1 Linux:Manual Restart with sysrq-trigger
2 =======================================
3 :author: Aaron Ball
4 :email: nullspoon@oper.io
5 :revdate: February 15, 2016
6
7
8
9 A little over a week ago I decided it would be fun to try to write my own
10 multi-threaded init system in c. Without going into too much detail (I'll write
11 a blog post that introduces it once it's further along), I needed to find a way
12 to shutdown and restart my system from c (init 0 and 6, respectively).
13
14 I looked around the internet and had a surprisingly difficult time finding what
15 I was looking for. Every post I found indicated I should use the system
16 function to call "shutdown -h -P now". While not a wrong or a bad answer, it
17 wasn't the answer I was looking for. I wanted to know how the shutdown and
18 reboot commands worked, or how my current init system executed runlevels 0 and
19 6, respectively.
20
21 Remembering something I found a ways back about kernel triggers found in and
22 /sys/power/ for controlling acpi states, I started searching for something
23 similar. After a bit of time, I found */proc/sysrq-trigger*. For further
24 reading beyond this post, please see the
25 https://en.wikipedia.org/wiki/Magic_SysRq_key[Wikipedia Article].
26
27
28
29 Simple Usage
30 ------------
31
32 The usage of sysrq-trigger is fairly simple. Before we get into exact usage for
33 the purposes of shutting down or rebooting though, please note that output from
34 /proc/sysrq-trigger can only be seen from an actual tty, and not from within a
35 terminal emulator.
36
37 That said, execute the following command to find out all of the options
38 available to you from sysrq.
39
40 # echo h > /proc/sysrq-trigger
41
42 If you don't see output from that, either sysrq isn't enabled, or you aren't
43 executing from within a tty.
44
45 To check if it is enabled, ensure that the following command yields a *1*.
46
47 # cat /proc/sys/kernel/sysrq
48
49
50
51 Reboot and Shutdown Sequences
52 -----------------------------
53
54 If you executed the _echo h_ statement above, you likely saw the output of
55 supported commands. Among them you should havE seen *b* and *o*, for reboot and
56 poweroff, respectively. _Note that it is a bad idea to execute either of these
57 right now._
58
59 If you read around the internet for what these two do, you'll see that both
60 cause a hard stop. Neither polietly asks processes to exit. They are roughly
61 equivelant to the hardware reset and power buttons. If rebooting or powering
62 off, *we want b or o to be the last command sent* to have as graceful a stop as
63 possible.
64
65
66 Here is the list of commands that should be sent to sysrq-trigger, in order,
67 for both a shutdown and a reboot.
68
69 echo r > /proc/sysrq-trigger:: (un*R*aw) Takes back control of keyboard from X
70
71 echo e > /proc/sysrq-trigger:: (t*E*rminate) Send SIGTERM to all processes. If
72 you aren't familiar with process signals, this
73 is basically a friendly kill command for every
74 process.
75
76 echo i > /proc/sysrq-trigger:: (k*I*ll) Send SIGKILL to all processes. If you
77 aren't familiar with process signals, this is
78 basically a kill -9 command for every remaining
79 process.
80
81 echo s > /proc/sysrq-trigger:: (*S*nc) Sync all cached disk operations to disk
82
83 echo u > /proc/sysrq-trigger:: (*U*mount) Umounts all mounted partitions
84
85
86 The previous commands all get us to a ready to shutdown or reboot state. To
87 shutdown or reboot, execute one of the following.
88
89 echo o > /proc/sysrq-trigger:: (p*O*weroff) Powers off the system
90
91 echo b > /proc/sysrq-trigger:: (re*B*oot) Reboots the system
92
93
94 The entire list in order, using the reboot command...
95
96 # echo r > /proc/sysrq-trigger
97 # echo e > /proc/sysrq-trigger
98 # echo i > /proc/sysrq-trigger
99 # echo s > /proc/sysrq-trigger
100 # echo u > /proc/sysrq-trigger
101 # echo b > /proc/sysrq-trigger
102
103
104 [role="datelastedit"]
105 Last edited: {revdate}
106 // vim: set syntax=asciidoc:
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