboard from X

echo e > /proc/sysrq-trigger:: (t*E*rminate) 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:: (k*I*ll) 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:: (*S*nc) Sync all cached disk operations to disk

echo u > /proc/sysrq-trigger:: (*U*mount) 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:: (p*O*weroff) Powers off the system

echo b > /proc/sysrq-trigger:: (re*B*oot) Reboots the system


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

    # 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


[role="datelastedit"]
Last edited: {revdate}
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