1 Linux:Manual Restart with sysrq-trigger
2 =======================================
3
4 A little over a week ago I decided it would be fun to try to write my own
5 multi-threaded init system in c. Without going into too much detail (I'll write
6 a blog post that introduces it once it's further along), I needed to find a way
7 to shutdown and restart my system from c (init 0 and 6, respectively).
8
9 I looked around the internet and had a surprisingly difficult time finding what
10 I was looking for. Every post I found indicated I should use the system
11 function to call "shutdown -h -P now". While not a wrong or a bad answer, it
12 wasn't the answer I was looking for. I wanted to know how the shutdown and
13 reboot commands worked, or how my current init system executed runlevels 0 and
14 6, respectively.
15
16 Remembering something I found a ways back about kernel triggers found in and
17 /sys/power/ for controlling acpi states, I started searching for something
18 similar. After a bit of time, I found ``/proc/sysrq-trigger``. For further
19 reading beyond this post, please see the `Wikipedia Article
20 <https://en.wikipedia.org/wiki/Magic_SysRq_key>`_.
21
22
23
24 Simple Usage
25 ------------
26
27 The usage of sysrq-trigger is fairly simple. Before we get into exact usage for
28 the purposes of shutting down or rebooting though, please note that output from
29 ``/proc/sysrq-trigger`` can only be seen from an actual tty, and not from
30 within a terminal emulator.
31
32 That said, execute the following command to find out all of the options
33 available to you from sysrq.
34
35 ::
36
37 # echo h > /proc/sysrq-trigger
38
39 If you don't see output from that, either sysrq isn't enabled, or you aren't
40 executing from within a tty.
41
42 To check if it is enabled, ensure that the following command yields a ``1``.
43
44 ::
45
46 # cat /proc/sys/kernel/sysrq
47
48
49
50 Reboot and Shutdown Sequences
51 -----------------------------
52
53 If you executed the ``echo h`` statement above, you likely saw the output of
54 supported commands. Among them you should have seen ``b`` and ``o``, for reboot
55 and poweroff, respectively. **Note** that it is a bad idea to execute either of
56 these right now.
57
58 If you read around the internet for what these two do, you'll see that both
59 cause a hard stop. Neither polietly asks processes to exit. They are roughly
60 equivelant to the hardware reset and power buttons. If rebooting or powering
61 off, *we want b or o to be the last command sent* to have as graceful a stop as
62 possible.
63
64
65 Here is the list of commands that should be sent to sysrq-trigger, in order,
66 for both a shutdown and a reboot.
67
68 * ``echo r > /proc/sysrq-trigger``: (un**R**aw) Takes back control of keyboard
69 from X
70
71 * ``echo e > /proc/sysrq-trigger``: (t**E**rminate) Send SIGTERM to all
72 processes. If you aren't familiar with process signals, this is basically a
73 friendly kill command for every process.
74
75 * ``echo i > /proc/sysrq-trigger``: (k**I**ll) Send SIGKILL to all processes.
76 If you aren't familiar with process signals, this is basically a kill -9
77 command for every remaining process.
78
79 * ``echo s > /proc/sysrq-trigger``: (**S**nc) Sync all cached disk operations
80 to disk
81
82 * ``echo u > /proc/sysrq-trigger``: (**U**mount) Umounts all mounted partitions
83
84
85 The previous commands all get us to a ready to shutdown or reboot state. To
86 shutdown or reboot, execute one of the following.
87
88 ``echo o > /proc/sysrq-trigger``: (p**O**weroff) Powers off the system
89
90 ``echo b > /proc/sysrq-trigger``: (re**B**oot) Reboots the system
91
92
93 The entire list in order, using the reboot command...
94
95 .. code-block:: sh
96
97 echo r > /proc/sysrq-trigger
98 echo e > /proc/sysrq-trigger
99 echo i > /proc/sysrq-trigger
100 echo s > /proc/sysrq-trigger
101 echo u > /proc/sysrq-trigger
102 echo b > /proc/sysrq-trigger
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